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Rear Admiral Albert P. Niblack, Commander, Patrol Squadron Based on Gibraltar, to Patrol Squadron Based on Gibraltar



               U.S.S. DECATUR, Flagship.           GWG

3 June 1918.






          1. There are certain general rules which apply to destroyers at sea, whether patrollong or escorting, which require to be always remembered and obeyed. A destroyer underway is always being watched by a submarine, with the intention of sinking the destroyer with a torpedo should popportunity offar.

          The watching submarine may be ahead, astern, or on either bow, beam or quarter.

          Her periscope may be three feet above the water, or six inches, or only showing for about 10 seconds in every minute.

          Be always ready to open fire with one gun in any direction. This does not mena [i.e., mean] that all guns are to be kept manned; two guns, each of which can fire through 180° ( the two arcs being opposite), are quite sufficient.

          2. Submarines are usually on the surface at night and by day when not kept down by air or surface patrols.

          In narrow waters, where air patrols are frequent, or when the visibility is low,submarines may remain submerged except for the perioscope during daylight hours. In this condition their range of vision is very limited, and consequently so as the area in which they can attack shipping. Submarines are not, however, likely to remain submerged unless forced to do so.

          They have also been seen on the surface in a fog. When travelling on the surface by day, their speed is usually about 10 knots unless schasing a ship, when they may travel about 15 knots.

          3. When going to the assistance of a ship being shelled by a submarine, have a gun ready to fire at long range, and a look-out aloft to report directly the submarine is sighted. The submarine will dive as soon as the gun is fired (if the shot fall within about 500 yards of her), and the steamer may thus be saved from being sunk or set on fire.

          4. When chasing a submarine trying to oscape on the surface, fire an occasional long-range shot, on the chance of holing her so that she cannot dive until she has repaired the damage, and thus the destroyer may overtake and sink her.

          5. When chasing a submarine on the surface, if the submarine dives steer at once at high speed for the place where she went under, follow the track of oil that comes to the surface, and drop a depth charge on her when overtaken, which will be ahead of where the oil track stops. It has been found by experiment that oil of specific gravity .886 takes about 85 seconds to rise from a depth of 100 feet. In this interval a submarine at 6 knots would travel 283 yards. Always be moving over 10 knots before the depth charge is dropped.

          A submarine under these circumstances will always dive deep – at least to 140 feet – and will probably use full helm as soon as she is under and full speed for a short distance, when making her escape after firing a torpedo at a ship or convoy, a submarine will probably turn away from the direction in which the ship fired at was heading unless there is good reason for her to do otherwise, such as the position of escorts or other ship.

6. It is known that a submarine can determine the bearing of ships or patrol craft by means of her dydrophones [i.e., hydrophones], but this becomes very difficult, if not impossible, if several craft are in the vicinity.

          7. Keep a good look-out at night for a submarine on the surface; her conning tower may look like a boat, a wrecked chart house, etc; if sighted, she will probably be at short range and there will be no opportunity for more than about two or three shots. Remember also that what you see may be a motor-launch, or a camouflaged destroyer, or other friend.

     Always keep an eye on the horizon by day; you may see unusual smoke, or flashes of gunfire, or sheel [i.e., shell] bursting; you may also hear the sound of guns or bombs when there is nothing actually in sight.

     8. Remember that, if the visability is good, a submarine steaming on the surface at cruising speed may be watching your mast from a distance of 9 1/2 to 10 miles, when you can see nothing of her from your bridge. (Height of eyes, 25 feet). Therefore, when weather permits, always have a lookout aloft with good glasses.

     9: A submarine is always likely to be disguised either with big sails, a funnel emitting smoke, or by some other means.

     Whether deliberately disguised or not, submarines may often be mistaken for small fishing craft, Very often the only proof that an object sighted is a submarine is that it unaccountably disappears.

     For this reason, if an object that has been definitely sighted cannot be picked up again and there is nothing in the prevailing weather conditions to account for its disappearance, there is a strong presumption that it is a submarine.

     10. Seeing the necessity of a very smarkt look-outs, it is advisable not to allow men to remain on the look-out for more than about one hour at a time.

     Everything that you see at sea may be of importance; it may be a dummy periscope with a mine attached, in the hope that you will ram it; it may be a boat with a bomb attached which will explode when you get hold of the boat; it may be a boat with three or four mines in it of whom one or two may be alive.

     As far as possible examine everything you can see, unless you are on very important duty which precludes it.

     11. A submarine attacking with gunfire, and frequently also with torpedo fire, attacks from the direction of the sun. If approaching to the assistance of a ship that is being shelled and there is time and space, approach in the sun’s rays, and you stand a better chance of getting the submarine.

     12. Remember that all wireless codes may be considered as compromised, and able to be deciphered by the enemy. In any case signals may be picked up by the enemy D.F. stations,1 from which your position can be plotted and the information passed to the enemy submarines. Wireless should, therefore, be used as little as possible, and positions should always be given with reference to a squared chart, a known rendezvous, or other position of that kind.

     13. When sent to patrol an area, remember that the ship is on patrol from the time she leaves the harbour till she returns.

     While on patrol ( except on dark nights) zigzag irregularly; do not always go from end to end of your area; steam in varying directions so as not to allow any inference to be drawn as to where you will be in an hour’s time. This makes navigation very difficult, and as careful a reckoning as possible should be kept. Compasses also should be corrected fairly often.

     14. Remember that your best chance of destroying a submarine at night is when she is between you and the moon. For the same reason the direction away from the moon is always the most dangerous to yourself. With a dark background a submarine may be within 500 yards of you and on the surface without your being able to see her? The foregoing applies generally to morning and evening twilight, and reports of attacks upon merchant shipping show ythat the hour of morning twilight is the most dangerous hour of the 24.

A. P. NIBLACK.     

Source Note: Cy, DNA, RG 45, Entry 517B. The signature is a stamp. There is an identifier in the top left-hand corner: “File-605.” Below the signature are two notations: “Alatl, 40. Referring to CSP 158 dash submarine contact code, table one third line change letter W to X. Issueing officers correct before issueing 00528 1074. Sims.” And “Alatl. 41. Director Naval Communi vcations circular letter file 24076-58-OR of May 15 regarding communications in the west at one A.M. June 6th 75 meridian Standard time 16003 1104. Sims.” There is also a handwritten note at the top of the first page: “Extracts from Admiralty Instructions.” These notations mean that Sims circulated this memorandum throughout his command and certain changes were made beforehand.

Footnote 1: D.F. means direction finder.

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