Chasing Subs: Instructions and Doctrine Editor's note: Scans of the document Instructions and Doctrine for Sub Chaser Detachment On are available in the Document Archives section.

As German U-boats came on the scene in WWI, the U.S. scrambled to put together the Sub Chaser fleet. The men who commanded the sub chasers trained in maneuvers with their new 100-foot wood-hulled boats; and their maneuvers were guided by a number of confidential documents outlining the strategy of chasing subs. This is a rough overview of the strategy of sub chasing, based on a Navy document, titled Instructions and Doctrine for Sub Chaser Detachment One, marked CONFIDENTIAL and circulated by L.A. Cotten, Captain, U.S.N., Commanding. Note that there are several similar documents, and that the tactics are described somewhat differently at different periods. This is because chasing subs was a wholly new enterprise, and to a large extent the tactics had to be developed and honed on the fly.

Taking Initiative. Early on in the description of tactics we learn that “While the fundamental principles of warfare are neither numerous nor complex, they must never be limited by a set of rules.” Those compiling these documents understood the novelty of anti-submarine warfare, and were keenly aware of the need to give the men “a certain independence in the execution of the tasks assigned,” and to allow them “to show initiative in meeting the different situations as they arise.” Yet, as one might expect, this expression of liberty to think independently was tempered by an admonition that “Everyone serving on Sub chasers must remember that inaction and neglect of opportunities will always warrant more severe censure than an error in the choice of means to be employed.”

Hunting Unit Organization and Communication. Sub chaser detachments were ordered as follows: A “Hunting Unit” was comprised of three Chasers. The Unit Commander’s Chaser was the “Leader,” and the other two were “wing Chasers.” A “Hunting Group” was two or more units operating together. Communications among the Chasers in a Unit was by means of radio telephone, radio telegraph, and visual signals. Radio telephones were a relatively new device, and offered incredible advantages over the other forms: communication was instantaneous, and didn’t require cumbersome deciphering. The crews developed code words, a kind of short-hand, that allowed them to communicate quickly. Because of this, the preferred device was the radio telephone. Yet Chaser crews were advised not to rely entirely on one means, but to remain proficient in all three. Code language was to be used at all times, except for “necessary conversation by phone in actual chase and attack,” and whenever it was practically possible to use visual signals instead of telephone or telegraph, instructions were to use the visual method. In action, however, the visual method was often utterly impractical, due to the distance between boats and the roll of these small boats on high seas, when wing boats would periodically slip out of sight altogether.

Hunting Subs. Sub Chasing was divided into “the hunt,” “the chase,” and “the attack.” Hunting was to begin by proceeding to areas where subs were most likely to be. Depending on the weather, Hunting Units would engage in a “drifting hunt” (in poor weather) or a “running hunt” (in clear weather). In this version of the hunt, Chasers in a Hunting Unit normally would maintain a distance of 800 yards from one another, and travel at right angles to the direction in which the sub was believed to be moving. The Chasers would alternate between silent and non-silent periods. Two standard formations for Hunting Groups are described: The first saw one Hunting Unit in front, followed by a Supporting Vessel, which in turn was followed by another Hunting Unit. Flanking the supporting vessel were two more Hunting Units, resulting in a diamond-shaped overall formation of 12 Chasers and one Supporting Vessel. A distance of five miles was maintained between the Supporting Vessel and each Unit. In cases of more than four Units, the additional Units would be positioned abreast of the flank units at five miles distance or less. In the second formation, the Units were arranged in a straight line, five miles or less distant from one another, with the Supporting Vessel ten miles or more behind, in line with the center of the line of Units.

Chasing. A Unit making contact would hoist a flag, and thereby become the Chasing Unit. The other Units would close in on the Chasing Unit. If the sub approached another unit, the Chasing Unit would turn the chase over to the other Unit. In standard chasing formation, the wing boats would stay at a distance from the leader such that they would be positioned at corners of an equilateral triangle with the Sub at the apex. This was called “the sixty-degree method.” The Leader was to head for the sub, the wing Chasers were to fall into formation, and when the leader had reached the last fix, the Unit would fall silent and listen; then repeat the process based on the new fix. The attack was to be made only when the Unit was within 300 yards of the fix. The chasers were well-designed for this process. They could stop their engines quickly and fall utterly silent, then quickly start up again, unlike the big warships, for which this would be utterly impractical. Interestingly, the engines of the chasers were connected directly to the propellers, with no clutch. This meant that the instant the engine started, the propellers started, and the instant the engines were shut down, the engines were shut down. The exception to this is that the middle engine (there were three altogether) was fitted with a clutch, of sorts, which allowed the engineer to disengage the propeller shaft, so that they could run on two engines without incurring drag from the center propeller. Another facet of this arrangement was that reversing engines meant shutting them down, moving a lever to change the cam shaft orientation, and re-starting them. In photos of the chasers, the time it took to do this is illustrated in the beating the bow of some chasers received by crashing into docks or other vessels. The standard attack involved a pattern of dropping 18 depth charges in a spread across the meeting point with the sub. In the approach, the wing Chasers would be in line with the Leader, the distance from wing to wing being 200 yards. Each Chaser would drop one depth charge 15 seconds before reaching the meeting point for the sub, and one at the meeting point. The Leader would then proceed dead-on, and the wing Chasers would turn away two points. 15 seconds past the meeting point, they would each drop one more depth charge. 30 seconds after the meeting point they would fire the Y-gun (which would be two depth charges per Chaser), and drop a final set of depth charges 45 seconds after passing the meeting point. Based on signs of the sub following the first attack, the Unit would turn and mount a home attack; then turn, stop and listen. While listening, one or more of the Chasers would be positioned bows-on to the probable location of the sub in order to attack with guns, if necessary, or to drop additional depth charges, should the sub surface. In cases where a sub was injured, the attack could be modified to the “Attack in Sequence” formation, in which one of the wing Chasers would be positioned 400 yards behind (and still to the right of) the Leader and the other 800 yards behind (and still to the left of) the leader. Upon seeing detonations from Chasers ahead, a Chaser was to look for the sub and sheer in promptly if they saw the sub.

Post-Attack. In the event that a sub was sunk, the Unit was to take accurate location readings and attempt to recover evidence of the sinking. “A few survivors is the best evidence,” they were advised. If they were unable to confirm the sinking, they were to remain in the vicinity for at least 24 hours, listening and in some cases dropping additional depth charges, all the while watching for oil or rising air. The following memory device was included in the document: S eek the sub always U tilize all information B e ready for every contingency C hase by sixty degree method H unt according to visibility A ttack en masse S ecure some evidence E xcercise patience R eport all useful information D evelop initiative O rganize for every contingency C o-operate in all things T hink alike and signal little R emember your guns I dentify suspicious sounds N ever assume the defensive E xcell in Sub Chaser spirit