Vice Admiral William S. Sims, Commander, United States Naval Forces Operating in European Waters, Circular Letter No. 87
U.S.NAVAL FORCES OPERATING IN EUROPEAN WATERS.
2 September 1918.
CIRCULAR LETTER NO. 87.
FROM: Force Commander.
TO : All Forces.
SUBJECT: Importance of following up contacts with enemy submarines.
ENCLOSURES: Issued herewith various information prepared by Intelligence Section Force Commanders Staff.
(a) Concerning typical attacks by U.S. and Allied vessels, following which submarine continued to operate or reached home port.
(b) Extracts from reports of Allied submarines concerning attacks by surface craft.
(c) Concerning balancing and silent running by submarines.
(d) Typical charts of daily submarine situations.
1. The Force Commander desires to impress upon all Commanding Officers the importance of following up all contacts with enemy submarines as long as there is the slightest possibility of inflicting damage or of re-establishing contact if lost.
2. During the War a number of claims for destruction of enemy submarines, where the attacking vessel has reported certainty of destruction, compares with the number of submarines actually sunk by such attacks as over six to one.
3. the pre-war belief that submarines were very vulnerable has been disproven in the most unmistakable manner. There is attached, for example, a few extracts from reports of Allied submarines concerning attacks made upon them by surface vessels. These reports happen to be the only ones available at the time of issuing this circular, but the experiences of the War have shown many other cases, in some instances remarkable ones of submarines withstanding very severe attacks and severe damage, and still being able to reach their Bases. One submarine, whilst submerged, actually had a mine explode on forward deck and suceeded in reaching its home port.
4. There are also attached various attacks made by our own and allied vessels, in which there is definite evidence that the submarine was not destroyed, but either continued to operate or succeeded in reaching its Base.
5. The case of the STERETT is known to all ships. The recent attack by the TUCKER gives very good evidence of seriously damaging the submarine, but from experience of the War there can be no certainty whatever that the submarine did not succeed in eluding the TUCKER and perhaps later carrying out repairs on the surface.
6. Numerous cases are known in which a damaged submarine actually remained in the immediate vicinity of the attack for a considerable time before coming to the surface.
Recently an enemy submarine in the Mediterranean had one engine damaged by a depth charge attack and came very near escaping with the other engine. Fortunately, however, a leak through the hull forced the submarine to the surface, and as the attacking vessel had been persistent in following up the attack, success was the result.
7. In the case of the STERETT which has been published to all Forces, It is to be noted that this attack was followed up throughout the night with very little evidence of the submarine’s location. The attack might easily have been dropped if it had not been for the very commendable persistence and good judgement of the Commanding Officer. Unfortunately, in this case, the final contact on the following morning found the STERETT with all depth charges expended.
8. The Force Commander fully appreciates the position of escort commanders when encountering submarines while on route to meet convoys, especially in the case of troop convoys. The decision is a difficult one indeed as to whether joining the convoy with the full quota of destroyers detailed is paramount to detaching a destroyer for a search and attack.
9. The relatively small number of submarines operating compared with the sea areas involved must be borne in mind (See attached charts). For example, if the latest intelligence when leaving port indicates that there are but a certain number of submarines at sea in certain general localities, it may be clearly justified to detach one or more destroyers in case of a contact, providing the evidence is reasonably good that the prospective route of the convoy is clear of other submarines. In other words, cases may arise in which the detachment of one or more destroyers might easily prove the best protection of the convoy.
10. It is fully realized that our intelligence information is not always complete. However, the game we are engaged in is a dangerous one and we cannot hope for one hundred per cent safety.
11. The Force Commander desires to assure all escort commanders of his support and of his willingness to accept responsibility in all cases where good judgement has been exercised upon the information available.
12. Various attempts have been made to estimate the value to the allied cause of the loss of one submarine. Considering the total number of enemy submarines on active service and the loss they have inflicted during the past year, one submarine destroyed may be estimated as representing the saving of 40,000 tons of merchant shipping per year; or a value of $30,000,000 to the Allied cause; or the supply on a war basis of 35,000 men in France; or the output for four days of the total of all German shipyards building submarines. These figures are, of course, not accurate, as they award equal offensive power to all submarines, which is not the case. The damage to enemy morale is probably of much greater importance, although it is one that cannot be estimated. The more dangerous the game becomes to the enemy the less efficient the operations of the remaining submarine.
13. The importance of following up contacts with the enemy and of improving our tactics cannot be over estimated. It is desired that all Officers give careful consideration to this circular letter and also to circular letters Nos. 16, 24, 26, 32 and 69 concerning depth charge policy and tactics and various other information circulars which have been issued on the general subject of submarine attacks. Base Commanders should require reports from all ships that those various papers have been read and are being given consideration.
(signed) Wm. S. SIMS.
A FEW U.S. DESTROYERS ATTACKS IN WHICH SUBMARINE CONTINUED TO OPERATE OR WAS NOT DESTROYED.
(a) Attacked a submarine with depth charges, bringing up oil and debris. Destroyer remained in the vicinity 1-3/5 hours and then rejoined convoy. There was a submarine in the vicinity and it continued to operate.
(b) Attacked submarine when missed by torpedo. Depth charges brought up oil and fresh debris. Submarine came to surface after first attack, about 500 yards from destroyer. Second attack brought up oil and debris. Destroyer remained in vicinity about two hours. Submarine in vicinity – continued to operate.
(c) Sighted wake and periscope of submarine and attacked with depth charges – no results. Attacked submarine again three hours later, second charge causing tremendous explosion and large oil slick and splintered wood. C.O. satisfied that submarine had been destroyed, and rejoined the convoy.
This submarine was able to reach Santander harbor.
(d) Attacked submarine about 4:30 p.m. First attack brought up oil in sufficient quantities to make following the submarine wake possible. Destroyer followed the submarine until 8:00 a.m. the next morning, using all depth charges. Another destroyer joined in the first attack but left on urgent duty. Track of submarine was lost in the morning. Submarine possibly slightly damaged. Returned to base.
(e) Attacked submarine with depth charges. Submarine distinctly heard on oscillator. After second attack no further noise was heard, save a peculiar buzzing sound. Three hours later another vessel saw submarine in the same vicinity. Submarine continued to operate.
During May, June and July U.S. Anti-submarine Forces in European Waters made 78 attacks on supposed enemy submarines. In 45 cases there was evidence of a submarine in the vicinity and in but one case, that of a sub.chaser attack in Otranto Straits is there any reasonable definite evidence of destruction.
A few typical attacks by Allied vessels on enemy submarines, which occurred during the first two quarters of 1918, in which the submarines were not destroyed or continued to operate.
1. A drifter sighted periscope of a S/m about 1/2 mile to the eastward, steering north. She immediately proceded full speed towards it, the periscope remaining in full view until within a drifter’s length, when submerged. The drifter passed immediately over the S/m, but did not succeed in ramming her. A depth-charge, set at 100 feet, was dropped on the spot and exploded. Within two or three minutes, as the drifter was turning round to port again, the S/m was seen to come to the surface and then sink. All that appeared was the rounded surface of a portion of the hull, about 40 feet in length. The drifter turned around and dropped a second depth-charge where the S/m sank. Large bubbles and patches of oil came to the surface. Two more depth–charges were dropped, and subsequently a yacht and another drifter joined and also dropped charges. The vessels remained in the vicinity with hydrophones out until 9:00 a.m. the following day. Nothing further was seen or heard, but oil continued coming to the surface during the night.
S/m continued operating.
2. A “P” Boat sighted the conning-tower of a S/m, which immediately disappeared,at about 500 yards distance. Ship was headed to pass just ahead of where S/m dived, and telegraph put to full speed. Just before the position was reached 3 depth-charges were dropped. When they had detonated a fourth was released. Immediately afterwards a broken periscope was observed by all on board, and course was headed to ram. When over the position, two hand depth-charges were released and helm was put hard over. The S/m was then observed to break surface, evidently on her side, as no conning-tower was seen. Course was shaped to ram but the S/m dived just before the spot was reached. Helm was again put hard over and the S/m again broke surface on her side and disappeared.
After the explosion of the first 4 depth-charges several of the crew state they saw a grey object resembling the bow or stern of a S/m rise about 8 feet out of the water and immediately disappear.
S/m continued operating.
3. A drifter sighted S/m on starboard beam. S/m was bearing S.W., distance about 3 or 4 cables. Drifter was heading S.S.E. at the time, speed 4 knots. She immediately increased to full speed and altered course to ram, but S/m submerged about 5 cables ahead of her. Position where S/m dived was then closed and starboard depth-charge, set at 100 feet, was dropped, depth of water being 22 fathoms. After the charge had exploded the S/m came to the surface on starboard bow. Another drifter fired two rounds which caused an explosion close to S/m’s conning-tower, after which S/m submerged. She then dropped a depth-charge and another explosion followed.
The first drifter remained in the vicinity for 6 hours with hydrophone out, but heard nothing further. At daybreak patches of oil were seen in the position where S/m had disappeared. The position was subsequently swept, but without results.
S/m continued operating.
4. While on patrol with a Hydrophone Division, a sloop sighted torpedo breaking surface about 30 yards on starboard quarter. Helm was put hard aport but before ship started to swing a second torpedo struck her on port side just abaft the foremast. The torpedo did not explode. The conning-tower of S/m was now sighted slightly on port quarter about 600 yards distant. She was seen to fire a third torpedo which the sloop avoided by starboarding her helm, and then submerged.
Passing over the spot where S/m had disappeared the sloop dropped 4 depth-charges together, 2 from the thrower and 2 from the poop, and 15 seconds later 2 single ones. A column of smoke shot up and a black shape rose out of the water and then disappeared. Twenty minutes later the Hydrophone Division returned and listened, but nothing further was heard.
S/m continued operating.
5. While escorting a convoy, a “P” Boat sighted periscope about 3 points on port bow, distant about 1/2 mile. Enemy appeared to be steaming on a similar course to convoy and was making approximately 7 to 8 knots. Engines were increased to full speed (23 knots) and course altered to head off the enemy. The periscope remained in sight for a very short time and submerged when ahead and about 100 yards away from the “P” Boat. When right over position in which S/m dived, the “P” Boat dropped a depth-charge, set at 200 feet, about 10 yards ahead of the swirl, and a calcium light was thrown overboard to mark position. Ship then turned under port helm, and almost simultaneously with the upheaval caused by explosion of charge the S/m broke surface about 100 yards distant on starboard side. Part of the hull of the S/m was distinctly visible. About 20 to 30 feet of her bilge showed about 6 feet above the surface of the water, and a few seconds later she seemed to turn over and sink. The “P” Boat immediately proceeded to position and two more depth-charges, both set at 200 feet, were dropped with an interval of between 10 and 15 seconds, both exploding satisfactorily. Six more depth-charges were dropped by the “P” Boat, who remained in the vicinity for about 45 minutes, but nothing further was seen.
S/m not destroyed. . . .
A FEW EXTRACTS FROM REPORTS OF COMMANDING OFFICERS AND ALLIED SUBMARINES.
“Both Destroyers continued to approach at high speed; the replies to the challenge were kept up continuously without result. Several rounds were fired, one of which struck us on the hull abaft the Conning Tower.”
8:10 a.m. Observed Light Cruiser & Destroyers. 8:15 Dived. 8:37 a.m. Loud explosion from depth charge. 8:40 a.m. Further loud explosion, similar to first. 8:50 a.m. Took bottom in 23 fathoms and stopped all motors. 9:05 a.m. Loud explosion. All these explosions were very loud and violent, shaking the boat considerably. The first two were apparently depth charges, whilst the last is believed to have been caused by an explosive sweep as a wire was heard gating alongside hull. By 11:30 nothing more could be heard of surface craft. 4:07 p.m. Left bottom and rose to 28 ft. 8:35 p.m. Surface and proceeded to billet, battery on charge. Arrived 1 p.m. B.S.T. Helm jambed at 90 feet, due to pressure – after hydroplanes were unworkable at 110 ft. Hull tight, no leaks. Boat was at 23 fathoms for 7 hours, as battery was low and it was presumed that torpedo craft would continue to hunt us. The third explosion was so violent that it broke all switches on auxiliary switchboard, putting out all lights, etc. propellers were heard in hydrophones up to 11:30 p.m. The first depth charge appeared to be immediately over the stern as it lifted all those in the after compartment off their feet. The second one to the westward of us and in line with the conning tower. The third one, starboard side forward.
Encounter between Submarine and Four Destroyers.
Durign the time of approach the southernmost destroyer was firing slowly, and after five shots had been fired, destroyers being then just over a mile distant, I told the signalman to get below and followed myself. Time was then 12:30 p.m. The boat took roughly a minute to get below water. Just after getting one clip on the conning tower hatch I felt a violent concussion close by, which may have been the shot on the periscope pedestal, for directly I got down I attempted to raise the periscope, but without success. At this time, by the noise, firing seemed to be very rapid and hits were being scored. The boat took up a big angle going down and No. 1. auxiliary was flooded to accelerate the operations. At 120 feet she was got under control, the after diving rudders jambing with about 5 degrees of dive helm and were never workable again. The boat had got a sinking trim which necessitated going very fast with the motors to keep her under control. My chief anxiety was that the destroyers might follow by my wash, although diving so deep, and I tried then to keep a depth corresponding to 75 lbs. After about 2 to 3 minutes the first depth charge was dropped followed at short intervals by three others, two the port side, one starboard, and one astern. They all made the boat shake violently, the last being the worst. Shortly after two or three depth charges were dropped a long way off. Numerous alterations of course were made in the attempt to shake the destroyers off, but according to the hydrophone with not much success. Diving. The boat had an inclination down by the stern of roughly six to ten degrees the whole time. I made various attempts by blowing water forward and shifting the crew about to better this state of affairs; but it seemed that if I decreased the angle at all she would commence to sink, so losing control, which I could only correct by more speed while I made another attempt to improve matters. For the first hour and a half motors were being worked anything from 2,000 ahead both to 500 grouper up. I was perfectly aware that I had negative buoyancy, but it was not until about 2:15 pm. that I dared pump out, for fear of oil or air bubbles showing on the surface. Owing to the inclinations I was doubtful whether the pump would suck from the tanks aft and so blew from No.1 Aux. into “A” main ballast, and pumped out from there blowing forward at the same time. My one aim and object was to get a “grouper down” trim, or approximately so, as then I might hope the battery would last out until it was dark enough for me to come up, which I thought would be about 7 p.m. The greatest pressure which showed on the guage was 95 lbs. The submarine took two big inclinations bows down, on the second occasion <and> motor shaft floated forward butting up against the engine clutch. About 4:15 p.m. after having blown out some of three and four main ballast I managed to get a “grouper down” trim and continued at about 300 on each till 5:15. p.m. when I broke surface involuntarily. Periscopes. Almost directly on getting down, water was observed to be coming through the centre of the after periscope very slowly, filling the well half full by the time of rising to the surface. As the periscope was almost right down it was difficult to observe exactly where the water was coming from, but I think from the gland in the bottom of the breech piece, through which the rod passes to change from unifocal to bifocal. The possibility of the window breaking was the source of much worry while submerged. Main Motor Wells. Owing to leakage in the after part of the submarine there was a constant danger of water over-flowing from the motor wells into the motor beds, which would be accentuated by any inclination down by the bows. To avoid this semi-rotary fuel pump with flexible hose discharge was rigged to discharge into the engine room bilges. Main Motor Bearings. Owing to the inclination at which the submarine was travelling, the bearing soon started warming up as it was found most difficult to keep them supplied with oil. The possibility of these bearings becoming overheated and causing serious trouble was another cause of anxiety. Leaks Only two places came under my personal observation which might have become serious, one was in the seam that had given trouble before, the other in the fore compartment and had doubtless been caused by the splinter of shell which was found in the free flooding space. Both were calked. The stern tube compartment was leaking but not enough to cause any apprehension of serious trouble. Every half hour I had the battery tanks examined for water but none was found. Hydrophones. Up to 5 p.m. I had an operator constantly listening and was maneuvering the submarine according to the reports I received from him. At 5 p.m. which was the first time I had left my position behind the man on the hydroplanes since diving, I went into the cabinet and listened myself. I remained there for about 10 minutes and on leaving, although I was willing to accept the verdict of the operator that Destroyers were still in the vicinity, I was a little doubtful. At 5:15 p.m. I lost control of the submarine and she rose to the surface when nothing was to be seen on the horizon. Whether this is due to the fact that the hydrophones had been put out of gear by the depth charges or to the constantly varying speed of the motors, I was unable to say. It may have been owing to lack of experience on the part of my operators. Clinometer. On first diving the submarine took a steep angle of about 18° which one could not measure on the 9-1/2° clinometer on the diving gages, but could be measured on one which in my personal property measuring 30° each way. As the submarine was going down very fast, I commenced to blow Nos. 1 and 2 main ballast in order to regain control. The big clinometer warned me directly the bubble started to go forward again and I was thus able to stop blowing before the submarine regained a tendency to shoot to the surface.
At 10:45 on the 21st. while on the surface, the fog suddenly lifted, and a destroyer and three trawlers were sighted about 3,000 <yards> to the north. The trawlers at once swept south in line and abreast, and I was compelled to dive underneath them to attack the destroyer that was circling at full speed in their rear. The fog was however again came down and they were lost sight of. Shortly after dark, when at 50 feet, a grating noise was heard forward, and the submarine dipped slightly by the bows. It was believed at the time to be a small piece of wreckage, but it is probable that it was the mooring rope of a mine which the submarine carried away. On the 22nd there was a strong n.easterly gale blowing with a rough sea, visibility during the morning about 3 miles. Position was checked by reaching the 10 fathoms line. Owing to the rough weather, a depth of 60 feet was kept, with frequent rises to 19 feet for observations. At 10:50 a.m. while at 60 feet a slight scraping was heard forward, followed by a violent explosion. The submarine immediately sank by the bows and struck the bottom at 85 feet, with an inclination of 25 or 30°. Motors were at once put to full speed astern, and Nos. 2 and 3 Main Ballast Tanks blown, No 1 being found to be open to the sea. The submarine came to the surface, and although there was now full visibility, nothing was in sight. After blowing fuel, and making some temporary internal repairs, courses were shaped as follows: W.N.W., 5-1/2 miles. West 15 miles, and then S. 62° W. Having investigated what happened after the explosion, I can with great pleasure bring to your notice the splendid conduct of the Officers and Crew. Her Commanding Officer, by his coolness and prompt action undoubtedly saved the boat; any delay in getting her to the surface would have been fatal. He reports that the Sub-Lieutenants and the entire crew, although it appeared obvious to all that the boat was lost,proceeded at once to their stations without any signs of excitement, and all orders were carried out promptly and correctly. I would submit that such conduct, in the face of apparent certain death, is an example of which the whole Service may be proud. The damage is serious. The mooring rope having evidently fouled, the mine exploded against the starboard forward hydroplane. Both forward hydroplanes and the bow cap are gone; the upper part of the hull in that vicinity, as well as both starboard torpedo tubes, are wrecked. All bulkheads appear to be strained, but luckily the one near the rear of the torpedo tubes,although leaking, did not give way. None of the four warheads or detonators exploded; but the fan and one detonator of one pistol was missing.
A heavy sea got up during the night, and on the morning of the 29th I decided to return. Owing to the rolling of the boat, the petrol had become so mixed with sand and dirt that no cylinders would fire, and it was not until 1:30 p.m. in spite of cleaning strainers and feeds, and drying spark plugs, that two cylinders started, by which time the battery was at 1,124 gravity. I finally reached port at 8:20 p.m.
By this time, ship had straddled us twice, and a certain amount of water came into the conning tower, being the spray from one shell. Boat brought up at 120 feet, badly out of trim. About 2 or 3 minutes later, two loud explosions were heard overhead, evidently depth charges, shaking the boat violently. No damage done. Dived to the Southward.
Captains of boats report that the manner in which the seagulls follow their periscopes in the Atlantic is very noticeable, far more so than in the North Sea. This information would perhaps be of assistance to patrols in locating periscopes, since one is very likely to neglect a spot which may have aroused suspicion on detecting a sea gull in the vicinity.
At 11:43 sighted destroyer coming out of the base and dived at full speed. Both masts were gotten down whilst diving, the main topmast being carried away by the wash. At 11:47, heavy underwater explosion was heard. No ill effects were felt from this, which proved to be a depth charge dropped by a destroyer. By the sound, it was estimated that the charge exploded within about 300 yards of the submarine.
MEMORANDUM CONCERNING BALANCING AND SILENT RUNNING OF SUBMARINES.
Intelligence Section, Force Commander’s Staff.
It has been put forward by scientific men that a submarine cannot balance. That she must either rise <or> sink. That on these occasions when an actual balancing has been reported, that a layer of water of greater density has been encountered etc. Never the less, they do balance, and for years the balancing of the boat between certain depth has been a U.S. requirement for acceptance trials, and for qualification of an officer to command; and it has always been possible to pass this requirement. Perhaps all sea water has layers of greater density as the depth increases.
1. The following is a quotation from a summary of operations of Submarine Division 5 for week ending June 1st, 1918:-
The A.L.1 during its last patrol lay between 65 and 80 feet depth for five hours and fifty five minutes with motor stopped, except for about two minutes on one occasion; during this time one ballast pump was run once for two or three minutes. Otherwise all her machinery was stopped for nearly six hours. The sea was rough and the difference in densities between the strata may have been due to aeration; we have no information that would indicate differences in densities for other reasons.
2. The following is a summary of a report obtained from the interrogation of prisoners taken from U-110 concerning the method of silent running:-
These prisoners stated that when surrounded with hydrophone vessels or when an enemy ship was in the neighbourhood the boat dived to a depth of 20 to 30 metres (65 to 98 feet) and the order “Silent running” was given. The port motor was then run at a speed of 100 revolutions; the starboard motor at 80 revolutions; the forward hydroplane was put in a central position and not moved again. The after hydroplane was left in power, but only worked <as> necessary; the periscopes were housed; the gyro compass remained running, but it was forbidden to use the pumps and toilets. After proceeding as stated above for <an> hour the boat rose to 11 metres (36 feet) and the periscope was raised to see if any hostile ships were still in the vicinity. Should this be the case the boat again dived to 20 or 30 metres (66 to 98 feet) and proceeded as before.
3. The following information was obtained from the prisoners taken from the U.C.55:-
Listening devices on German submarines can detect sounds at a distance of ten miles. Devices work satisfactorily however, only up to a distance of three miles. When observing the movements of vessels with this apparatus it is comparatively easy to distinguish as many as three vessels, especially is they are of different types, as the sound produced by each vessel is heard as a different note. Should the number of vessels in the vicinity be more than three it is impossible to establish the exact number as the sound transmitted becomes too blurred. When pursued by vessels fitted with hydroplanes German submarines stear a zigzag course, stopping occasionally to listen.
4. Prisoners from U.B. 72 when interrogated gave the following information concerning silent running:
In the case of U.B. 72 the silent running speed was 100 revolutions per minute with both motors, giving a speed of 1-8/10 knots. It was not considered necessary to stop the gyro compass which made so little noise that it was inaudible at the end of the boat.
In the case of U.B. 52 the practice was to run at 60 revolutions per minute on one motor only. At this speed the consumption of electric energy was only 25 amperes per hour. As the battery had a capacity of 8000 ampere hours air was the principal factor in limiting the submarine radius. The Captain of the U.B.52 stated that when attacked it was always his policy to keep at periscope depth, and allow the depth charges to explode beneath him. He stated that he had no fear of Allied craft equipped with listening devices.
5. The following account of submarine manoeuvres when attacked is taken in the interrogation of the survivors from the U.C. 65.
In February 1917 the U.C. 65 was surrounded by net divers in the Irish Sea. The submarine cast a screen, however, by submerging to a depth of 197 feet. To avoid being heard and chased all auxiliary machinery was put into hand gear. As soon as the search formations were sighted the trim was regulated by blowing one regulator tank, and flooding the other, the engines never going more than slow speed.
6. Another account of submarine manoeuvres when attacked is given in the report of the interrogation of the survivors of U.B. 40.
The U.B. 40 at the end of February 1917 was sighted by two armed trawlers. The submarine, unable to escape on the surface, dived and lay on the bottom for two hours. The motors and even the gyro compass were stopped. After two hours the submarine rose to periscope depth and found the patrol vessels still close at hand. While emptying the trimming tanks the pump made more noise than usual and the patrol seemed to have heard the noise. The submarine then submerged and made off at full speed at right angles to its former course. An hour later on coming to the surface again the patrols were found to be still steering their former course. The submarine then escaped.
7. Still another similar report is to be found in an account on the interrogation of the survivors of U.C. 75:
At the end of April 1917 the U.C. 75 encountered a number of patrol craft which seemed to be equipped with listening devices. The submarine commander as a precaution ordered silent running. The submarine then proceeded slowly with one engine. The hydroplane vertical rudder and pump were working as usual. After some time the submarine came to the surface and sighted the patrol craft coming in its direction. It then submerged at utmost speed to 40 metres and proceeded as quietly as possible in the opposite direction. In this way the submarine escaped from the patrol.
EXTRACTS FROM REPORT OF COMMANDER U.S.SUBMARINE DIVISION FIVE.
(a) TO AVOID SURFACE CRAFT
On several occasions the AL-boats have avoided surface craft by running directly away submerged at 60-feet at high speed for five minutes and then continuing at slow speed. This was done where it was considered improbable that the surface craft had sighted the periscope. Also on several occasions the surface craft have been avoided while the submarine was on the surface, by the latter altering course as soon as the other craft was distinguishable as friendly.
(b) HOSTILE LISTENING CRAFT.
Under conditions where listening boats made contact the general opinion is that constant changes of course at slow speed be made until the listeners are thrown off. It is the opinion that, if on soundings in smooth water to go to the bottom is the best manoeuver unless the indications are that the enemy suspect the move and are searching for you. Then the best thing is to run away at slowest speed. The foregoing is the procedure which in the opinion of the submarine officers should be followed when the periscope has not been sighted and listening contact has been made accidentally or after scouting. If the periscope has been sighted by ships close aboard the opinion is that a fast run should be made immediately to get out of the listening range or out of bombing area. Then as it is possible for listeners to follow a submarine when the latter is making six to eight knots submerged, it is best to slow down and run as nearly silently as possible on various courses.
(c) ABILITY OF SUBMARINES TO BALANCE.
Actual experience in the Bristol Channel shows that it is possible to balance between fifty and eighty feet while stopped for from half an hour to four hours. This has been done by the AL-boats on several occasions and from ten to fifteen minutes many times. The AL-4 stopped for four hours except for two minutes. This is entirely an accidental occurrence. In the open sea off the Irish Coast the usual procedure is to obtain a fine trim and run at slowest speed which is about 1.7 knots. Every ten minutes the boat is stopped to listen in. After stopping the boat will not change depth for ten minutes if the trim is good and then only very slowly so that by starting at slow speed again as soon as the boat starts to settle or rise the depth is maintained. On this form of patrol no machinery except the main motors and the gyro compass motors are used, and the boat is practically silent after the main motors are stopped. About every hour the pumps are used to pump out about one hundred pounds which usually leaks in. Balancing by running the periscope out and in is not attempted but it is considered feasible. However this would require running the periscope motors and sooner or later would require pumping so pumping is considered to be the most practical way of controlling the trim.”
“In the water north-east of the Scilly Islands and just south of St. George’s Channel there appears almost always to be a certain amount of stratification of the water, though we have found it more noticeable on roughish days with a breaking sea, which has usually been accompanied by a westerly wind, than on calm or nearly calm days. However, even on calm days we have often remained stopped at about 70ft. for as much as half an hour. I am now inclined to believe that the stratification may be due to differences in the temperature of the water near the surface and of the water well below the surface due to surface currents caused by the prevailing westerly winds, through aeration of the water near the surface due to combing waves may add to the effect. But whatever the reason may be we have not found this stratification at all, even on rough days, when we have been well off shore to the south and south-west of Ireland. <There> we have found it difficult to remain stopped for a much as 15 minutes without rising or sinking 30 ft. or more, and as the boat rises she appears to grow lighter and as she sinks she appears to grow heavier. In fact with a neutral trim at 70 ft. we find that we need to take in about 700 lbs. to get a neutral trim at periscope depth, and with a neutral trim at 70ft. we can hardly hold the boat at 100 r.p.m. on both at 100 ft because she is then so heavy. It is possible that with a very sensitive depth gauge and the boat very tight one would balance by running the periscope up and down, but that remains to be proved. However, in spite of our bow rudders being so much too small we can keep our depth pretty well at say 70 ft. with our propellers turning over so slowly that we can barely hear them on our own K and C tubes. It is true of course that our K and C tubes are rather screened from our own propeller sounds by the hull and conning tower, but on account of his relatively larger bow rudders a German submarine can undoubtedly run more slowly than we can. it has been proved that a Lake boat with hydrophone can, and I don’t believe that a destroyer with K-tube in her tank would be able to hear a submarine with proper diving rudders, running dead slow even if she were right on top of the submarine. On the other hand I, at least, am pretty well satisfied that as long as we are in deep water, 50 fath. or more, even on a pretty rough day we can hear any sort of surface craft running at a fair speed on her propellers a good bit farther than we see her with the best visibility using our 6 x Grubb periscope with 12 ft. of it above the surface”.