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CHAPTER XIII

IMPAIRED STABILITY
 

13-1. Foreword. The stability of a vessel may be impaired by one or more of the following causes:

1. Addition of topside weight.
2. Removal of low weight.
3. Deterioration of reserve buoyancy.
4. Flooding.

Impairment may be due to our own mistakes, or it may result from enemy action. The purpose of this chapter is to analyze stability and its consequences.

13-2. Addition of topside weight. The addition of appreciable amounts of topside weight may be occasioned by:

1. Deck cargo.
2. Provisions, ammunition, or stores not struck down.
3. Icing of the topside.
4. Large numbers of survivors topside.
5. Alterations, etc.

When a weight of considerable magnitude is added above the existing location of the ship's center of gravity the effects are as follows:

1. Consuming a part of the reserve buoyancy.
2. Reduction of GM and right arms due to raising G.
3. Reduction of GM and righting arms due to lost freeboard.
4. Reduction of righting arms if G is pulled away from the centerline.
5. Increase in righting moment due to increased displacement.

The net effect of added high weight is always a reduction in stability. The reserve buoyancy lost is the added weight in tons. The new metacentric height can be obtained from the following formula:

G1M1 = KM1 - KG1.
KM1 from curves of form, for new mean draft.
KG1 from formula, Article 7-7.

Stability is determined by taking a new stability curve from the cross-curves and correcting it for AG, sin θ (per Art. 7-3) and G1G2 cos θ (per Art. 7-4), then multiplying the residual righting arms by the new

  displacement (W + w) to obtain a curve of residual righting moments. For this purpose the general stability diagram is helpful.

Problem:

Analyze the stability characteristics of U.S.S. MIDDLETOWN if 800 tons of ice are added to the topside. The windward side is icing more heavily than the leeward side. Assume that the center of gravity of the ice is 50 feet above the keel and that the ship has a 13° list to port. The following conditions existed before icing:

Displacement, 12,400 tons.
KG, 23.6 feet.

Answer:

Figures 13-1 and 13-2 are a stability data sheet and general stability diagram which have been used to analyze this problem. The answer is tabulated at the bottom of figure 13-1.

13-3. Removal of low weights. The removal of appreciable amounts of weight from the underwater body may be occasioned by:

1. Using fuel oil and not ballasting as prescribed.
2. Using large quantities of ammunition, as in a bombardment.
3. Removing machinery, heavy spare parts, etc.
4. Jettisoning fuel, feed water, ammunition, and other low weights.
5. Pumping out damage water from low compartments.
6. Alterations, etc.

When a weight of considerable magnitude is removed below the existing location of the ship's center of gravity, the effects are as follows:

1. Increase in reserve buoyancy.
2. Increase of GM and righting arms due to increased freeboard.
3. Reduction of GM and righting arms due to raising G.
4. Reduction of righting arms if G is pulled away from the centerline.

 

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Figure 13-1. U.S.S. MIDDLETOWN; stability data sheet for problem in Article 13-2.
Figure 13-1. U.S.S. MIDDLETOWN; stability data sheet for problem in Article 13-2.
 

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Figure 13-2. U.S.S. MIDDLETOWN; general stability diagram for problem in Article 13-2.
Figure 13-2. U.S.S. MIDDLETOWN; general stability diagram for problem in Article 13-2.
 

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Figure 13-3. U.S.S. MIDDLETOWN; stability data sheet for problem in Article 13-3.
Figure 13-3. U.S.S. MIDDLETOWN; stability data sheet for problem in Article 13-3.
 

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Figure 13-4. U.S.S. MIDDLETOWN; general stability diagram for problem in Article 13-3.
Figure 13-4. U.S.S. MIDDLETOWN; general stability diagram for problem in Article 13-3.
 

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5. Reduction in righting moment due to reduced displacement.

The net effect of removing a low weight usually is a reduction in stability. The reserve buoyancy increase is the removed weight in tons. The new meta-centric height may be found from the following formula.

G1M1 = KM1 - KG1.
KM1 from curves of form, for new mean draft.
KG1 from formula, Article 7-7.

Stability is found by taking a new stability curve from the cross-curves and correcting it for AG, sin θ (per Art. 7-3) and G1G2 cos θ (per Art. 7-4), then multiplying the residual righting arms by the new displacement (W - w) to obtain a curve of residual righting moments. For this purpose the general stability diagram is useful.

Problem:

Analyze the stability characteristics of U.S.S. MIDDLETOWN if the following ammunition is removed:

A-506M, 117 tons, centerline, 7 feet above keel.
A-507M, 213 tons, 12 feet to starboard, 7 feet above keel.
A-508M, 74 tons, centerline, 6 feet above keel.
D-501M, 92 tons, centerline, 7 feet above keel.
D-502M, 188 tons, 14 feet to starboard, 8 feet above keel.
D-503M, 105 tons, centerline, 8 feet above keel.

Before the removal, the ship had no trim, no list, displacement was 13,600 tons, and KG was 24.7 feet.

Answer:

Figures 13-3 and 13-4 are a stability data sheet and a general stability diagram that have been used to analyze this problem. The answer is tabulated at the bottom of figure 13-3.

13-4. Loss of reserve buoyancy. There are two ways in which reserve buoyancy can be lost to a ship:

1. It is used up by the addition of weight.
2. It is rendered ineffectual if its boundaries are ruptured.

After a portion of the reserve buoyancy has been lost due to addition of weight, a rough gauge of the remaining reserve buoyancy is provided by the ratio of the remaining mean freeboard to the total depth of the hull. This is true only if the boundaries of the above-water body remain intact. However, if the shell of the ship is pierced above the waterline, or if a portion of it is carried away by an enemy hit, the affected part of the reserve buoyancy is vitiated,

  In preventing rupture of the shell and weather deck above the waterline, armor and ballistic plating serve the important function of protecting reserve buoyancy. Once the external boundaries of the ship have been damaged, subdivision of the above-water body limits the extent of reserve buoyancy loss. In order to make the ship functional and habitable, however, subdivision must be pierced by access and ventilation. The practical solution is to provide closures which restore watertight integrity to the boundaries that are pierced.

Reserve buoyancy may be lost due to our own errors, or as a result of enemy action. The following possibilities are included:

1. Own mistakes.

a. Poor maintenance (leaving boundaries non-watertight).
b. Failure to close fittings properly.
c. Improper or lax classification of fillings.
d. Overloading the ship.

2. Enemy action.

a. Blast from a hit carries away boundaries.
b. Fragments or missiles penetrate boundaries.
c. Blast blows closed fittings open.
d. Blast warps open fittings, preventing closure.
e. Flooding overloads the ship.

When the above-water body is riddled, some reserve buoyancy is destroyed. Also, when the ship rolls it can no longer immerse buoyant volume on the damaged side, toward which it is rolling. The immersion of buoyant volume is necessary to the development of a righting moment as the ship rolls. The effect of riddling the above-water body is analogous to losing a part of the freeboard, thus reducing stability. When this happens, if the ship takes water aboard on the roll, the combined effects of high added weight and free surface operate to cut down the righting moment.

Figure 13-5 shows the probable effect on the stability curve of a riddled shell. The highest portion of the group of curves represents the righting arms before the hit. As soon as the shell is opened up, all the freeboard from the bottom of the shell hole upward is lost. The effect is to cut down the curve from the intact value to AC. In other words, the difference between curves 0AB and 0AC is occasioned by lost freeboard. As soon as the vessel rolls beyond the angle at which the lower edge of the hole is immersed, she will ship water on the second deck. The added high weight and the free surface effect raise G, producing a final curve that will resemble 0DE.

 

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Figure 13-5. Probable effect on stability curve of a shell hole above the waterline.
Figure 13-5. Probable effect on stability curve of a shell hole above the waterline.
 
The practical conclusion from this discussion indicates the necessity for not only plugging and patching the shell and sub-division of the underwater body, but also of exerting every effort to restore the watertightness of external and internal boundaries in the above-water body.

13-5. How flooding takes place. There are various ways in which flooding water may enter a ship. Included among them are the following:

1. Underwater damage (torpedo, mine, or near miss).
2. Shell or bomb bursting below decks.
3. Topside hit opening shell at or above the waterline.
4. Collision.
5. Water from fire fighting.
6. Ruptured water piping.

  7. Sprinkling (or flooding) of magazines, handling rooms, etc.
8. Counterflooding.
9. Leakage in voids, bilges, etc.

The ballasting of fuel tanks with sea water after the oil has been removed is not considered to be in the category of taking on flooding water. Rather, it is a case of replacing one liquid with another for the purpose of maintaining the ship in its maximum condition of resistance to damage.

Regardless of how flooding water enters the ship, it is convenient to classify it in three general categories, each of which may be further broken down, as follows:

1. With respect to boundaries:

a. Solid flooding.
b. Partial flooding with boundaries intact.
 

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Figure 13-A. Riddled bulkheads result when fragmentations fly with projectile violence from a bursting bomb, projectile, of torpedo. The relationship of such damage to progressive flooding is obvious.
Figure 13-A. Riddled bulkheads result when fragmentations fly with projectile violence from a bursting bomb, projectile, of torpedo. The relationship of such damage to progressive flooding is obvious.

 

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c. Partial flooding in free communication with the sea.

2. With respect to height in the ship:

a. Center of gravity of the flooding water is above G.

b. Center of gravity of the flooding water is below G.

3. With respect to the ship's centerline.

a. Symmetrical flooding.

b. Off-center flooding.

13-6. Solid flooding. The term solid flooding designates the circumstance in which a compartment is completely filled from deck to overhead deck. This means that every available cubic foot of the compartment is occupied, either by flooding water, or by impermeable matter which was in the compartment before it was bilged. In order to flood solidly, a compartment must be vented. Venting can take place through an air-escape, through an open scuttle or ventilation fitting, or through fragment holes in the overhead.

If a compartment whose overhead deck is above the waterline is to flood solidly, the bulkheads and deck of the compartment must be undamaged, and the source of flooding must be internal. However, solid flooding with ruptured boundaries can occur if the compartment is completely below the waterline.

Solid flooding water behaves exactly like an added weight, and has no other effect than so many tons placed exactly at the center of gravity of the flooding water. The effects of an added high weight have already been covered in Article 13-2. Solid flooding, however, is more likely to occur below the waterline, where it has the following effects of any added low weight:

1. Loss of part of the reserve buoyancy.
2. Reduction of righting arms due to lost freeboard.
3. Increase in GM and righting arms due to a lowering of G.
4. Reduction of righting arms if G is pulled away from the centerline.
5. Increase in righting Moments due to increased displacement.

Inasmuch as G usually is a little above the waterline in warships, the net effect of solid flooding below the waterline is most frequently a gain in stability, unless a sizeable list or a serious loss of freeboard results in a net reduction. The reserve buoyancy consumed is, obviously, the weight of the flooding water in tons. The new GM and stability characteristics are found in the same fashion as that employed for added high weights in Article 13-2.

  13-7. Partial flooding with boundaries intact. The term partial flooding refers to a condition in which the flooding water would lie somewhere between the deck and the overhead if the ship were on an even keel. The compartment is partially but not completely filled. The expression "with boundaries intact" means that the deck on which the water rests, and the bulkheads which surround it, remain watertight. If the boundaries remain intact, water will neither run into nor out of the flooded compartment as the ship rolls.

Partial flooding can be brought about in any of several ways, for example, the following:

1. Leakage to compartments beyond the damaged area through riddled or distorted bulkheads and decks, or through defective fittings.
2. Seepage through riddled or distorted shell plating.
3. Shipping water on the roll through holes in the above-water body.
4. Draining of damage water down from higher levels.
5. Loose water remaining in the ship after plugging a large hole in the side.
6. Partial filling of a compartment due to a low hole admitting water which compresses entrapped air above it (this borders on free communication).
7. Loose water from fire-fighting efforts, sprinkling systems, etc.
8. Broken fire main, or rupture of other piping which handles liquid under pressure.

Partial flooding has all the effects on stability of an added weight, and in addition, the free surface effect reduces GM and righting moments. The results of partial flooding are:

1. Loss of a part of the reserve buoyancy.
2. Reduction in GM and righting arms due to lost freeboard.
3. Increase (or reduction) in GM and righting arms due to the added weight moving G down (or up).
4. Reduction in GM and righting arms due to free surface effect.
5. Reduction in righting arms if the added weight pulls G away from the centerline.
6. Increase in righting moments due to increased displacement.

Except where the free surface is relatively narrow (athwartship) and the weight is added low in the ship, the final net outcome of partial flooding is likely to be

 

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Figure 13-B. Another common type of damage. Observe the warped bulkhead, the severed electric cables, and the damaged vent duct. The integrity of this boundary has been seriously impaired.
Figure 13-B. Another common type of damage. Observe the warped bulkhead, the severed electric cables, and the damaged vent duct. The integrity of this boundary has been seriously impaired.
 

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a decided loss in overall stability. Both surface permeability and pocketing tend to reduce the adverse effects of partial flooding, so that the net loss is not quite as great as it might otherwise be. In any case the reserve buoyancy expended is the added weight in tons, while the new metacentric height may be computed from the formula:

G1M1 = KM1 - (KG1 + i/V).

With:
KM1 from the curves of form for the new mean draft. KG1 by calculation as in Article 7-7.
i/V the free surface effect, computed as in Article 8-4.

The expression (KG1 + i/V) may be regarded as the virtual height of the ship's center of gravity after partial flooding has taken place. It is for this virtual height of G that the new righting-arm curve should be corrected after taking it from the cross-curves. In other words, a curve of (AG1 + i/V) sin θ will be deducted from the new uncorrected stability curve. (This curve of righting arms is not apt to be accurate at large angles of heel inasmuch as the effect of pocketing has been neglected.) Subsequently G1G2 cos θ should be deducted from the righting-arm curve if the flooding is off-center, before multiplying residuals by the new displacement to obtain the final curve of righting moments.

When free surface exists, and there is known to be a significant surface permeability factor, the results will be more accurate if a reasonable estimate of the surface permeability is used. In this case free surface effect is represented by si/V.

Problem:

Analyze the stability of U.S.S. MIDDLETOWN after partial flooding of compartment C-204L, if the flooding water measures 4 feet deep by 64 feet athwart-ship by 32 feet fore-and-aft. The displacement before damage was 12,700 tons and KG was 24.1 feet. The ship has no list or trim either before or after damage. The flooding water rests on the second deck, which is 28 feet above the keel.

Answer:

Figures 13-6 and 13-7 are a stability data sheet and a general stability diagram that have been used to analyze this problem. The answer is tabulated at the bottom of figure 13-6.

13-8. Partial flooding in free communication with the sea. Free communication occurs when a free

  surface exists and the shell is so badly ruptured that the sea is free to flow in and out as the ship rolls. Free communication does not exist unless there is a condition of partial flooding, and the water level in the compartment remains at sea level regardless of the movements of the ship. This condition usually occurs when a large hole in the side extends above and below the waterline, as in the case of damage caused by a shallow torpedo hit. It can also occur when there is a large hole in the shell below the waterline and the partially flooded waterline compartment is vented as the ship rolls.

Free communication is modified by several factors which tend to constrict or interfere with it. These include:

1. Size of hole. A small hole in the shell, or inadequate venting constrict communication, in which case the condition approaches partial flooding with boundaries intact.

2. Surface permeability breaks up the surface and reduces the amount of water that moves in and out.

3. The location of intact decks with respect to the waterline causes pocketing as they submerge and emerge on the roll.

The above factors (three) reduce the potential adverse influence which free communication has on stability. This adverse influence is evaluated as a free-communication effect (ay2/V) which is used in conjunction with free-surface effect (i/V).

The effects of partial flooding in free communication with the sea are exactly the same as those for partial flooding with boundaries intact, except that wherever free surface effect occurs, free communication effect is added to it. In finding the height of the ship's center of gravity above the keel, for the purposes of determining the new GM and making a sine curve correction of the righting-arm curve as taken from the cross-curves, the sum of i/V and ay2/V should be added to the new KG1. Both effects are regarded as causing a virtual rise in G. The expression ay2/V should be multiplied by s where an important value of surface permeability is known to exist.

Partial flooding in free communication with the sea is regarded as an added weight (whose effects are based on even-keel measurements) with a reduction in stability for free surface effect plus another reduction in stability for free communication effect. The latter reduction accounts for the weight of water that moves in or out of the compartment as the ship either

 

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Figure 13-6. U.S.S. MIDDLETOWN; stability data sheet for problem in Article 13-7.
Figure 13-6. U.S.S. MIDDLETOWN; stability data sheet for problem in Article 13-7.
 

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Figure 13-7. U.S.S. MIDDLETOWN; general stability diagram for problem in Article 13-7.
Figure 13-7. U.S.S. MIDDLETOWN; general stability diagram for problem in Article 13-7.
 

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Figure 13-8A.
Figure 13-8A.
 
rolls or lists away from an upright position (see Chapt. VIII).

Problem:

Analyze the overall stability of U.S.S. MIDDLE-TOWN after the following damage: a torpedo hit at frame 70 on the starboard side floods the engine room, C-1, and wing tanks C-907F, C-909F, and C-915F in free communication with the sea (see fig. 13-8). The flooding water is an average of 8 feet from the overhead. At about the same time a bomb passes through the main and second decks, and out through the shell on the starboard side, flooding ice machine room, D-305E, in free communication with the sea (see fig. 13-9). The ship finally lists at 8° to starboard. Prior to damage, the ship was in the following condition:

Displacement, 13,038 tons.
KM, 27.5 feet.
KG, 23.5 feet.
GM, 4.0 feet (not corrected for free surface).
Free surface effect in ship's tankage, 0.18 feet.
GM, corrected for free surface, 3.82 feet.

After damage the mean draft was observed to he 23.0 feet.

Answer:

Figures 13-10 and 13-11 are a stability data sheet and a general stability diagram that have been used to analyze this problem. In figure 13-10, the weights

  in column 3 were obtained from the flooding effect diagram of the Damage Control Book for U.S.S. MIDDLETOWN. Unflooded depth was recorded in column 4. Values in column 5 were computed by subtracting unflooded weight from column 3.

Unflooded weight =
(length X breadth X unflooded depth)/35

Unflooded weights are subtracted, because the irregular shape of the compartment bottoms prevents proportioning the weight of flooding water to the depth of the sounding. The answer is tabulated at the bottom of figure 13-10.

13-9. Additional factors to be considered. There are several factors which modify the effects of flooding and which have not been evaluated numerically in the foregoing discussion. Their influence can either be applied numerically, or can be estimated and evaluated qualitatively. These factors include the following items:

1. Volume permeability.
2. Surface permeability.
3. Pocketing.
4. Constriction of free communication.
5. Excessive trim.
6. Free surface in ship's tanks.

Volume permeability is the ratio of the number of cubic feet of liquid that actually can flow into a

 

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Figure 13-8B.
Figure 13-8B.
 
compartment to the number of cubic feet of volume it would have if completely empty. The volume permeability ratio is reduced by watertight solid objects that are already in the compartment before it is bilged. Whenever w, the added weight of sea water taken aboard is calculated, it should be multiplied by volume permeability. A note is sometimes placed on the flooding effect diagram, indicating that certain volume permeabilities have been used in obtaining the weights of flooding water shown on the diagram.

Surface permeability is the ratio of the moment of inertia of an actual free surface to the moment of inertia that the free surface would have if there were no solids projecting through it. Some surface permeability factors are given in Chapter VIII. When

  available, and if significant, these factors should be used by multiplying i/V and ay2/V by s.

Pocketing occurs whenever free surface or free communication is broken up (or reduced in effective area) due to the rise or fall of undamaged decks as the ship lists or rolls. No method of numerically evaluating pocketing is suggested. Its effect is to diminish the loss of stability which is indicated by deducting a curve of (i/V) sin θ from the stability curve. With pocketing, the loss calculated as (ay2/V) sin θ is diminished in the same fashion.

Constriction of free communication occurs when the size of the hole admitting the sea is reduced. As the hole is made smaller and smaller, ay2/V is cut clown further and further, until finally it disappears and

 

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Figure 13-9A.
Figure 13-9A.

Figure 13-9B.
Figure 13-9B.

 

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Figure 13-10. U.S.S. MIDDLETOWN: stability data sheet for problem in Article 13-8.
Figure 13-10. U.S.S. MIDDLETOWN: stability data sheet for problem in Article 13-8.
 

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Figure 13-11. U.S.S. MIDDLETOWN: general stability diagram for problem in Article 13-8
Figure 13-11. U.S.S. MIDDLETOWN: general stability diagram for problem in Article 13-8.
 

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only i/V is left. Another case of restriction occurs when air is trapped above the surface of the liquid.

Excessive trim superimposes a change in transverse stability on all of the effects enumerated above. Trim by the bow in some ships causes a loss of righting arm. If a broad stern is lifted out, this becomes appreciable (see Chapt. X).

Free surface in ship's tanks used in its broad sense refers to the i/V for loose liquids in any tank that is not pressed full; including fuel oil, reserve feed water, and fresh-water tanks. The i/V for all the tankage in the

  ship must be added to KG whenever GM is sought, and whenever a righting-arm curve is to be corrected for the height of G. Free surface effect for various conditions of loading is given in the ship's Booklet of Inclining Experiment Data.

13-10. Note. The foregoing discussion of methods used in calculating the effects of flooding should not be construed as a tool for computing stability in the heat of action. For this the reader is referred to Chapter XVII-"Estimate of the Damaged Ship's Situation."

 

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CHAPTER XIV

LIST
 

14-1. Foreword. As used in this text, the term list refers to any permanent angle of athwartship inclination, whereas heel is temporary (involving motion) and roll is temporary, involving recurrent motion from side to side. A ship in a seaway will roll about its angle of list. This Chapter is devoted to a discussion of how list due to different causes influences the stability characteristics of the rolling ship.

A severely damaged ship may or may not take on a permanent list. If the vessel remains upright, but stability characteristics are seriously affected, the ship will be logy and a series of heavy beam seas may capsize her. In such case GM is small but positive, and there are no off-center weights. However, it is more common for a seriously damaged ship to take on a list, and it is vitally important that the ship's company understand the cause of the inclination before they attempt to correct it. The list that a damaged ship takes on may be attributed to one of three basic causes, as follows:

1. Off-center weight with GM positive.
2. Negative GM.
3. Off-center weight with GM negative.

OFF-CENTER WEIGHTS
(List due solely to off-center weights, with GM positive.)

14-2. Nature of list due to off-center weights. Under designed operating conditions, a ship's loading is so distributed that the moment of all weights on the port side equals the moment of all weights on the starboard side. When this condition exists, G is on the centerline and the ship remains upright.

The term off-center weight means that this balance has been disturbed by one or more weights whose moment has not been compensated. Such an unbalance may be created by adding, removing, or shifting weights. The consequence of an athwartship alteration is to pull G off the centerline by an amount wd/W, and this forces the ship to list at such an angle that B has moved off the centerline and has become centered under G again. If disturbed now, as in a seaway, the ship will roll about its angle of list.

  Off-center weights may be introduced in a ship in various ways, such as the following:

1. Flooding of compartments which are off the centerline.
2. Flooding of compartments which are not symmetrical with respect to the centerline.
3. Flooding of centerline compartments which have off-center volume permeability.
4. Severe explosions which shift machinery or other heavy weights.
5. Severe explosions which remove machinery or parts of the ship.
6. Liquid running out of damaged side or wing tanks whose upper boundaries are above the waterline.
7. Pumping liquids across the ship.
8. Pumping side or wing tanks overside.
9. Pumping flooding water overside from off-center compartments.
10. Removing large quantities of stores, ammunition, etc., from off-center compartments.
11. Jettisoning topside weights from one side only.

To show the effect on stability characteristics of list due to off-center weights, superimpose a curve of inclining moments on the righting-moment curve, with the angle of list occurring where the two curves cross. If both the inclining and righting moments are divided by displacement, the result is a curve of inclining arms superimposed on a curve of righting arms.

14-3. Variations in the angle of list. Obviously, there are two ways in which the angle of list due to off-center weights may be changed, as follows:

1. By variation in the righting moments.
2. By variation in the inclining moments.

Assuming that successive increments of free surface effect reduce the stability curve in a series of steps, a family of righting-moment curves is produced, each lower than the preceding one. A given off-center weight produces a given cosine curve of inclining moments, which will intersect each of these righting-moment curves at a successively greater angle of list (see fig. 14-1). A small increase in free-surface

 

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Figure 14-A. The fate of the LAFAYETTE indicates the importance of considering stability. Her capsizing was the result of high weight and free surface, rather than off-center weight, since she had no fore-and-aft bulkheads where the enormous quantities of fire-fighting water were poured into her.
Figure 14-A. The fate of the LAFAYETTE indicates the importance of considering stability. Her capsizing was the result of high weight and free surface, rather than off-center weight, since she had no fore-and-aft bulkheads where the enormous quantities of fire-fighting water were poured into her.
 

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Figure 14-1.
Figure 14-1.

Figure 14-2.
Figure 14-2.
 

effect beyond that of curve 4 will place the righting-moment curve below the inclining-moment curve and the ship will capsize. Note the small amount of residual stability in the case of curve 4.

So long as GM remains positive, the curve of righting moment starts out above the base, and any list that the ship takes on will be due to off-center weight producing a cosine curve, such as that of figure 14-1.

  Assuming that successive increments of off-center weight increase the inclining arm in successive steps, a family of cosine curves will be produced, each higher than the preceding one. A given righting-moment curve will intersect each of the cosine curves at a successively greater angle of list, as in figure 14-2. A small increase in off-center weight beyond curve 4 will place the inclining-moment curve above the righting-
 

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Figure 14-3. The shaded area shows residual stability of a ship which has listed to one-half the angle of maximum righting arm.
Figure 14-3. The shaded area shows residual stability of a ship which has listed to one-half the angle of maximum righting arm.

Figure 14-4. Same ship as figure 14-5, listing at a given angle, with unimpaired GM, a large cosine curve, and high rate of development of righting arm.
Figure 14-4. Same ship as figure 14-5, listing at a given angle, with unimpaired GM, a large cosine curve, and high rate of development of righting arm.
 

moment curve and the ship will capsize. Note the small amount of residual stability in the case of curve 4.

From figures 14-1 and 14-2 it may be deduced that a ship cannot safely list to the angle of its maximum righting arm. if stability is unimpaired, but off-center weights cause the ship to take on a permanent list as great as the angle of the maximum righting arm, little or no residual dynamic stability is left. A small impulse will cause the ship to roll beyond her range of stability and she will capsize. On the other hand, if righting arms are reduced by lost freeboard, added high weights, or free surface, a relatively small off-center weight will cause a relatively large permanent list. Figure 14-1 indicates that this list could not become as great as the angle of maximum righting arm on the original curve, without resulting in capsizing.

  Note the relative amount of residual stability remaining to the ship in figure 14-3, which has listed to exactly one-half the angle of maximum righting arm.

14-4. The "feel" of a listed ship. A ship which is listing due to off-center weights may feel stiff and roll with a snap, or it may feel tender and have a slow, logy roll. Two identical ships may list at a given angle, one with unimpaired GM and a large off-center weight, the other with impaired GM and a small off-center weight. The first ship is likely to be fairly stiff and stable, whereas the second is likely to be tender and sluggish in returning from a roll, even though the angle of list is the same in both cases.

This may most easily be shown if the rate of development of righting arm is found on the residual curve in both cases. Figure 14-4 shows the first case, with

 

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Figure 14-5. Same ship as figure 14-4, listing at a given angle, with low stability curve (impaired GM), small cosine curve, and lower rate of development of righting arm.
Figure 14-5. Same ship as figure 14-4, listing at a given angle, with low stability curve (impaired GM), small cosine curve, and lower rate of development of righting arm.

Figure 14-6.
Figure 14-6.
 

unimpaired GM and a large cosine curve. Here the rate of development of righting arm is high. Figure 14-5 depicts the second case, where impaired GM has produced a low stability curve, so that a small cosine curve produces the same list. Here the rate of development of righting arm is smaller.

It is the rate of development of righting arm that makes a ship feel stiff or tender. The foregoing statements indicate that the feel of a listed ship is a further index to her relative stability.

NEGATIVE METACENTRIC HEIGHT
(List due solely to negative metacentric height.)

14-5. Why ships may list without off-center weight. It is possible for a damaged ship to list even though all solid weights in it are symmetrically disposed and all flooding is symmetrical with respect to the

  centerline. Under such conditions G is on the centerline but has risen above M. When the center of gravity is above the metacenter, upsetting arms instead of righting arms develop for the first few degrees of heel. The ship cannot remain upright, and must either capsize or list to some angle where positive righting arms again develop.

When negative GM thus creates upsetting arms, positive righting arms can again develop at some angle of heel due to one or both of the following two causes:

1. Form and weight characteristics of ship.
2. Pocketing of loose water.

In the case of some ships which have a cargo type hull, the form and weight characteristics produce a stability curve resembling the broken line curve of figure 14-6. The initial portion of the curve is markedly concave upward. Assuming that high weights are

 

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Figure 14-7.
Figure 14-7.
 
added, raising the center of gravity, a curve of AG1 sin θ may be superimposed and deducted from the righting arms. The sine curve of loss of righting arm in figure 14-6 lies above the GZ curve during initial angles of heel. At about 20° it crosses the righting-arm curve; then lies below it up to about 50°. Thus, from 0° to 20° upsetting arms develop, and from 20° to 50° righting arms develop. In calm water the ship will list at 20°, and in a seaway it will roll about this angle of list.

Residual values of righting arm have been plotted with respect to the base of figure 14-6, producing the solid line curve. This is the curve of stability for a ship which is listing due to negative GM, with positive stability restored by reason of its form and weight characteristics.

The cause of the negative GM may be a large amount of loose water which produces a virtual rise in G. If the loose water is pocketed as the ship heels over, the effect of the pocketing is to diminish the correction curve of (i/V) sin θ, as shown in figure 14-7. The diminished correction curve intersects the stability curve at two points, about 22° and about 47°. Below 22° the correction curve is greater than the stability curve, and upsetting arms develop. Over 22° and up to 47° the correction curve is lower than the stability

  curve, and righting arms develop. In calm water the ship will list at 22°, and in a seaway it will roll about this angle of list.

The solid line curve of figure 14-7 shows the residual righting arms plotted with respect to the base. This is the curve of stability of a ship which is listing due to negative GM, with positive stability restored due to pocketing of loose water. (If the free surface were not pocketed, the full value of the sine curve would be in effect and the ship would capsize).

Note that in both cases the residual dynamic stability is relatively small. A ship with negative GM may have seriously impaired stability, although this is not necessarily the case for all types of vessels.

14-6. Causes and effects of list due to negative GM. A list due to negative metacentric height may be brought about by one or more of the following causes, any and all of which contribute to reducing the ship's GM:

1. Added high weights.
2. Removed low weights.
3. Loose water, causing a virtual rise in G due to free surface and free communication effects.

Loose water is the major contributing cause of negative GM in warships. In the combatant types, changes of loading in excess of the design limits are rare. How

 

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Figure 14-8. Cosine curve of inclining arm superimposed on the righting-arm curve of a ship with negative GM.
Figure 14-8. Cosine curve of inclining arm superimposed on the righting-arm curve of a ship with negative GM.
 
ever, severe damage frequently introduces large areas of loose water, which render GM negative and would cause the ship to capsize if it were not for the beneficial effect of pocketing and the shape of the hull. Negative GM is more likely to occur in the smaller ships, and in cargo and auxiliary types. It is not to be expected in battleships and large aircraft carriers, although it might be a possibility in the most extreme cases.

A ship with negative GM and no off-center weights will list with equal facility to either port or starboard, and in a seaway or after a hard turn, may "flop" over to the other side. A damaged ship with negative GM is frequently in jeopardy. A damaged ship with very low GM (almost negative) can also be in a very critical condition. The following conditions should lead damage-control personnel to suspect the existence of negative metacentric height:

1. Listing with no off-center weights and no external inclining moments such as those due to wind and rudder.
2. A list to either side, or flopping from side to side.
3. Large areas of broad free surface, particularly if near the waterline.
4. Enormous amounts of unusual topside weight.

A policy of minimum liquid loading requirements to assure adequate stability as well as underwater protection is prescribed by FTP-170B. Such liquid loading requirements are based on avoiding a serious reduction of stability due to lightening the ship, and insuring against undue list after damage. The necessary amount of ballasting to place the ship in its optimum condition of resistance to damage is prescribed by the Bureau of Ships for vessels of various types and classes.

 
OFF-CENTER WEIGHTS WITH NEGATIVE METACENTRIC HEIGHT
(List due to a combination of negative meta-centric height and off-center weights.)

14-7. Causes and effects of the combination. It has already been indicated that the most common cause of negative metacentric height is damage involving large amounts of loose water. It is rare that a damaged ship has no off-center weights whatsoever. Consequently, a combination of negative GM and off-center weights is more likely to occur than is negative GM alone. The effect on stability of such a case can be shown by superimposing a cosine curve of inclining arm on the righting-arm curve of a ship with negative GM, as has been done in figure 14-8. The angle of list now occurs where the inclining-arm curve crosses the righting-arm curve. Note the small amount of residual dynamic stability.

In the case of a ship like that represented in figure 14-8, the center of gravity has risen above the meta-center and moved off the centerline. The ship is now likely to list to one side only, and to remain listing to that side even after a hard turn, although she may lurch heavily to the opposite side. She will be logy and sluggish in returning from a roll on the down side, but may "flop" in coming back if heeled toward the high side. Failure to follow ballasting instructions is an invitation to this condition in case of extensive flooding.

Example:

U.S.S. MIDDLETOWN has used most of her fuel and failed to ballast any of her tanks. As a result, displacement is 9,600 tons and GM is 2.6 feet. Gunfire damage at the waterline causes her to ship loose water

 

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on the second deck. The breadth of free surface is 60 feet, and its total length is 88 feet. Flooding is to an average depth of 2 1/2 feet.

Added weight = (60 X 88 X 2.5) / 35 = 377 tons.

New displacement = 9,977 tons.

Assume that there is no appreciable shift in G due to added weight or shift in M due to change of draft. However, a large free surface effect exists:

i/V = b3l/12V = (60 X 60 X 60 X 88) / (12 X 35 X 9,977) = 4.5 feet.

The 4.5 foot free-surface effect is larger than the original GM of 2.6 feet. Hence, GM after damage is negative. If, now, there is off-center flooding in one of the unballasted fuel tanks, the result will be a list due to the combination of off-center weights and a free surface which pockets as the ship heels over.

A list due to a combination of off-center weights and negative initial stability cannot always be identified by "flopping" in a seaway, since the ship probably will list to one side only. Damage-control personnel can suspect this condition, however, if one or more of the following circumstances exists:

1. Large areas of broad free surface, accompanied by some unsymmetrical flooding or other off-center weights.

2. Considerable amounts of topside weight or high

  flooding, accompanied by unsymmetrical flooding or other off-center weights.

3. A considerably greater list than the known amount of off-center weight should produce.

4. The ship listing to one side only, but very logy and sluggish in its response to wave action or other disturbance.

14-8. Correcting list due to off-center weight and negative GM. Recognition of the cause of a list involves identifying conditions which could render GM negative, and at the same time identifying conditions which could introduce off-center weights. In attempting the correction of a list it is advisable to compensate only for the known amount of off-center weight when using measures which create a transverse moment. In fact, it is advisable to slightly undercompensate, and subsequently make up the difference as required. If an attempt is made to correct immediately both the list due to negative GM and the list due to off-center weights, by any measure which creates a transverse moment (such as pumping fuel oil across the ship), one of two things will happen:

1. The ship will flop over (and list to an even greater angle on the other side).

2. The ship will capsize.

A more complete analysis of corrective measures will be found in Chapters XVIII and XIX.

 

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CHAPTER XV

UNDERWATER EXPLOSIONS
 

15-1. Foreword. Preceding chapters have described what happens when stability and reserve buoyancy are impaired. Now we shall examine one of the primary causes of such impairment-underwater explosions, and the protection embodied in ship designs to counteract such explosions.

GENERAL

15-2. Types of underwater explosions. Underwater explosions affecting ships may be classified in four groups as follows:

1. Contact explosions at the side.
2. Non-contact explosions at the side.
3. Contact explosions under the bottom.
4. Non-contact explosions under the bottom.

15-3. Principal fields of research in progress. A long series of caisson experiments to investigate the effects of underwater explosions on ship structure was started in our Navy in 1908. This work has progressed almost continuously since that date, and has been greatly intensified in the last few years. The present program includes the following principal fields of investigation:

1. The fundamental phenomena of underwater explosions.

2. The damaging effects of underwater explosions upon ship structures.

3. Practical means of increasing the resistance of ship structures to damage from underwater explosions.

With respect to item 1 above, high-speed photography and scientific measuring devices, especially developed for such work, are being used to determine what takes place in an underwater explosion in terms of fundamental quantities on a time scale, with respect to pressure, energy, and momentum associated with the explosive charge, and deflections and stresses experienced by the structure. Theories have been developed which, in special cases, have shown close agreement with experimental results. Much remains to be done, however, in exploring basic phenomena before the damaging effects of an underwater explosion can be calculated with a sufficient degree of accuracy for practical purposes. With respect to items 2 and 3, tests are being conducted with models of ship structures, and the results of these tests are being correlated

  with cases of actual war damage. In this connection, the war damage reports submitted by the forces afloat are of great value. The basic research mentioned under (1) has given a clearer understanding of the characteristics of underwater explosions, and the whole program of investigation has provided a firm basis for certain improvements in ship structure. Still further improvement is hoped for as the program continues.

15-4. Nature of underwater explosions. Detonation of a heavy charge of a high explosive underwater results in the following effects:

1. A high velocity shock wave or pressure pulse travels out through the water in all directions from the explosion. This is followed by the formation of an expanding globe of incandescent gases. The shock wave and gases may have any or all of a number of effects in turn:

a. Initial destruction of structure and machinery, the devastation decreasing as the distance from the explosion increases.

b. Creation of a large hole in the shell plating. In ships without torpedo-protection systems, this will cause flooding of one or more principal compartments.

c. Formation of large structural fragments. In ships with no liquid protection along the side, these fragments may penetrate inboard bulkheads to permit progressive flooding, start fires if inflammables are present, or possibly cause the explosion of thin-walled ammunition. Whenever practicable, therefore, ships without torpedo protection-systems are given a measure of protection by providing a liquid layer abreast certain spaces.

d. Violent movement of the ship as a whole.

e. Often there is initial whipping of the ship, if the explosion is near one end, causing a flexural vibration.

f. If the explosion occurs in contact with the ship, the expansion of gases takes place in and up through the ship's structure, with flash and incendiary effect.

g. There is a rapid flow of water out in all directions, as it is pushed out of the way by the expanding globe of gases.

 

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Figure 15-A. A destroyer which survived contact explosion (torpedo) at the side.
Figure 15-A. A destroyer which survived contact explosion (torpedo) at the side.

 

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h. The gas globe contracts and expands several times, sending out additional shock waves which may augment the whip or flexural vibration initiated by the first shock wave.

15-5. The effects of underwater explosions which endanger a ship's ability to survive. The results of an underwater explosion may be such as to endanger a ship's ability to survive. The factors involved are as follows:

1. Ability to keep the ship afloat. The entrance of damage water into the ship will reduce transverse stability, reserve buoyancy, and longitudinal stability. The ship may be lost by capsizing, or by sinking. Also, in addition to local structural damage, the longitudinal strength of a light ship may be so impaired that she will break in two near the middle. If the hit is near one end, that end may break off.

2. Ability to control and extinguish fires. Fuel oil from damaged tanks will spread through the damaged area. Fire may result from the incandescent gases and the heated structural fragments. Fire pumps may be destroyed and fire mains ruptured, making it difficult to control and extinguish fires. These circumstances eventually may lead to magazine or gasoline explosions.

3. Ability to stay in action or repel attack. Turbines, boilers, shafting, and steering gear may be destroyed, causing the ship to lose mobility and maneuverability. Turrets may be immobilized and fire-control gear shattered. Electric power and illumination may be lost due to the throwing of circuit breakers and other shock damage. Excessive list and trim will reduce speed, and limit or prevent gunnery and aircraft operations. Whip or shock effect may rupture brittle fittings and ruin delicate instruments.

4. Ability to reach a safe haven. If the ship can float but cannot fight it may be necessary to beach her immediately, if possible, or make for the nearest safe haven. Extensive or progressive flooding, or serious structural damage, may make such an operation hazardous.

15-6. Contact explosions at the side. In World War I and in World War II, explosions at the side have been far more frequent than under-bottom explosions. The former type of explosion may produce any of the possible effects described in Article 15-5 in varying degrees of intensity, depending upon the type of ship and the point of impact. The effects upon ships with and without torpedo-protection systems will be discussed in subsequent Articles.

  15-7. Non-contact explosions at the side. "Near-miss bombs constitute the majority of cases of non-contact explosions at the side. Such an explosion very close to the side produces essentially the same effects as a contact explosion, with one important difference. If the non-contact explosion is even four or five feet from the side, and at an appreciable depth below the surface, there will be interposed between the ship and the explosion a liquid layer which will prevent or greatly reduce the formation of fragments of ship's structure, and will impede the actual bomb fragments so that they will do little or no damage. If the explosion is very close to the surface, fragments from the bomb will penetrate the ships' shell plating at or above the waterline. As in the case of under-bottom shots, and probably to a more pronounced degree as the distance of the explosion from the ship increases, the local structural damage decreases rapidly. If the side explosions occur at depth greater than ship draft, the effects approach those of distant under-bottom explosions. The effects of such explosions are again manifested as bodily movement, and vibration of the ship as a whole."-FTP-170B.

15-8. Contact explosions under the bottom. Explosions under the bottom produce essentially the same effects as explosions at the side.

"When a torpedo or contact mine explodes under the bottom of the ship, the result is widespread serious damage to structure and machinery in the affected area, since it is impracticable to provide the necessary torpedo-protection layers underneath the ship to completely guard against this type of attack. Dependence is placed on transverse subdivision and distribution of vital activities to limit the effects of any one hit. Such a hit near an end of the ship will also cause violent whip, with consequent serious shock damage to electrical equipment and delicate fire-control gear, as well as any brittle fittings such as cast iron foundations, etc. Under-bottom explosion of an influence type mine or torpedo, or of a bomb that passes clear through the ship's bottom, will have an effect similar to contact under-bottom explosions. There are no cases on record of contact explosions under the flat bottom of the middle portion of a large ship such as a battleship. There would be advantages in developing a weapon to produce a contact explosion in this location, but there would also be disadvantages in that special control devices would be required for this particular purpose. A more probable attack is one in which the explosion takes place only a few feet under the bottom, but not actually in contact. Such attacks have already occurred from (1) charges placed under the bottom by hand;

 

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or (2) a large bomb passing clear through the ship and detonating just below the bottom. The charge weight is quite definitely known in all of these cases, and it is estimated that the damage is not materially greater than it would be against the side of a ship having side structure equivalent to the bottom structure here attacked. In other words, for very close under-the-bottom explosions, the explosive charge does not gain appreciably in effectiveness by virtue of being under the bottom rather than at the side.

"There is one case on record of a contact explosion well under the bottom of an aircraft carrier, near the stern where the ship's bottom has a distinct V-shape. Here again the structural damage was not appreciably greater than would be expected from a contact explosion at the side, against similar structure. There was, however, a pronounced vertical whipping motion induced at both ends of the ship, as sometimes happens with contact explosions against the ship's side, near one end.

"It has not been found possible, within the limitations of weight and space, to devise a structural system of ship protection which will prevent rupture of a ship's bottom by a fairly large charge (of the order of charges in modern torpedoes) detonating in contact or a few feet below a ship's bottom. Even the heavy triple-bottom construction of recent battleships is insufficient for this purpose. A great deal can be done, however, in localizing damage from this cause by providing heavy triple-bottom structure and by providing heavy transverse bulkheads closely spaced, which are very effective in sharply limiting structural damage from any underwater explosion. There is some benefit to be gained by carrying liquids in bottom tanks, but this is not sufficient to warrant a requirement that bottom tanks be kept ballasted for this purpose. The benefits are, in fact, much less than those to be gained from liquids in side tanks, and the increase in average operating displacement would be very appreciable if the bottom tanks were always kept full. In those cases where bottom tanks are required to be ballasted this is done primarily for stability reasons."-FTP-170B.

15-9. Non-contact explosions under the bottom. "As the distance below the hull of an under-bottom explosion is increased, the local structural damage is decreased and the affected area is increased. Thus a point is reached where there will be no actual rupture of shell plating, but severe dishing of plating will occur over a large area. The initial shock wave from the explosion will produce a violent upward movement of the ship as a whole, and may induce whipping. It has been shown in small scale that the gas globe from

  a deep underwater explosion will expand and contract several times before it is finally dissipated or comes to the surface. With each contraction of the gas globe a new shock wave is produced, so that a ship subjected to a deep under-bottom explosion will experience a series of distinct shocks. This was known to be the case with several ships attacked by magnetic mines, but it was some time before the true explanation was found."-FTP-170B.

15-10. Damage due to intense vibration or whipping of the ship as a whole. "The conditions necessary to set up this whipping motion are not definitely known, but they are functions of the ship structure and the location of the explosion relative to the nodal points of the ship as a vibrating beam. The whipping of the ship, i.e., the vibration of the ship as a whole, may reach, in its most intense forms, such proportions as to cause buckling and permanent deformation of structural members-not necessarily in the region nearest to the explosion and thus seriously impair the strength of the ship girder. Such cases have been infrequent, but in light ships subjected to underwater attack, careful inspection must be made throughout the ship to determine whether any structural impairment has occurred. Conditions under which such damage may occur cannot be classified, but the probability of its occurrence is more likely on light ships, such as destroyers, with explosions toward the ends of the ship, and with non-contact under-bottom explosions of large mines. In many cases of contact explosions there has been no report of whipping or even of significant shock damage. The effects of whipping and shock on equipment are varied and numerous; e.g., breakage of materials having low impact resistance, misalignment of machinery, malfunctioning of relatively delicate mechanisms such as circuit breakers, relays, tripping and latch mechanisms, derangement of relatively delicate equipment such as gauges, meters, radio and sound equipment. Much had been done in our service in the years before the war in the elimination of materials having low impact resistance, and more recently a great deal more has been done in the development of equipment, especially electrical, having greater inherent shock resistance. In special cases, shock resistant mountings are being provided for equipment and elements of equipment. It is impracticable, however, under present conditions, to accomplish much improvement in this connection to equipment already made and installed. The possibility of such shock damage must be recognized and such precautionary measures as practicable should be taken by the forces afloat to minimize its effects, such as

 

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Figure 15-B. When the shell is carried away decks and bulkheads within the hull become the skin of the ship, and must withstand sea pressure and wave action.
Figure 15-B. When the shell is carried away decks and bulkheads within the hull become the "skin of the ship", and must withstand sea pressure and wave action.

 

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Figure 15-1. Diagram to illustrate a modern torpedo-protection system.
Figure 15-1. Diagram to illustrate a modern torpedo-protection system.
 
isolating idle branch electrical circuits, giving close attention to the operation of equipment especially subject to shock damage, etc."-FTP-170B.

SHIPS WITH TORPEDO PROTECTION SYSTEMS

15-11. Classes of ships designed with torpedo-protection systems. All battleships following BB-42 and all large aircraft carriers (CV-2 and 3, CV-5, 6, and 8, and subsequent CV's) were designed with torpedo-protection bulkhead systems. Certain ships originally designed with torpedo-protection systems have had blisters added to improve torpedo protection, buoyancy, and stability."

15-12. Purpose and extent of torpedo-protection systems. The purpose of torpedo-protection systems in larger ships is to prevent flooding or any of the other effects mentioned in the preceding discussion from reaching vital compartments of the ship.

Torpedo-protection systems generally extend over the vital areas of the ship, including the magazines and main machinery spaces. In the case of some ships, however, varying degrees of protection are provided in way of different spaces. Certain auxiliaries have torpedo-protection systems only in way of cargo ammunition. The principal object of such torpedo-protection bulkheads is to prevent flooding of, and other damage to the vital space protected. It is also extremely important to limit the fore-and-aft vertical extent of flooding into other spaces in order to minimize angle of list and loss of buoyancy.

  15-13. Design features of torpedo-protection systems. The present type of torpedo-protection system has been developed as a result of very extensive research, experimentation, tests, and practical experience with heavy ships hit by torpedoes. It consists of a series of parallel fore-and-aft bulkheads, several feet apart, just inboard the shell. Every 16 to 20 feet there is a transverse watertight bulkhead traversing this system; and every 36 to 40 feet a main transverse bulkhead extends from the outer shell on one side to the outer shell on the other. The inboard bulkhead is called the holding bulkhead; it is expected to withstand damage and deflection without leakage, even though bulkheads outboard of it are ruptured. The holding bulkhead extends from the shell at the bottom of the ship to the third deck. A lighter bulkhead continues upward to the second deck, one deck height higher than the other protective bulkheads (see fig. 15-1).

15-14. Functions of torpedo-protection systems. Figure 15-1 shows the type of torpedo-protection system currently in use. Present designs vary somewhat, but the basic theory is the same. These systems perform the following functions in resisting damage to vital spaces:

1. Suppressing blast. When the explosion takes place the shell is ruptured with great force. As the distance into the ship increases, the destruction of structure diminishes. The intent of the design is to construct the system so that each of

 

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the torpedo-defense bulkheads will stretch as far as possible, and absorb a maximum of the energy of the explosion before letting go. This so attenuates the explosion that by the time the remaining force impinges upon the holding bulkhead, the latter is strong enough to withstand the resulting distortion without failure. Thus, flooding of the vital inboard spaces is prevented, although wing voids may flood over a considerable length.

2. Suppressing fragments and flash. At the same time, it is necessary to suppress the fragment and flash effects mentioned above. Experience has proved that one deep layer of liquid, either oil or water (several feet in transverse depth) , or two shallower layers of liquid somewhere in the system, are required to impede large fragments of shell plating and other structure produced by the explosion sufficiently to prevent fragment damage to interior bulkheads and extension of flooding. Flash effect should also be eliminated.

3. Suppressing list and trim. When voids on one side of the ship are flooded as a result of the explosion, the ship lists and changes trim due to added off-center weight. The adverse effects of list and excessive trim are principally concerned with:

a. Impaired speed due to:

(1) Increased propulsion resistance.
(2) Increased difficulty in operating the main-propulsion plant.
(3) Possible improper immersion of screws.

b. Impaired maneuverability.

c. Impaired transverse stability.

d. Increased difficulty in servicing and operating guns.

e. On carriers, impairment of flight operations. (There also may be some reduction in stability due to reduced freeboard and loss of reserve buoyancy, but for capital ships this factor is not usually a controlling consideration.)

The fore-and-aft spread of flooding through damaged structure is limited by the main transverse bulkheads, and by the transverse bulkheads in the torpedo-protection system. It also has been found that structural damage to shell plating and ship's structure falls off sharply at a heavy transverse bulkhead.

Carrying one or two layers of liquid outboard has a very important effect in reducing the angle of heel after damage; because:

1. The fore-and-aft extent of flooding is apt to be less.

  2. The possible amount of off-center weight due to flooding is decreased.

3. Flooding inner void spaces in the liquid-backed shell arrangement produces less heeling moment than does flooding outer void spaces in the older arrangement with an air-backed shell.

15-15. Optimum distribution of liquids. When there is liquid backing of the shell, the damage to shell plating is reduced, and the formation of fragments from the structure is greatly restricted. Further, a liquid-backed shell is less likely than an air-backed shell to be folded outward by the explosion. When the shell plating is blown outward it causes increased resistance to propulsion, and may spread the damage as portions of the plating are torn off by action of the sea.

The deflection of the holding bulkhead is not greatly influenced by the transverse position of the protective layer of liquid, provided that it is not immediately against the innermost or holding bulkhead. Liquid in the space immediately outboard of the holding bulkhead causes increased deflection of that bulkhead, which leads to failure of joints or plating.

Carrying three layers of liquid outboard would have an advantage in further reducing the angle of heel after damage. Generally, however, such practice results in unacceptably heavy displacements, with reduction of armored freeboard and reduction of speed. In most cases the practice of carrying two layers of liquid outboard offers the best available compromise in meeting all requirements.

The optimum amount and distribution of liquid for each class of ship depends upon a number of factors, and is determined by special study in each case. The results of these individual studies have been and are being furnished to the forces afloat. For new ships the best distribution of liquids for the optimum battle condition is shown in the Damage Control Book flooding effect diagram. This subject will be discussed more thoroughly in Chapter XXI.

15-16. Impracticable nature of expansion spaces and venting. Frequently suggestions are offered that liquid layers be carried at partially filled levels, with an air space above to provide a cushioning effect, ostensibly to reduce damage to the deck above. Similarly, arrangements for "venting" the explosion to reduce the destructive effect have been proposed. It is an unfortunate fact that no such schemes have yet been found of value, upon test. Similarly, blisters outboard of the shell merely increase the depth and extent of the torpedo-protection system. They do not induce the explosion to vent itself upward, outside the ship.

 

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15-17. Effect of torpedo depth. There have been a few cases of torpedoes striking external heavy armor. The result has been bodily displacement inboard of the section of armor plate in question, with loosening of watertight backing structure and supports above and below. The consequent flooding is less than that normally resulting from a torpedo hit. This refers to armor outboard of the protective layer.

Torpedoes striking above mid-depth, i.e., just below the armor belt, or striking near the turn of the bilge, are likely to cause more severe damage than those striking near mid-depth. Shallow torpedoes are likely to cause more serious damage to the third deck; deep ones will cause the holding bulkhead to leak at its lower boundary.

15-18. Importance of careful maintenance. A torpedo-protection system cannot be depended upon of itself to limit the extent of flooding, but must be supplemented by careful, continuous attention to the watertight integrity of nearby boundaries. For example, the third deck over the torpedo-protection systems, as shown in figure 15-1, usually will leak after a torpedo explosion below it. The watertight bulkhead shown between the third and second decks is provided to limit the transverse extent of flooding from this leakage, and similarly transverse bulkheads are provided to limit fore-and-aft flooding on the third deck. Failure to maintain the integrity of watertight closures in these boundaries has resulted in extensive flooding of third deck spaces. Damage-control preparations should be based on the possibility that some leakage, possibly serious, will take place through the holding bulkhead in way of a torpedo hit.

SHIPS WITHOUT TORPEDO PROTECTION SYSTEMS

15-19. Classes of ships designed without torpedo-protection systems. Because of the great weight and space required to provide a fully effective torpedo-protection system, as described above, it is out of the question to include such systems in any vessels other than battleships and large aircraft carriers. The vast majority of Naval ships, therefore, are without such torpedo-protection systems. The types in this category vary so widely that discussion of their resistance to underwater explosions can only be presented in the most general terms.

15-20. Elements of resistance in all types of ships. As discussed in Chapter I, all types of Naval ships have the following elements of protection against underwater explosions:

  1. Transverse subdivision.
2. Adequate stability.
3. Adequate reserve buoyancy and freeboard.
4. Hull strength.
5. Segregation of machinery units.
6. Armor on cruisers and light carriers.

15-21. Ability of ships without torpedo-protection systems to withstand underwater explosions. Even the rather small fishing craft have withstood fairly violent shocks from nearby underwater explosions. As size increases, hulls become stronger, and are able to resist larger explosions and those that occur close aboard. A number of destroyers have returned to port after receiving single torpedo hits. Merchant ships of cargo type have in many cases withstood one torpedo hit, and in fewer cases have been lost from one hit either by breaking in two, capsizing, or sinking bodily. The results depend largely on the size of the ship, its subdivision, and the location of the hit. Cruisers should survive one torpedo hit very readily and usually can survive two. Some of the newer large cruisers can probably withstand three torpedo hits if the damage is favorably located.

15-22. Effect of torpedo at mid-depth: ships without torpedo-protection systems. An average torpedo hit on a cruiser normally will bring about flooding of two and possibly three major central spaces inboard, rupture of the deck over the explosion probably with some distortion of the main deck, and possibly fire from ignition of fuel oil thrown throughout the damaged area and on the topside by the hit. If machinery or shafting is in the damaged area, main propulsion is affected. If a fuel-oil service tank is ruptured, fires may be temporarily extinguished in boilers not put out of action by the hit. If high explosive (bomb, warhead, or depth charge) magazines are in the way of the explosion they may be detonated with catastrophic results. Torpedo hits in gasoline stowage will throw gasoline all about the damaged area, with probably secondary ignition of the fuel, and fierce resultant fires. Shock damage to electrical equipment, machinery, and fire-control instruments is probable. Vital piping systems, electric cables, communication, and ship-control systems will be disrupted in way of the explosion. A list usually develops, and the ship may trim. Lights go out, and the damaged area is filled with smoke and highly toxic gases from the combustion of the high explosive.

15-23. Effects of shallow and deep torpedoes. ships without torpedo-protection systems. A torpedo which strikes the ship's side near the waterline can

 

140
 
Figure 15-2. A represents incorrect ballasting procedure, B correct ballasting procedure.
Figure 15-2. A represents incorrect ballasting procedure, B correct ballasting procedure.
 
cause all of the effects outlined above, and in addition, is very apt to tear the sheer strake and the main and second deck entirely across the ship, with serious decrease in hull strength. A deep torpedo can similarly tear the bilge strake, bottom, garboard strakes, and keel with comparably disastrous effects.

15-24. Effects of underwater hit near ends: ships without torpedo-protection systems. A hit in the bow region may blow off the entire bow section, usually at a heavy transverse bulkhead. (This may be the effect of a secondary gasoline or magazine explosion).A hit near the stern, although less likely to blow off an entire section of the ship, probably will carry away one or all of the screws, and may destroy or render inoperative the rudder and steering gear. In the case of a destroyer, a hit near either end causes flexural vibration that may produce compression failures in the waist of the ship, with the result that she may breakup later in a seaway. Extensive flooding due to a hit near the bow or stern will cause a heavy trim, with an attendant reduction in speed, interference with gunnery due to a wet deck, and serious loss of transverse stability if the weather deck is immersed at either end.

15-25. Value of a liquid layer in wing tanks. In certain types of the ships under discussion, wing tanks are fitted outboard of machinery spaces, and in cruisers, outboard of magazines and other vital spaces throughout the greater part of the ship's length, as shown in figure 15-2. Present instructions call for carrying these tanks filled with fuel, or ballast sea water to the waterline level. There are three reasons for this:

1. Minimizing of list. If the ship receives under-water damage, any resultant list will be much less if wing tanks were filled to normal waterline.
 
Only a relatively small additional amount of flooding is possible in an off-center tank that is filled to the waterline. Figure 15-2 shows what happens when an underwater explosion occurs in way of an empty wing tank. A large list will occur due to the added off-center weight of water which enters the damaged wing tank. On the other hand, B is a case of similar damage after correct ballasting procedure had been followed previously; very little list will occur only that due to flooding over the original waterline.

2. Suppression fragments, flash, and damage to the shell.

3. Protection of high explosive magazines.

Some ships which are not fitted with this type of wing tank have liquid stowage provided in the compartments outboard of the magazines. Directives call for maintaining these tanks in the proper ballasted condition at all times. The liquid layer acts to suppress fragments and flash; penetration of magazines by white-hot fragments obviously is undesirable.

Damage Control Books for combatant types contain a flooding effect diagram which shows the tanks that should be kept filled or ballasted. In addition, specific instructions are separately issued as necessary to ships larger than patrol craft. If there is any doubt as to what tanks should be ballasted in a given ship, the matter should be referred to the appropriate administrative command.

15-26. Conclusion. All ships possess some degree of resistance to underwater explosions, and there is, for each type, some borderline condition in which the ship may be very seriously threatened but will be saved by proper damage-control measures.

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