A very good design was produced, and some of the later patrol boats were converted and called “P Q’s.”  These vessels had the appearance of small merchant ships at a cursory glance.  They would not, however, stand close examination owing, again, to their fine lines, but being better sea boats than the “P’s,” by reason of their greater freeboard, the design was continued, and they met with considerable success against submarines (especially in the Irish Sea) by ramming and depth charge tactics, the submarines when submerged probably not realizing when observing the “P Q.’s” through a periscope the speed of which they were capable.

During 1917, when the unrestricted submarine warfare was in progress, many of the decoy vessels were fitted with torpedo tubes, either above water or submerged, since, as the submarine commanders became more wary, they showed great dislike to coming to the surface sufficiently close to merchant ships to admit of the gun armament being used with certainty of success.  A torpedo, on the other hand, could, of course, be used effectively against a submarine whilst still submerged.  The use also became general of casks or cargoes of wood to give additional flotation to decoy ships after being torpedoed, so as to prolong their life in case the submarine should close near enough to allow of effective gunfire.

Another ruse adopted was that of changing the disguise of a decoy ship during the night, so that she could not be identified by a submarine which had previously made an attack upon her.  In all cases of disguise or of changing disguise it was essential that the decoy ship should assume the identity of some class of vessel likely to be met with in the particular area in which she was working, and obviously the courses steered were chosen with that object in view.

Again, since for success it was essential to induce the submarine to come within close range so that the decoy ship’s gunfire should be immediately effective, it was necessary that her disguise should stand the closest possible examination through the periscope of a submarine.  German submarine commanders, after a short experience of decoy ships, were most careful not to bring their vessels to the surface in proximity to craft that were apparently merchant ships until they had subjected them to the sharpest scrutiny at short range through the periscope, and the usual practice of an experienced submarine commander was to steer round the ship, keeping submerged all the time.

Not only was it essential that there should be no sign of an armament in the decoy ship, or a man-of-war-like appearance in any respect, but when the “panic” signal was made to lead the submarine commander to think that his attack had succeeded, precautions had to be taken against the presence of more than the ordinary number of men in the boats lowered and sent away with the supposed whole ship’s company; also the sight of any men left on board would at once betray the real character of the decoy ship and result in the disappearance of the submarine and the probable sinking of the disguised craft by torpedo fire.