Lieutenant (j.g.) Harold S. Vanderbilt, Submarine Chaser No. 271, to Vice Admiral William S. Sims, Commander, United States Naval Forces Operating in European Waters
U.S. NAVAL FORCES OPERATING IN EUROPEAN WATERS,
SUBCHASER DETACHMENT THREE,
U.S.S.C. No. 271, FLAGSHIP.
BASE SIX, [Queenstown, Ireland]
CONFIDENTIAL. 13 September, 1918.
From: Lieutenant (j.g.) Harold S. Vanderbilt (T) U.S.N.R.F.
To: Force Commander,
Via: Commander Subchaser Detachment Three,
And: Senior U.S. Naval Officer, U.S.S. MELVILLE.
Subject: Relation of Depth Charge Attacks to the submerged depth of submarines.
1. Attention to date has been largely concentrated on increasing the size and effectiveness of the depth charge bar[r]age in the horizontal plane. The safety of attacking vessels, in view of the danger of countermining has necessarily been the limiting factor governing the number of depth charges that could be dropped in a given time. The depth setting at which depth charges may be set to explode has been increased from time to time, and it is understood that depth charges with a 300 ft. setting are about to be issued. The necessity for this increased setting was obviously due to the greater depth to which enemy submarines could submerge in view of their increased structural strength, and to the limited effective radius of the present 300 lb. depth charge. The problem is becoming more and more that of the effective bombing not only of the horizontal but also of the vertical plane in which submarines may be operating. If a 300 lb. Depth charge is dropped directly over a stationary submarine submerged at from 15 to 300 ft. the submarine has slightly better than an even chance of escaping destruction, unless her submerged depth is approximately known, which is not usually the case. One submarine commander was recently reported as having stated that he always remained near the surface and let the depth charges explode underneath him.
2. In view of the above it would seem advisable that:
(a) A plan be devised for dropping depth charges which will effectively cover not only the horizontal, but also the vertical plane in which a submarine may be operating.
(b) A listening device be perfected for installation on vessels fitted with sound detecting apparatus which will enable such vessels to determine the approximate submerged depth of a submarine.
The following plan for dropping depth charges is suggested:
Drop two depth charges at the same time, one set at 80 ft. one at 200 ft. (when maximum setting 200 ft.); one at 120 ft. one at 250 ft. (when maximum setting 300 ft.). Attach by a snap-hook to the depth charge set at 80 ft. or 120 ft. a small wooden or metal buoy of sufficient buoyancy to retard the rate of sinking of that charge so that it will reach its exploding depth at about the same time that the charge with the greater depth setting reaches its exploding depth. By this means almost twice the number of depth charges could, if desired, be expended in making an attack as at present, and the chances of getting the submarine in the area bombed would be increased nearly 100%. This method would be particularly valuable when, as is often the case, it is desired to bomb as effectively as possible, a certain area in the shortest possible time. Vessels equipped with both Y gun and Thornycraft throwers could in like manner buoy a depth charge fired from one of these should it be desired to fire both the Y gun and thrower simultaneously. It is understood that destroyers at present seldom expend more than 50% of their depth charges in making an attack.
Should the above method be adopted it is a question whether the proposed 600 lb. depth charges are desirable. It is understood they will have an effective radius of 95 ft. and thus cover an area of 28,353 cu. ft. Two 300 lb. charges employed as above would have an effective radius of 70 ft. each and cover an area of 30,788 cu. ft. It is not believed however that these figures form a true basis of comparison. There seems to be a far better chance of disabling a submarine with two 300 lb. charges dropped as above, than with one 600 lb. charge.
It is also a question whether or not a 300 lb. depth charge is the most effective depth charge, and whether or not three smaller charges set at say 100, 200 and 300 ft. and all dropped at the same time in the manner set forth above would not be more effective.
4. The following listening device to enable vessels equipped with sound detecting apparatus to determine the approximate depth of a submarine is suggested:
Install through the hull a vertical housing M.B. tube with compensator attached, (Fig. 1).
This tube to be used in the following manner on subchasers. Tube would be lowered only when unit has closed to within attacking distance. Its use at greater distances would be unnecessary and subject to great inaccuracy owing to the smallness of the vertical angle (x) (Fig. II) and consequent small changes in angle (x), due to relatively great changes in depth of submergence of submarine. Tube when lowered would be set (from bearings obtained from listeners on horizontal tube) so that the receivers face the submarine (thus eliminating as far as possible, interfering noises). During the plotting operation the listener on the vertical tube would center the submarine through the compensator. When the command “underway” is given the listener would communicate the vertical angle obtained to the plotting officer, who would enter a table with the vertical angle and the plotted distance of the submarine as factors, and take out the depth charge setting. This setting would be communicated to the unit while getting underway. Not a seconds delay in attacking would be occasioned by this simple procedure. The vertical tube would be installed only on the boat of the unit leader.
It is believed that this vertical listening tube would be particularly valuable to small vessels such as subchasers, because the limited number of depth charges carried would not ordinarily permit their expenditure as outlined in paragraph three.
The M.B. tube is selected in preference to the S.C. tube on account of its greater selective power. The S.C. tube could of course, also be adopted for use as a vertical tube.
Attention is invited to the usefulness of the vertical tube in the case of a vessel acting singly against a submarine operating at a considerable depth. In this case a large angle reading would be a sure indication of a close approach to the submerged submarine.
Harold S. Vanderbilt