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Commander Nathan C. Twining, Commander, Nantucket Division Patrol Force, to All Vessels





20 April, 1917.

FROM:     Detachment Commander.

TO:       All vessels

SUBJECT:  Anti-submarine tactics.

     1. When a submarine is sighted by a patrol vessel the primary object is to destroy it; the safety of the patrol vessel is a secondary matter.

     2. Offensive action may be taken against submarines by:

          (a) Ramming,

          (b) Gun-fire,

          (c) Torpedo fire,

          (d) Dragging with nets, bombs or grapnels.

     3. (a) Ramming.– It is difficult to ram a moving vessel that is endeavoring to avoid the ram; the submarine has the ability to evade by submerging as well as by maneuvering on the surface. Ramming should not be attempted except at close quarters since the chance of success is small when the submarine is far away and the bow presentment reduces the volume of gunfire that can be delivered.

     4. A submarine suddenly coming to the surface ahead or on the bow and close aboard might be rammed; an attempt, would, at least, be justified; opening fire with guns should not be delayed, however. If nothing more were accomplished the submarine would be obliged to maneuver or to submerge, the aim of her torpedoes would be spoiled, and the target presented would be a minimum.

     5. A vessel steaming at 15 knots advances 500 yards in a minute during which time a submarine sighted as supposed in paragraph 4 would probably not be able to submerge so deeply as to escape the ram. At less speeds or at greater distances it would probably be useless to try to ram and it would be better to turn so as to bring all guns of a broadside to bear and open fire.

     6. The blow delivered by a vessel in ramming increases in force as the square of the speed; at a speed of twenty knots the blow would be four times as heavy as at ten knots. The reaction on the ramming vessel increases in the same proportion but the end in view is damage to the submarine and light blows would probably do none while a heavy blow might sink or disable her.

     77. British destroyers have rammed at ten knots with damage to themselves but none to the submarine.

     8. Dummy periscopes are sometimes attached to mines for the purpose of destroying vessels that attempt to ram. A German U-boat sank a British destroyer by such means, the mine being towed about 100 yards astern of the totally submerged submarine. Patrol vessels should not attempt to ram a submarine when nothing but a periscope has been seen.

     9. (b) Gun-Fire.– To destroy a double hull submarine such as the German U-boats the inner hull must be perforated but such a boat may be seriously damaged by perforation of the outer hull and may be rendered inoperative except on the surface by destroying her periscopes or riddling her conning tower.

     10. No gun of less than 3-inch caliber is likely to pierce the outer and inner hulls but damage may be done by a gun of any caliber, even shoulder rifles, machine guns, pistols or shotguns. Every available gun should, therefore, be used in the attack.

     11. After a submarine has been sighted there will be but little time for gun fire against her before she submerges; guns must, therefore, be always ready. It is not humanly possible to keep all guns loaded and manned continuously but one or two should be kept in that condition so that the fire of at least one gun may be opened with delay on any bearing.

     12. When R. F. guns are loaded the breech blocks should be kept open and loaded B. L. R. guns should be kept unprimed to avoid accidental discharge.1

     13. No gun that is not kept manned should require more “clearing away” than can be done by one member of the crew on watch while the rest of the crew is coming at call. The condition of the gun should be such that it can be loaded and fired as soon as the crew arrives. The crews of such guns should sleep near their guns at night with one man constantly on watch at the gun, on his feet, awake, and keeping a lookout.

     14. Most of the work of the German U-boats has been done at short range but their torpedoes probably have a possible range of 6000 yards or more at a speed of 30 knots. As periscopes will rarely be sighted beyond 3500 yards a torpedo may have already been fired before the periscope is seen but there is no less reason for trying to destroy the submarine.

     15. Attempts to get the range by means of range finders cause delay and it is thought desirable, while not neglecting the use of the range finder, to have all sights set a 2500 yards and give pointers instructions to aim at the periscope or at the water line in wake of the conning tower if the latter is visible. The range can then be corrected by spotting if necessary.

     16. “Shorts” are better than “overs” as they facilitate getting on and they may do damage by ricochet or under water run.

     17. While firing at a submarine attempt should always be made to approach her – not heading directly for her (unless near enough for ramming) – but at such an angle that the largest number of guns will bear. Meanwhile sharp lookout should be kept for a possible mate as submarines frequently “hunt in pairs”. All disengaged guns should be ready to open fire instantly if a periscope appears.

     18. As soon as a periscope disappears so that it can be no longer fired at the patrol vessel should if within 1000 yards head directly for the spot at which the periscope was last seen using all available speed in the hope of ramming. If more than 1000 yards away back at full speed to avoid torpedoes. Backing at full speed is also to best way to avoid a torpedo seen approaching; if it is not avoided altogether the hit is more likely to be well forward there the damage to the ship will be the least.

     19. (c) Torpedo Fire. Torpedo fire against a submarine is not likely to prove effective; if the boat is submerged with periscope showing the depth of the hull may be approximately estimated from the fact that the two periscopes usually carried have heights of about 8 and 20 feet, respectively, above the hull. As they are usually telescopic and self-housing their height above the hull at any time cannot be known with certainty. The higher of the two, when both are up to full height, is usually the after one, but they are so close together as to give but little clue to the boat]s course

     20. A submarine on the surface can doubtless be torpedoed with about the same facility as a surface craft but the depth setting of the torpedo would best be for not more than 6 or 8 feet. A vessel having three inch guns or larger would have more chances of destroying or damaging the submarine with them than with a torpedo.

     21. (d) Dragging. As the vessels of the patrol force are not furnished with appliances for operations of this kind the tactics of their use need not be discussed. It might be possible to locate a submarine on the bottom or running submerged by me[a]ns of towed grapnels but operations of this sort are more within the province of the inshore patrols.


     22. The sharpest possible lookout, all around, the compass must be kept at all times, day and night. By day there should be at least two lookouts aloft, each having half the horizon under observation and not less than four at the main deck level, each watching in one quadrant.

     23. Search should not be confined to the horizon but the entire area between the ship and the horizon must be constantly swept.

     24. On the bridge there should be two lookouts whose special duty is the search from ahead to four points on each bow.

     25. At night the deck and bridge lookouts should be the same as by day. The lookouts aloft may be dispensed with at night unless it is bright moonlight. The extent of the ability of a lookout aloft to see anything under the light conditions existing should determine.

     26. The man on watch at each gun should also keep a lookout in some definitely described direction. All lookouts should be given certain definite sectors to cover and be required to watch them continuously.

     27. All lookouts should be supplied with glasses both day and night; if prismatic binoculars are used each man should be taught how to focus them for his own vision. Such glasses are of little use at night.

     28. A searchlight should be kept ready for use and in electrician or other person at hand who can start it and manipulate it. A searchlight must not be turned on for search purposes.

     29. The importance of being ready to open fire in any direction as soon as a submarine is sighted cannot be too strongly emphasized.

     30. All officers and men should be constantly impressed with the fact that we are at war and that, amongst against an enemy having so many advantages as a submarine, the best each man can do is none too good.


Footnote 1: “R.F.” designates “rapid fire” guns, and “B.L.R. guns” are “breach loading rifle guns.”