Naval History and Heritage Command

Naval History and Heritage Command

Ensign William H. Perot, Commander, Submarine Chaser. 325, to Captain Arthur J. Hepburn, Commander, Submarine Chaser Detachment 3


5 Sept 1918   

From: Commanding Officer.

To  : Commander Submarine Chaser Detachment # 3.

Subject: Monograph on Tactical Dispositions for 110 foot Submarine Chasers engaged in Anti- Submarine Warfare.

     1.It is believed that destroyers have been the type of patrol vessel hitherto most successful in conducting Anti-Submarine warfare.

     It is believed, that, rather than any mechanical advantages of the vessels or their apparatus, it has been the ceaseless vigilance of the officers and men, and the indomitable tenacity with which they have followed ev<e>ry clue, which has inspired the submarines with that fear of destroyers which so weakens their morale and intimidates their effectiveness.

     Submarine chaser personnel would do well to cultivate these qualities, remembering that if they never destroy a submarine, never injure a submarine, never even have a contact, that they will have accomplished their mission if their effectiveness has inspired respect in the submarine such that the mere threat of their presence in an area will cause the submarine to curtail it’s operations or to depart. Of course to do this they must possess weapons dangerous to the submarine, and it is believed that they do in the 3” Gun, themachine guns, depth charges, and listening devices.

     A. 110 foot submarine chasers possess the following advantages over destroyers for anti- submarine warfare:- they are of too shallow draught to be torpedoed under normal conditions. This enables them to lie to with comparative security from torpedo attack.

     All machinery that may interfere with listening devices can be stopped in less than five seconds from full speed ahead on all engines & they are xxxxxxx ready to get underway <again> immediately.

     They are equipped with listening devices possessing the following advantages and limitations:-

          A submarine running on the surface on it<s> Diesel engines , can be heard at least ten miles distant on the “K” tube, and on days when the water noises are not too bad, it can be heard at least five miles distant on the M.B. and S.C.C. tubes.1

          A submarine running awash or at periscope depth at any but his “silent speed” on its electric motors, can be heard at least a mile away on the “K” tube , and at least 800 yds. distant on the M.B. and S.C.C. tubes on good listening days. For high speeds the range would be incre[a]sed to at least 2 1/2 miles on the “K” tube and 1500 yds to one mile on the MB. and S.C.C. tubes. If a submarine is running at greater than periscope depth, the range of all listening devices would be slightly increased.

     If submarine is running at“silent speed” on electric motors, he can proba<b>ly be heard at 400 yards <on K tube> when submerged to 60 ft. But at this short distance the “K” tube’s di<r>ectional qualities are weakened, and it can not be centered on any two scales within 35 degrees. On the M.B. and S.C.C. tubes, under the best listening conditions, when at a depth of 60 ft., a submarine running at“silent speed” can certainly be heard directly underneath,and probably at a distance of 200 yds. The M.B. and S.C.C. tubes can be centered within 5 degrees at the shortest ranges, unless the sound is very faint, when the S.C.C. tube cannot be centered within 15 degrees.

     Compared to most other noises encountered, that of a submarine awash or under the surface is very faint. Any other noise will distort it, though sounds of slight intensity coming from a direction at least 20 degrees away from the submarine will usually not drown out the submarine on the “K” tube or the M.B. tube. But intense or close sounds will always drown out a submarine submerged, even when the submarine is very close. A steamer two miles away has been observed to drown out on the M.B. tube, a submarine at periscope depth, 200 yds. distant, proceeding at 50 R.P.M. faster than silent speed, over an arc of 60 degrees toward the steamer and 60 degrees away from the steamer( the 180 degreex error ). On the remaining arcs the submarine, when it crossed to them, could be faintly heard, and with difficulty centered to 10 degrees. On the “K” tube when the steamer was at one end of the scale and the submarine at the other, the latter could be faintly heard, but not centered to within 50 degrees. On the S.C.C. tube the steamer drowned out the submarine for the entire circumference. A steamer or gas engine within 1000 yds. will drown out a submarine on all tubes, unless the latter is very close or the listener is skillful enough to pick it out and center it from behind the interference.

     A submarine on it’s Diesel engines is a better proposition, and is on a par with a chaser, destroyer, or a small steamer, and is louder than most trawlers. The accidental noises made by a submarine, such as, hammering, dropping heavy gear, <anchoring,> using oscillator, can be heard for long distances, but because of short duration, are hard to center and identify. The comparatively faint sounds of bilge and air pumps are heard for at least 600 yards on all tubes,even when a submarine is submerged to 60 ft.<;> and probably for a much greater distance on the “K” tube. The only difficulty with such sounds is that they are so rare that the listeners are apt to confuse them with some well known sound that they resemble slightly.

     The sounds made by a submarine, as recorded b y the above listening devices, are distinctive within certain limits. A small steamer with a slow steady, regular beat, going at high speed; a distant sub. chaser; destroyer; P- boat; or a motor launch, all going slow speed, and a trawler; all resemble a submarine sufficiently to be considered as “suspicious sounds”. A distant chaser, P-boat, motor launch or trawler, each going at high speed, and a certain type of motor auxiliary used on sail vessels, all sufficiently resemble a submarine running awash or submerged to be considered “suspicious” for a short time.

     Also certain combinations of sounds, as a P-boat heard behind a steamer, resemble sufficiently to be considered suspicious for a short time.

     Submarine Chasers are also equiped with an electric trawl device , which rings a bell upon contact of the wire with any metal object in the water. This device is prone <to get> out of adjustment and considerable time is usually required to find and repair the trouble. Furthermore any metal object in the water will cause the bell to ring, and even if circuit is completed and the bell rings, it does not necessarily indicate the presence of a submarine. Also it is possible for the submarine to hear trawl weights dragging along the bottom,and thus avoid contact. This device is best used when a submarine is believed to have been destroyed,for the trawl’s contact can be anchor-buoyed and thus give divers an accurate fix.


     110 ft. submarine chasers are in the following respects inferior to destroyers for anti- submarine warfare:-

     Inferior speed, cruising radius, armament, crew capacity, provisions capacity, ammunition capacity, and less dependable engines.

     2. The mission of the submarine chaser is self evidently- a patrol in areas in which it has the best chance of injuring, worrying and destroying the enemy submarine; a patrol of such a nature as to best utilize itself and its equipment under the various prevailing conditions, in order to be able to seize every opportunity to worry,injure and destroy the enemy submarine.

     A patrol close in-shore within the traffic lanes, or at scenes of reported suspicious sounds,by shore hydrophone stations, is not believed to be such a patrol, if those areas are already covered by adequate hydrophone vessels and visual patrols. For although submarines are more likely to be present in these areas than any other, interference is so frequent and so loud that but few contacts can be made; definite fixes are rare indeed, and no contact can ever be maintained.Furthermore the tactics of a well advised submarine in waters where it expects hydrophones will enable it to elude chasers every time, unless there is no interference and the chasers have information sufficiently definite to form a close sound barrage.

     Hence, unless assigned to in-shore patrol, nothing but allocations of submarines by sight, by attack on allied vessels, or radio direction fin<d>er near enough to enable chasers to arrive in time to form a sound barrage,should ever lead chasers into in-shore waters; and then a removal of interference by diverting traffics and visual patrols outside of interference <range> would be invaluable.

     Therefore the normal patrol stations of chasers in districts where in-shore waters are closely patrolled, is well off shore, in areas where submarines are likely to come to pump bilges, charge batteries, obtain fresh air, send radio messages, take observations and make repairs;- beyond the stations of the close visual and hydrophone patrols. The mere p resence of chasers in such areas would tend, even though few contacts were made, to drive the submarine further off shore to charge batteries, get fresh air, etc.-thus relieving the traffic lanes;- or would compel it to do these things in the areas of close in-shore patrol.

     In districts where patrols are thin, the chasers would have to resort to inshore patrol. And herein wou ld lie the advantage of two three boat units operating together. One could assume in-shore patrol and the other off-shore patrol. Another advantage would be in proceeding to an allocation. Both units could steam there in close formation and one could stop while the other continued underway, or better still both could stop, and after a short period one could get underway and either resume patrol or proceed to the limits of signal distance(three to five miles) and stand by to chase suspicious sounds, or take up silent patrol at si<t>e of allocation, while the other unit followed the suspicious sound. Furthermore six vessels would make a more effective sound barrage, and it is believed that six vessels<,>handled as two units<,> could be taught to cooperate in such work as well as three vessels now do [word crossed out and unreadable] in one unit.


     A. When no definite report has been received qua location of submarine by sighting, attack on vessel, or radio direction finder, within reasonable steaming distance, or when there is no reasonable proof that an enemy submarine is on patrol in an area within reasonable steaming distance.

     (1) In daylight, when visibility is good:-

          Under these conditions the submarine can see the chaser at least two miles before<t>he <latter> can see it. When stopped submerged, it can hear chasers at least ten miles, when underway submerged, at slowest speed, it can hear chasers [words crossed out and unreadable] about five miles distant, at half speed its listening range would be about two miles, and even at full speed <submerged> it xx could probably hear chasers several hundred yards away, and the sound of chaser engines starting would carry still further. Also the deeper the submarine is, the more effective his listening apparatus. Hence he can well synchronize his runs with the xxxxx chasers, starting when they do, and stopping almost as soon. Thus there would be a very few seconds in which to obtain a bearing of a sound which is very faint under the most favorable circumstances.The submarine could therefore use any speed it desires with almost equal security. A submarine seeing a patrol vessel stopped, would use anti-hydrophone tactics. A submarine hearing patrol vessels underway would use anti-destroyer tactics or combined anti-hydrophone and anti-destroyer tactics. In any case it would have little difficulty in avoiding contact. Therefore on clear days, contacts would be rare and uncertain.

     There are two possible types of patrol, viz- in-shore and off shore

     (a) In-shore patrol-to be <assumed> where other patrols are thin or inadequate. In such areas on clear days, the submarine would be bottomed, balancing, anchored, proceeding at silent speed,or cruising at slow speed and using its periscope cautiously to observe patrols and prey. The chances for sound contact are slight<,> for German submarine<s> are not careless when on patrol. The chances for visual contacts are better. Therefore,under conditions 2 A (1-)(a),the running patrol is recommended. Periodsunderway should average two to four times the <length of> periods for listening. While underway the sharpest lookout should be kept for oil patches, air bubbles wakes, discoloured water, surface disturbances, and periscopes. All which cannot be identified should be followed and thoroughly invstigated. If circumstances warrant it , the most probable position should be thoroughly bombed, making due allowance for the rising of oil,air slack water, tides and currents. In absence of such circumstances, the unit should listen on all tubes at the best position ascertainable, for such period as seems best, considering on the one hand the need for maintaining a running patrol over the whole district and on the other hand ,the fact that after a 24 hour silent patrol, it is reasonably certain that no submarine is nearby, that after six hours the above is reasonably probable, and that any shorter period of listening without result is not indicatory of the submarine’s absence.

     If none of the above phenomena are encountered, listening xxxx should be conducted at irregular intervals for short periods-10 to 20 minutes inplaces where a submarine might be on a fine clear day and where interference is remote.

     On this type of patrol it is recommended that every suspicious sound be followed until it’s identity be ascertained.

     In xxx case any suspicious sound be lost, the following procedure is advised:-

     If sound is followed on a one boat bearing or other uncertain fix; hunting unit should proceed to best obtainable predicted position and listen until satisfied that the sound is lost.As soon as it is evident that sound is lost, two vessels should remain listening on all tubes for six hours at least- unless need for patrolling entire area is considered more important-and one vessel should steam away to a distance of not less than three miles nor greater than five miles,(the limit of dependable phone range).If possible he should shape course to second best predicted position, and then listen on all tubes for the six hour period. The six hour wait is based on the theory that submarines cannot balance for longer than five hours , unless it finds layers of salt water of different density- (a rare phenomenon in Irish waters.)-and on the probabilities theory that an uncertain fix will not come near the submarine, that it will hear patrol vessels approaching and stop at a point not dangerously near; presumably to listen, and that it will hear them get underway and [word crossed out and unreadable] proceed xxxxxxxx in another direction, and when they are at a safe distance it will resume courses and speed, or will come to periscope depth for observations. In case either group of chxasers did approach near enough to x scare it into balancing or bottoming , it is believed that the submarine would ordinarily make at least one audible noise during six hours. In case it took to silent speed it would get x away anyhow, unless very close indeed. I<f> there are two or more hours remaining of daylight at the end of the six hours, it is believed that it would be advisable to resume the running patrol,- for if there had been a submarine at fix, it might have escaped at silent speed, and the only chance of regaining contact would be by running patrol. If the six hour period terminates at darkness, the drifting patrol could be assumed at position of fix, unless it was not a satisfactory location for such patrol. It is not believed that depth charge attack would be advisable, for probabilities favor scaring the submarine into silent speed or other tactics , rendering pursuit most difficult and do not favor damaging it sufficiently to improve pursuit.

     In case a good fix is obtained, if sound is then lost, it must be assumed that submarine is either balancing or bottoming, at, or near fix, or proceeding at silent speed,( in which case it cannot be far away.) If run has been for a mile or longer, area between fix and predicted position should be thoroughly bombed. I f run has been less than a mile fix itself should be bombed, unless need for maintaining running patrol covering the whole district is acute. All three vessels should then listen on all tubes for six hours. If nothing is then heard , one vessel should then steam away to a distance of five miles towards enemy’s most probable position, assuming <it> escaped from bombed area at silent speed:- for in six hours <it> would cover from 3 1/2  to <9> knots, according to the R.P.M. of <its> silent speed. Two vessels should remain listening on all tubes, and when the third reaches position it also should listen on all tubes. This silent patrol should continue for 18 hours more- 24 hours in toto. <T>his period is recommended, because a submarine must have fresh air, must must pump bilges, and can hardly help making some accidental noises in 24 hours.; and since it is reasonably probable that there is a submarine in the vicinity, it is advisable to remain until it is reasonably certain that <it> was sunk, escaped or never existed.

     In such instances the electric wire trawl might be used to advantage, one chaser listening and two chasers trawling, with frequent stops to permit the third to listen. Its use should be limited to localities where submarine could lie on the bottom. Its advantages and disadvantages should be borne in mind and weighed:-   

Advantages:-    Contact with metal somewhere in water signified by ringing of bell.

Disadvantages:- Metal encountered might not be submarine. Bell might ring due to<“>short<”> or loose connection.

                Submarine could escape by synchronizing runs with trawling chasers.

                Submarine could hear weights dragging over the bottom, and might thus be able to avoid trawl.

                <Submarine would have no doubt as to presence & intention of Chasers>

(b.)Off-shore patrol:- to be assumed when in-shore areas are adequately patrolled by other vessels.

     Under conditions 2 A. (L), the submarine can see stopped chaser long before the latter can see or hear it. Submarine can hear chaser underway at sufficient distance to keep out of xxx range of periscope visibility or of chaser’s listening devices. The problem is therefore to take stations-

I. In a locality where a submarine would not dare to show its periscope, but where it wouldnot expect a hydrophone patrol, and

where there would be no interference- object to obtain contact or to worry.

II. In a locality where a submarine which had already reached a position of safety from shore watchers,visual patrols and hydrophones, and was running awash at loud speed on electri-c motors, or on surface with Diesel engines, could be heard by chasers before it could see them- object to obtain contact or to worry.

III. In a locality where submarine would be expected to come to charge batteries, obtain fresh air etc.- object not primarily to obtain contact but to worry enemy, to compel him to go farther out ( still farther off his station), or to wait for darkness or fog, thereby running risk of engine trouble; sickness among crew, etc., or to chance it in area of close patrol.

In cases I and II. a modified running patrol is recommended. That would be listening on all tubes for long periods and underway for short periods purpose to permit the submarine to run the difference between its listening range for chasers and their listening range for it at slow speed submerged, and at same time to cover sufficient area to worry submarines attempting such a mano<e>uvre, but who have eluded contact. This patrol should be about 30 minutes listening and 15 minutes underway<(> 2 1/2 miles at half speed.)

     Best position for case I, would be just inside limit of visibility of shore watchers, and visual patrol, and beyond limit of range of hydrophone patrol:- normally 15 to 20 miles off-shore. It would be successful only when an aircraft co-operated. The latter could fly low over water to shoreward of chaser, just beyond range of visibility of chaser to a periscope. Submarine would dive deep and remain submerged for some time, he would hear nothing and having seen no patrols to seaward, he would run thither at moderate speed (3.5 to 5 knots) well submerged. This would bring him in range of listening chaser. It is not believed that submarine running toward shore from sea would be encountered in daylight. Submarine would prefer to run in by night and when it could run on the surface, flush fresh air through ports, and count on reduced effectiveness of visual patrol.

     Best position for case II, would be beyond the xxxx limit of effective visibility of shore watchers, and between five to ten miles beyond visibility of in-shore patrols. This would normally be 25 to 30 miles off-shore.Modified running patrol is recommended.

     Best position for patrol in case III, would be beyond visibility and listening range of in-shore patrols, and although not beyond visibility of shore-watchers, too far away from patrols to render them dangerous in case they were to follow a report of shore watchers. This would normally be twenty to twenty five miles off shore. Running patrol is recommended,with twenty minutes underway and twenty minutes listening on all devices.

     In cases I. II. and III. all suspicious sounds should be followed until identified, and if they are lost, procedure in 2 A (1) a, pars, 4, 5, 6, should be followed. The trawl would be of no value since submarine could not bottom.

     2. At night.

          Drifting patrol is recommended, unless through inadequacy of inshore patrols , the inshore running patrol is needed to keep traffic lanes clear. Position should be taken wellx off shore, at limit of range of visibility of some well known light. Engines should not be started except to follow some suspicious sound. In case sound is lost, procedure should be the same as in 2 A, (1)(a) pars. 4,5,6, and 7.

     3. In thick weather.

          Unless through the inadequacy of inshore patrols, the inshore running patrol is necessary to keep traffic lanes clear, position should be well off shore, outside traffic lanes, Modified running patrol should be used on theory that submarine would be lying on the surface charging batteries, or proceeding at high speed on the surface, depending on thick weather as a protection from destroyers and visual patrol, and its off shore position as a protection against hydrophones. In any case,its listening devices would not be as effective as when submerged. On this type of patrol all suspicious sounds should be followed and in the event of one being lost, 2 A (1)(a) pars. 4,5,6, and 7 should be followed.

     (4) In very heavy weather.

          Submarine could not operate and would prefer to bottom or to balance, using engines just enough from time to time to maintain depth, or to cruise at low speed. In this manner, they could probably ride out a three days gale without having to come to the surface to charge batteries, but they would certainly need fresh air. They could not“open up” in the open sea for fear of xxx shipping water, but would either wait for a lull in the <storm> or make their way to the lee of a headland or island. Also the seas might dammage their diving rudders, or they might sustain other dammage which would compel them to come to the surface or would prevent them from diving or maintaining depth. Furthermore,in heavy weather many patrol craft are forced to seek harbor, while those that remain suffer loss both in speed and mano<e>uvring qualities. Submarines know this and count on it, feeling more secure when running on the surface in heavy weather. Unit Commanders should bear these facts in mind when thinking of taking shelter from a severe storm. If they decide to remain on station, they <should> remember that submarines are apt to be met with anywhere, but more probably <on the surface> in the lee of a weather shore, where there is plenty of deep water and no dangerous rocks or shoals.

B. WHEN REPORT HAS BEEN RECEIVED – by sighting, attack on allied vessels, or radio direction finder, placing submarine within reasonable steaming distance.

     Upon first receiving such a report, all patrol xxxxx work not of first importance should be dropped,- this includes chases after indefinite suspicious sounds, drifting patrols at uncertain fixes or oil patches,- and unit should proceed to follow up report at full speed. It is not believed advisable to proceed to the position of the allocation itself, but rather to a position the group Commander thinks the most probable location of the submarine. In reaching this decision,he must take due allowance for the time elapsing from the report to <his> arrival at the position, for tides, currents, the weather conditions at the time of the allo, the tactics of submarines in general, and those in particular which frequent this area<,> of the patrol conditions at the scene of report, - whether closely or thinly pat<x>roled and whether other patrols can reach the scene before his unit.- what type of patrol vessels are apt ot be in the vicinity and their general characteristics. In short<,>the group Commander must be able to estimate just where the submarine could be by the time he is able to reach the vicinity, and by his special and peculiar knowledge of local existing conditions and submarine tactics , be must be able to select with reasonable certainty the most probable position of the enemy at the time he can bring up his unit. It would be best to arrive at this vicinity with a destroyer, or another unit of chasers in close formation, then after a short stop the <destroyer or> other chasers could steam away ,leaving the hunting unit to maintain a drifting , silent patrol, listening on all tubes. If this is not possible<,>one of the chasers in the unit, could after a short stop, steam away to three miles, and then stop and listen. This silent drifting patrol should be maintained until definite information is obtained that the quarry is elsewhere, or until contact is made. All suspicious sounds should be checked carefully and should not be followed on any mere guess, as a short wild goose chase could undo hours of patient waiting. In this case two units could work well together, one to follow suspicious sounds and the other to remain on station. If all three vessels have been <reta>ined on this station,it is advisable that two pursue suspicious sounds while the third remains on station. With one unit operating without without any other aid, the whole mano<e>uvre could be executed thus:- an allo is received, the unit proceeds at full speed to the most probable position of the submarine, and stops for a short time. One vessel gets underway and goes to a distance of three miles, while the other two remain on station. The vessels on station hear a suspicious sound and signal to the other one, who immediately proceeds to station and assumes drifting patrol, while the others pursue the sound. Having found <it> to be a small trawler they return to station. After a short period of silence the third vesse-l once more gets underway and proceeds thre<e>miles, then stops and listens.

     No definite limit can be set to reasonable steaming distance. It is probable that any greater distance than six hours <at full speed> in daylight, and three hours at night,would offer no chance of success but circumstances can be conceived wherein nine hours would not be too far distant.

     It is not considered advisable to follow up reports of shore listening stations, unless unit is on in-shore patrol in short distance of report,(not more than 30 minutes), and site of report is comparatively free from interference. For “submarines” on shore listening stations are on a par with “suspicious sounds” on chaser’s listening tubes. Although the shore stations can xx equally well obtain direction and nature of sound, their estimate of distance is open to considerable xxx error, and should be considered the equivalent of an “uncertain fix” of a [“]suspicious sound” by one boat bearing on the “K” tube.(Not centered within 15 degrees.)

     C. WHEN THERE IS GOOD REASON TO BELIEVE THAT A SUBMARINE IS ON PATROL IN A CERTAIN AREA, although no definite report has been received.

     Proceed precisely as in 2B/, excepting “suspicious sounds”. In this case all suspicious sounds should be followed immediately until their identity is ascertained, or until a definite reliabl-e report is received, when case falls under B.

     3. On all these various types of patrol txxxxxxx the M.B. and the S.C.C. tubes should never be used exxxx without the “K” tube e-xcept on short range or loud noise chase.

4. In order to promote the study of outguessing the submarine, it is recommended that a list be procured from the Brittish of all the submarines reported during the war, in the are<x>as to be xxxxx patrolled by chasers. Unit Commanders should make an exhaustive analytic study of this list, noting the time of day, season of year , the weather conditions, the location and the nature of currents, tides and sea bottom, depth of water, and distance off shore,the tactics of the submarine (if attacking), together with those of vessel attacked, the type of vessel and success of attack; if under escort, the type of vessel escorting, the tactics of submarine if attacked, together with those of attacking vesse-l, the type of vessel attacking and her success; above all they should observe the trend of submarinex tactics during the course of war, what phases of tactics have been substantial<ly> the same throughout, what tactics have been changed, whether changes were frequent or seldom, abrupt or gradual, and where the changes have lead and are leading.

     5. The foregoing is intended as no more than a working hypothesis for the guidance of unit Commanders, to be developed by actual patrol, and to supplement and bring up to date the admirable tactical doctrine, “ Submarine Hunting for Submarine Chasers”, promulgated by the Force Commander.

     6. Respectfully submitted.

William Hannie Perot


( to monograph on Tactical Dispositions )


     This depends on the primary object of patrol.

     There are three such:- viz- to obtain sound contact, to obtain visual contact, to keep a certain area clear.

     1. To obtain sound contact. 400 to 800 yards is considered to be the best interval.

     There are three criteria:-

          (a.) Interval should not be greater than twice the maximum effective range of listening devices for submarine going at slowest speed.

     Since K tube cannot pick up submarine proceeding submerged at “silent speed” beyond 400 yards, interval between ships should never be more than 800 yards. Thus a submarine could never pass through a listening unit, and an effective continuous sou<n>d barrage 2400 yards long and 800 yards wide at its minimum extent is thus presented, which is more likely to obtain contact and also in a better position to run one down, than three separated sound barrages of 800 yards square in <maximum> extent.

          (b.) Interval should not be greater than the limit of effective manoeuvering formation for running down a sound contact.

     It is believed that no interval greater than 800 yards will satisfy this condition for the following reasons:-

              I. With interval of 800 yards or less, a fix, certain within at least 300 yards, can be obtained on submarine making silent speed – for the range of the K tube for silent speed of submarines is known to be not over 400 yards, and the range of the M.B. and S.C.C. tubes under like conditions is known to be not over 150 yards; also a distant sound can usually be picked up by two vessels at least, when interval is 800 yards or less, whereas faint sound , close by, such as submarine on silent speed, <are often> heard only by one vessel. Hence,if vessels take interval of 800 yards, and one hears faint sound on K tube , but not on the M.B. or S.C.C. tubes, and this <so>und is not heard by another vessel<;> said sound must be close by the hearing vessel. If it is made by a submarine, the latter must be 100 to 460 yards distant from the hearing vessel, making due allowance for length of K tube cable, & for bearing of [word crossed out and unreadable] K tube from ship-as compared with bearing of sound from ship.> If interval greater than 800 yards be taken, this process of elimination is greatly weakened, if not entirely lost. Furthermore, with interval of 800 yards, two, and even three boat fixes on such sounds are possible, but with any greater interval nothing but a one boat bearing is possible.

              II. Any interval greater than 800 yards renders improbable the concentration of two boats, and renders impossible the concentration of three boats to bomb a fix close aboard, and this is the very situation when concentration is most likely to insure success.

              III. On loud sound contacts or distant fixes, any interval greater than 800 yards is of no advantage.

     800 yards is a sufficient base line for the most distant fix; a loud sound could be heard with equal effectiveness at 800 yards or two miles; in order to run down a distant fix, an interval of 800 yards or less would have to be assumed to bring unit into effective attacking formation.

     (c.) Interval should not be too small to give a good base line for a distant fix.

     Any interval less than 400 yards would offer <a> too small base line.

     2. To obtain visual contact.

          Interval depends on weather conditions, and should vary from two miles on very <calm> days to 1000 yards on days when white caps are numerous. This is based on visibility of periscope under different conditions of sea and on probability that no submarine would show periscope between patrol with interval less than 1000 yards.

     3. To keep a certain area clear.

          Interval of three miles is recommended. This would enable unit to sweep a wide area and still retain quick intercommunication.

     4. Respectfully submitted.

William Hannie Perot

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

Footnote 1: For more on M.B., or Mason tubes, see: William S. Benson to Sims, 19 November 1917. For more on S.C. tubes, see: Tube Specifications, 22 November 1917.

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