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5
BORESIGHTING SUBMARINE TORPEDO TUBES
 
A. INTRODUCTION
 
5A1. Importance of boresighting. Torpedoes, compared to guns, are relatively inaccurate and there may be a tendency toward slipshod boresighting. At 4,000 yards, an error of 1 degrees in boresighting results in an error of 210 feet, about half the length of an average ship. Thus it can be seen that boresighting must be sufficiently accurate to prevent errors greater than   1/4 degrees, and even greater refinement is desirable.

In two-power periscopes, the line of sight usually is changed when low power is thrown in. Consequently, in both torpedo firing and boresighting the high power of the periscope should be used in which no movable lenses are in the line of sight.

 
B. BORESIGHTING PROCEDURE
 
5B1. Boresighting submarine torpedo tubes. When a periscope azimuth circle reads 0 degrees (or 180 degrees for the stern tubes), the line of sight through the periscope should be parallel to, or coincident with, the mean axis of the torpedo tubes. The object of boresighting is to determine whether this condition exists, and if it does not, to determine the relative alignment of the forward and after tube nests and the periscope with the azimuth circle.

Boresighting is done while the submarine is on the ways of a marine railway or in drydock, preferably on a cloudy day or at night with low wind velocity. Boresighting should not be done with strong sun on one side of the vessel since

  the unequal heating produces a slight curvature in the vessel's centerline. To create a condition of low wind velocity, it may be necessary to erect canvas shields, extending from the main deck to the ground on both sides of the bow and stern, to protect the outside plumb bob lines from the wind.

To establish the mean axis of each set of tube nests, two main targets must be erected for each nest of tubes. The usual arrangement consists of one set of targets inside and another set outside, fastened to the ship's structure. The outside targets must be as far away from the ends of the torpedo tubes as practicable (Figures 5-2 and 5-3). The two inside targets must be

Figure 5-1. Construction of four main targets and four auxiliary targets in perspective.
Figure 5-1. Construction of four main targets and four auxiliary targets in perspective.
 
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located under the torpedo loading hatch openings (Figure 5-1), so that a plumb line can be dropped from topside to it. Small auxiliary targets on which to plumb up and sight, are located topside above the four main targets.

A set of boresight gages, which fits snugly into the tube bore at each end, is necessary. One boresight gage has a small peephole in its center axis (Figure 5-4) while the center axis

  of the other (Figure 5-5) is bored to receive a boresight gage bushing of a press fit. The bushing is threaded in the bore to receive the external threads of the boresight telescope (Figure 5-6).

5B2. Establishing the centerlines. The first operation is to establish the centerline of each tube on both inside and outside targets.

1. Starting with the outside target, set up the boresight gage with the small peephole

Figure 5-2. Construction of bow main outside target and auxiliary target.
Figure 5-2. Construction of bow main outside target and auxiliary target.
 
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Figure 5-3. Construction of stern main outside target and auxiliary target.
Figure 5-3. Construction of stern main outside target and auxiliary target.
 
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Figure 5-4. Boresight gage with peephole.
Figure 5-4. Boresight gage with peephole.

(Figure 5-4) at the muzzle end of the tube, and the boresight gage for the boresight telescope at the breech end (Figure 5-5).

2. Engage the external threads of the boresight telescope in the internal threads of the boresight gage bushing (Figure 5-6) and secure the boresight lock ring tightly with a spanner wrench.

3. Focus the telescope and adjust the crosswires to the peephole at the muzzle end of the tube.

4. Illuminate the peephole by holding a portable light or flashlight behind it.

5. Remove the boresight gage with the peephole from the muzzle end of the tube and focus the telescope on the target.

6. Have a helper mark the intersection of the crossline on the target.

7. Carry out this procedure with the remaining tubes of the nest. Project the tube

  Figure 5-5. Boresight gage for boresight gage
bushing and boresight telescope.
Figure 5-5. Boresight gage for boresight gage bushing and boresight telescope.

centers on the inside targets in like manner, with the boresighting telescope located in the muzzle of the tube and the peephole in the breech end.

8. Repeat the procedure used for the forward nest with the stern nest of tubes. At this point there are four targets, each bearing marks corresponding to the number of tubes in each nest; that is, for the current 310-foot submarine, there will be two targets with six marks forward (Figure 5-7), and two targets with four marks aft (Figure 5-8).

5B3. Marking off the targets. The next operation consists of marking off the individual targets.

1. Join the projected intersection points with straight lines as indicated. Measure distances on targets as shown in the appropriate figure.

5B4. Finding mean axes. 1. Upon completion of target marking, drop a plumb line from the plumb line adjustment adapter attached to the target to the lower target (Figure 5-9).

 
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2. Have the plumb line adjusted until it splits the various bisecting and diagonal points. Mark this line, representing the horizontal point of impact, by inserting a nail in the top of the upper target.

3. Follow the same procedure for all four targets. At this point, there are four nails, one on each of the upper targets, and a line between the nails on the two targets of any nest to represent the mean axis of that nest.

Figure 5-6. Boresight gage bushing for insertion
in boresight gage Find attachment of boresight
telescope.
Figure 5-6. Boresight gage bushing for insertion in boresight gage Find attachment of boresight telescope.

5B5. Paralleling the mean axes. With the periscope removed from the submarine, provide a transit holding fixture on the periscope opening atop the periscope support. (See Figure 5-10 for construction of this fixture.) With the transit parallel to the mean axis of the forward nest of tubes, as represented-by a line through the nails referred to above, the paralleling is, done as follows:

1. Sight the transit on the nail established on target 2 (Figure 5-8) and project this point to target 1.

2. Measure the distance m. Then lay off distance T, which is equal to a/b X M. Mark this point and project it down to target 2.

  3. Measure distance U. It must be equal to T to satisfy the condition of parallelism. If it is not, swing the transit in azimuth slightly to one side or the other until U is equal to T. Then the transit line of sight is parallel to the mean axis of the forward tubes. Mark this line on top of targets 1 and 2, then project it to a permanent place on the ship's structure, and centerpunch a benchmark.

4. Swing the transit through 180 degrees in azimuth. This azimuth swing of 180 degrees should be done by the double reverse method to obviate any error which the transit itself may have.

a. Sight the transit on the forward benchmark just established.

b. Depress the transit telescope and lightly mark a point p on target 3.

c. Turn the transit 180 degrees in azimuth and resight on the forward benchmark, elevating the transit telescope.

d. Depress the transit telescope and lightly mark another point p on target 3. If the transit is in proper adjustment, points p and P should coincide. If they are apart, the proper mark is halfway between them.

e. Mark this point on target 3 and then project it aft to target 4. Continue the line to a permanent place on the hull and prick punch another benchmark. At this point there is, besides the nails already mentioned above a mark on each upper target. A line through these marks represents a line parallel to the mean axis of the forward tubes. Two benchmarks on the hull represent the same line.

5. Measure the distance V and W. The tangent of the angular difference between the mean lines of the forward and stern tubes is ((W - V) / 12D), if W and V are measured in inches and D is measured in feet. This tangent represents the total angle of error, and if it is less than 0 degrees 5', it is considered negligible. If this error is greater, the officer in charge of boresighting should split the errors between the bow and stern nests and establish the permanent benchmarks accordingly. The required transverse distance in inches between the temporary and final forward benchmarks is equal to Re

 
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(W - V), where e is the horizontal distance from the center of the periscope to the benchmark measured in feet, and R is the ratio of the final bow tube error to the total error. The permanent stern benchmark must be 180 degrees from the forward benchmark. If the error is less than 0 degrees 5', the benchmarks as established originally are considered permanent.

6. If periscope supports are not erected, the same operation may be performed by setting up a transit on the periscope support foundation. Set the transit in such position that the nails in targets 1 and 2 line up. This places the transit on the mean line of the forward tube nest. Project this line aft. Turn the transit 180 degrees in azimuth and proceed as before, placing the

  temporary benchmarks at a convenient location and calculating the angle between the line of the forward and after tube nests. After the periscope supports are installed, it is necessary only to erect the transit atop the periscope supports on the periscope line and parallel the line between the temporary benchmarks. This operation may be performed as follows:

a. Set the transit on the fixture (Figure 5-10). This should be done on a rigid table.

b. Level the base of the fixture, using a level and shims; then level the transit plate.

c. Carry the whole assembly atop the periscope supports and install it in the periscope upper bearing.

Figure 5-7. Two main targets of six tube nests in perspective.
Figure 5-7. Two main targets of six tube nests in perspective.
 
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d. Sight on the forward benchmark, project a line aft using the double reverse method described before, and mark a point next to the stern benchmark.

e. Measure distance S between this point and the benchmark.

f. Next to the forward benchmark, lay off a distance l/n XS, where l is equal to the distance from the forward benchmark to the periscope position, and n is the distance between the after benchmark and the periscope position. Mark this

  forward point on the ship on the same side as the after mark from the after benchmark.

g. Project this point aft 180 degrees with the transit and mark another point next to the after benchmark. The distance from this point aft to the original after benchmark must be equal to the distance between the original forward benchmark and the mark laid off forward, and must be on the same side of the ship. The permanent benchmarks are then established on this parallel line; this line being parallel to the mean axis of the forward tubes.

Figure 5-8. One four-tube nest target and one two-tube nest target showing position of auxiliary targets and
paralleling mean axes.
Figure 5-8. One four-tube nest target and one two-tube nest target showing position of auxiliary targets and paralleling mean axes.
 
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Figure 5-9. Plumb line adjustment adapter.
Figure 5-9. Plumb line adjustment adapter.

7. Periscopes, when installed, should be trained on the forward benchmark and the azimuth circle set on 0 degrees.

8. If the ship has a list, it is necessary to place the axis of the transit in the longitudinal centerline plane of the vessel and to project the mean target points upward parallel to the same line.

9. Final angles of error less than 5' of arc maybe considered as negligible.

  10. When any submarine has undergone extensive depth charging, or the periscope has been damaged, because of a collision or an ice flow, the repairman must check the periscope alignment to the bow and stern benchmarks to ascertain the alignment of the periscope supports.

Figure 5-10. Transit fixture.
Figure 5-10. Transit fixture.

 
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