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Vice Admiral William S. Sims, Commander, United States Naval Forces Operating in European Waters, to Commanding Officers of Submarine Chasers




U.S.S. MELVILLE, Flagship.

London, England.

29 August, 1918

From:  Force Commander.

To  :  Commanding Officers Sub Chasers.

SUBJECT: Anti Submarine Tactics

Reference: Force Commander’s Circular Letter No. 32.1

Enclosure: Issued with this letter “Submarine Hunting by Sub                                     Chasers.”

     1.   The enclosed notes, which have been prepared by the Force Commander’s Staff, are a revision of those issued in reference above.

     2.   They are issued as a matter of general instruction and are not intended to set forth rigid rules requiring strict compliance.

     3.   The work on hunting submarines by using listening devices is still in its infancy and the proper tactics for the use of these devices are not yet developed. The Force Commander can not impress too strongly upon all officers, regardless of rank and regardless of the duty upon which they may be temporarily engaged, that they each individually bear a share of the responsibility in solving the problems which confront the service.





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     The following principles shall, in general, govern the employment of sub chasers. . . . 2

          (g)  Those forces, like all other U.S. Naval Forces in these waters, operate under the direct military control of the Senior Allied Commander Present.


          In addition to being organized into squadrons and divisions, all sub-chasers will be organized into units of three vessels each, to be known hereafter as HUNTING UNITS. As many units as possible should operate in co-operation in establishing contacts. Experience to date has indicated that not more than three vessels can efficiently coordinate their efforts in following up contacts. The vessels of each unit will habitually operate in tactical support of each other; and must at all times be prepared to:-

     (a) To engage a submarine on the surface.

     (b) To attack successfully a submerged submarine when it has been located.

     (c) To prevent a submerged submarine from coming to the surface and escaping by superior speed.

     It is essential that the organization of units shall be permanent so that the same vessels shall always work together. This will permit the development of real team work in tactics and in signals. The utmost skill in operation can be obtained only by continuity of association of ships and personnel.

     In the matter of recognition of service rendered, it should be a principle that all vessels of a unit that actually participate in an operation, shall share equally in the honour of success.


     The tactics of submarine hunting by sound may be divided into three stages:-

          (1) Search.

          (2) Chase.

          (3) Attack.


     Information of the approximate position of an enemy submarine may be gained by:-

     (a) Reports from shore listening stations.

     (b) Reports from directional radio stations.

     (c) S.O.S. calls.

     (d) Reports from aircraft and vessels at sea or reports from coastal stations.

     When reports of the above nature are received, hunting units should be designated to search the area near the reported position of the submarine. In the absence of such reports, the hunting unit will seek enemy submarines in areas or along routes assigned. Three methods of search will be considered:-

          (1) Anchored Patrol.

          (2) Drifting Patrol.

          (3) Running Patrol.

A N C H O R E D  P A T R O L .

     The Anchored Patrol may be used to establish a sound barrage along a line or around an area in which an enemy submarine has bottomed.

     Advantages are :-

          (a) Ease and certainty of maintenance of position.

          (b) Each vessel knows bearing and distance of all other vessels of units at night or in thick weather.

          (c) No necessity for using lights for position signals.

          (d) Possibility of a continuous watch on all short  range listening equipment, and, except at turn of tide, a continuous watch on other listening equipment.

     Disadvantages are:-

          (a) Impracticable in rough sea.

          (b) Probable delay in getting underway for Chasing.

          (c) At slack water there is a period of about one hour when directional quality of certain types of listening devices (streaming type) disappears.

          (d) Loses submarine if it drifts along the bottom.

     When Anchored Patrol is decided upon –

          (a) Use a hawser instead of chain for anchor cable as handling chain betrays you to the submarine.

          (b) Be ready to slip instantly, making no attempt to get anchor.

     The best formation for Anchored Patrol is in line normal to the probable course of enemy submarines. In the case of a bottomed submarine the best formation is a triangle enclosing probable position of enemy submarine.


     The Drifting Patrol may be used to establish a sound barrage along a line that shifts with the current, or around an area in which an enemy submarine is believed to be drifting. It is particularly applicable off soundings in an area where a submarine has been seen to submerge and within which the submarine must surely be.

     Advantages are:-

          (a) No necessity for using lights at night. Relative bearings can be ascertained by tapping a pre-arranged signal at specified times on vessels hull inside, below waterline. Bearing within three to five degrees of accuracy may be obtained by this method.

          (b) Possibility of a continuous watch on all listening equipment, due to fact that own noises are not present to interfere.

          (c) Enemy receives no sound warning from noises of hunting group.

     Disadvantages are:-

          (a) Hunting unit will drift out of touch with a bottomed submarine. Remedy by day is to anchor a buoy as a guide. In planting buoy, speed up engines to drown sound of buoy anchor and cable; or lower anchor by hand quietly.

          (b) Difficult to maintain position. Effort to regain position will betray presence of units. If all vessels move simultaneously one might continue out of sound range of the submarine to convince submarine that area was clear.

Note.    In both the Drifting and Anchored Patrol, extreme caution is necessary to avoid making noises. Do not throw things about the deck or against the hull of the ship. Do not break up coal while drifting. Do not hammer.

     The best formations for Drifting Patrols are the same as for Anchored Patrol.


     The Running Patrol_is for use in searching a large area. It should be used in going to and from station unless proceeding to intercept a reported submarine, when the Running Patrol need not be taken up until within the area of possible contact with the submarine reported.

     The Running patrol may sometimes be used in advance of, and out of sound of a Convoy, as a measure of protection.

     In the Running Patrol vessels proceed on course assigned stopping engines and auxiliaries for listening observation simultaneously at pre-arranged intervals. The efficient working of a running patrol requires that time pieces be kept in exact step.

     The Running Patrol is particularly applicable for day use in that vessels are less exposed, and if various other vessels are operating in the vicinity, those latter may be avoided so as to prevent sound interference.

     Advantages are:-

          (a) Covers a large area.

          (b) Easy to maintain position.

          (c) Engines ready for emergencies at all times.

     Disadvantages are:-

          (a) Enemy submarines have opportunity to hear hunting units.

          (b) Listening is not continuous.

F O R M A T I O N S .

          (a) For a UNIT acting singly.

     The best formation for a hunting unit of three vessels to take in Running Patrol is Line Abeam. This formation should be used in proceeding to the patrol area and while on patrol.

     If there is a supporting vessel such as a destroyer or similar vessel, it should zigzag within supporting distance in rear of the listening vessels at a distance sufficiently great to prevent its noises from interfering with the sound detection of submarines.

     The distance between vessels in the case of units acting single, should, generally speaking, be not greater than one and a half miles. Any greater distance requires too much time for vessels to close in to attacking distance; which must be done promptly when a “sure” or “suspicious” sound is heard.

          (b) For a Group of Units.

     The best formation for a group of units to take in running patrol is also Line Abeam; although at times it may be desirable to assume other formations, such as forming a square, or triangle, about a supporting ship.

     The distance between units should be five miles generally, but in case of bad visibility this distance should be decreased. Units should keep within sight contact of adjacent Units. The distance between the vessels of a unit should be as when acting singly.

T H E  C H A S E .

          The chase of an enemy submarine by a hunting unit requires a thorough understanding of the game by all concerned. There must be teamwork in listening, communicating and manoeuvring. Each vessel of the unit should require practically no direction from the flagboat.

          Each Ship Commander must be kept fully informed of all matters that bear upon the success of the Chase. The Commander of each Unit he is responsible for the development of the “team spirit” and “team work” of his Unit. He should hold frequent conferences of the Officers of the unit while in port, for an interchange of ideas, for discussing improved methods, for eliminating causes of failure, or lack of complete success in team work. He should propose situations and ask Officers in turn for their decisions to meet the situations proposed, correcting decisions and explaining corrections as necessary. It is an invaluable practice for the Unit Commander and his subordinate to work out tactical problems on a manoeuvre board and to discuss each successive phase of each problem until they all are thoroughly conversant with like situations and the ways of meeting them.

          Once sound contact with a submarine has been made, nothing but bad weather should be accepted as legitimate reason for losing sound contact. The detection instruments already provided and about to be provided should enable a competent personnel to run the submarine down.

          When a Submarine is Heard, the vessel hearing submarine reports immediately, -

     (1)  Submarine, or suspicious sound heard.

     (2)  The magnetic bearing of sound.

     (3)  Estimated distance and speed.

(4)  Whether submarine is on surface or submerged. Vessels manoeuvre to take position in line abeam of vessels that made sound contact. All vessels observe the silent interval of the Chase, if not otherwise signalled by unit leader. The silent interval of the Chaser should be sufficiently frequent to prevent any possibility of,-)

(1)  Submarine passing beyond hearing between silent intervals.

     (2)  The submarine being overrun between silent intervals.

     Immediately upon the reporting of an enemy submarine, all vessels must keep a specially sharp look-out for signals from the Unit leader. The unit leader should keep plotting the bearing of the submarine from the leader, the probable position of the vessels of the unit. The position of the submarine may be determined more accurately if the unit leader is given simultaneous bearings of the submarine by two or more vessels; these bearings plotted on cross-section paper will give a very good estimate of the submarine’s position. The unit leader should keep all chasers informed of estimated positions of enemy.

     In making the initial contact, experience has shown that if possible a good fix should be secured before taking up the chase. It is best in the beginning to be sure and therefore generally one or more sets of bearing should be gotten before making the first run towards the sound. In case the sound should be lost before a second set of bearings can be obtained, or in cases where only one vessel has made contact, a run of considerable distance, depending upon hearing conditions should be made in the direction of the sound.

     In moderately rough weather, it is advisable to keep “weather gauge” of the submarine, as water Noises interfere considerably when attempting to head into the sea, but are negligible when drifting or running before the sea. In rough weather when stopping to listen, place the vessel stern to wind, thus materially increasing the range.

     In chasing the submarine, assume its speed is 4 knots per hour, unless listener can give a good estimate of submarine’s speed, by counting the number of revolutions of its propellers.

     Overrunning a submerged submarine places the hunting unit at a distinct disadvantage, as noise astern, under good weather conditions, cannot be heard with anything like the same efficiency as noises on the beam or ahead. Neither can the bearing of the noises heard be so accurately determined.

     During the chase in line abeam, vessels of the hunting unit should close gradually, providing the submarine is heard clearly.

     In the chase if the enemy submarine attempts to synchronize use of his propellers with the hunting unit, so as to avoid danger of sound detection, one vessel of the unit should stop her engines a minute ahead of the other vessels so that it may begin its observations instantly the other vessels stop. The enemy will, if he knows he is being chased, resort to silent running and great care must be used by the listeners not to be misguided by such a manoeuvre. Listeners should be taught to always make an effort to ascertain the speed of the submarine’s engines and should at every “hearing ” report speed of engines as well as intensity of the sound.

     In enclosed or crowded waters it will be difficult to chase a submerged submarine to exhaustion. It is therefore very essential so to manoeuvre, as to bring about the attack as soon as possible after contact.

     If during the case sound contact is lost, the circumstances should indicate whether the submarine is beyond range of the listening devices, resorting to “Silent running”, balancing or bottoming; a judicious use of depth charges may cause a submarine to betray its position.

A T T A C K .


     Preceding the attack, vessels should have been so manoeuvred as to close the submarine to within 300 yards, (but with the submarine still ahead,) should be in line abreast, distance not more than two cables apart. The support, when there is any, should not close the attacking vessels, until specifically so ordered, as the noise of the machinery of the support interferes too seriously with the tracking of the submarine. Time for attack should be chosen when the indications are that the submarine is on a steady course and when the submarine is as close aboard as it is practicable to get it, without overrunning it. The signal for attack should immediately follow the expiration of a silent interval/ Upon the signal to attack, All vessels should proceed at maximum speed, the centre vessel towards the estimated position of the submarine, the flank vessels closing on the centre vessel to a distance of 100 yards, maintaining line abeam. The centre vessel should drop first depth charge 20 yards short of estimated position of submarine at time of attack and successive depth charges as rapidly as possible, being careful to drop depth charges at least 200 feet apart in order to prevent countermining of the preceding depth charge. The flank vessels should drop their depth charge 50 yards beyond the position of the first depth charge of the centre vessel, and successive depth charges according to the rule just laid down for the centre vessel. This is not to be considered a hard and fact rule for dropping depth charges. The Commanders of the various detachments should devise a number of patterns which should be used as directed; depending upon the existing conditions.

     Time for ordering the attack will depend entirely upon the judgment of the Commander of the Hunting Unit; unless the submarine shows itself in position for attack, when the nearest vessel should attack immediately and without signal. Vessels should not expend all their depth charges in an attack that is guided by sound alone.

     If during the chase touch is lost with the submarine, the Commander of the Hunting Unit must determine upon his procedure, having in mind the capabilities of his various listening devices, the advantages and disadvantages of the various forms of patrol, and the probabilities as to what the submarine would do under the circumstances.


     If within gun fire range attack immediately without orders from Unit leader then report sighting. All vessels close as quickly as possible from different directions being careful not to get in line of fire with one another. Use three inch and rapid fire guns closing to ram if possible, being always ready to use depth charges should the opportunity present itself where there is a possibility of damaging the submarines in this way. Have a boarding party consisting of one officer and at least three men well drilled and always ready for boarding should the opportunity offer.



(1)  See all instruments in working order.

(2)  See that listeners are trained in their duties.

(3)  Unit Commander assemble Commanding Officers and explain plans; then by question and answer, and by instruction on manoeuvre board, make certain that no tactical signals will be necessary in pursuit except:-

     (a) Submarine heard.

     (b) Bearing and distance.

     (c) Stop and start.

     (d) Attack.

     (e)  Course to be steered.

(4)  Each Commanding Officer of ship assemble ship’s officers, petty officers, and listeners, and instruct them in plans so that they will be able to work together as a team and each one will know exactly what to do under all conditions.

(5)  Set all clocks by time signal.

(6)  Determine and announce frequency and length of silent interval when on running patrol.

(7)  Determine and announce frequency and length of silent interval in chase.


(1)  As soon as clear of harbor, form line abeam.Unit leader in centre; distance between vessels as predetermined; support zigzagging a predetermined distance astern.

(2)  Proceed to station carrying on running patrol unless special circumstances make it necessary to arrive on station as soon as possible.


(1)  Begins first silent interval on signal.

(2)  Start and stop thereafter by clock time.

(3)  Use only those instruments ordered.

(4)  First vessel hearing submarine hoists “hearing” flag; report Vessels conform to the change of course as ordered, directing their movements to get in line abeam on new course at chase distance.

(5)  All vessels at once take up frequency and length of silent interval ordered. Watch Unit-leader’s movements and follow.


     (1)  Centre vessel keeps submarine ahead. Right flank vessel keeps submarine on port bow. Left flank vessel keeps submarine on starboard bow.

     (2)  Regulate speed so as not to overrun submarine previous to decision to attack.

     (3)  Vessels of chasing line close gradually to 400 yards distance between vessels.

     (4)  All vessels signal bearing and estimated distance and speed of submarine at end of each silent interval.

     (5)  All vessels change course to conform to movements of submarine and requirements of sub-paragraphs (1) and (3) above;without signal. Guide on flag boat of chasing line.

     (6)  All vessels keep sharpest possible lookout for submarine. If sighted attack immediately.


     (1)  Get as close to submarine as possible and locate its position as accurately as possible.

     (2)  Begin attack at full speed, centre vessel heading for last reported position of submarine: other vessels closing to 100 yards on centre vessel and taking parallel course.

     (3)  Centre vessel drops first depth charge 100 yards short of estimated position of submarine. Drops succeeding depth charges as rapidly as possible avoiding danger of countermining; or in accordance with specially prescribed pattern.

     (4)  Flank vessels drop first depth charges in accordance with plan, avoiding danger of countermining. All vessels conserve a part of their depth charges unless attack is based upon “close aboard” sighting of the submarine.


          The principal cases of submarine chasing will be:-

     (1) Daylight – on soundings.

     (2) Daylight – off soundings.

     (3) Night – on soundings.

     (4) Night – off soundings.

          During Daylight in crowded waters the submarine operates, as a rule submerged. If pursued on soundings during daylight while submerged, the submarine may:-

     (a)  Attempt to escape by proceeding at maximum speed.

          (b)  Attempt to escape by proceeding at slow speed, or by “silent running”.

          (c)  Attempt to escape by resting on the bottom. Submarine will probably not attempt this operation in water more than thirty or thirtyfive fathoms deep, and will always seek bottom free from rocks and other dangers to bottoming.

          (d)  If near bottoming ground, may attempt to escape by proceeding slowly (silent running), stopping and balancing occasionally to listen, or stopping synchronously with the hunting unit.

          (e)  May anchor submerged – submarines frequently rest on the bottom; when so doing they are apt to drift slowly.

NOTE.     A submarine proceeding submerged can probably continue underway as follows:-

          Speed 3 knots                 36 hours.

          Speed 5 knots                 12   "

          Speed 7 knots                 3   "

          Speed 8 to 9 knots.        1-1/2   "

          Speed 10 to 11 knots.         1   "

These are rough estimates based on the capabilities of the average submarine. Later types of enemy submarines have greater submerged radius. The surface speed of enemy submarines varies in different classes from ten to eighteen knots.

     At Night submarines are usually to be found on the surface charging batteries, cruising to new stations, or operating. One of the first concerns of a submarine is to keep its battery fully charged. In crowded waters the submarine finds it too dangerous to charge batteries except at night.

     When operating far off shore, it has much more latitude and doubtless charges its batteries while cruising on the lookout for victims. When on the surface submarine will probably have a little of the upper deck showing She may be stationary or underway depending on circumstances. If underway she can submerge in from 30 to 40 seconds; if stationary time to get underway must be added.

Source Note: TDS, DNA, RG 45, Entry 520, Box 340.