In order properly to appreciate the difficulties of the Q-ships, it is necessary to understand something of the possibilities and limitations of the U-boats. No one could hope to be successful with his Q-ship unless he realized what the submarine could not do, and how he could attack the U-boat in her weakest feature. If the submarine’s greatest capability lay in the power of rendering herself invisible, her greatest weakness consisted in remaining thus submerged for a comparatively short time. On the surface she could do about 16 knots; submerged, her best speed was about 10 knots. As the heart is the vital portion of the human anatomy, so the battery was the vital part of the submarine’s invisibility. At the end of a couple of hours, at the most, it was as essential for her to rise to the surface, open her hatches, and charge her batteries as it is for a whale or a porpoise to come up and breathe. It was the aim, then, of all anti-submarine craft to use every endeavour to keep the U-boat submerged as long as possible. Those Q-ships who could steam at 10 knots and over had a good chance then of following the submarine’s submerged wake and despatching her with depth charges. If she elected not to dive, there was nothing for it but to tempt her within range and bearing of your guns and then shell her. To ram was an almost impossible task, though more than one submarine was in this way destroyed. The difficulty of anti-submarine warfare was increased when the enemy became so wary that he preferred to remain shelling the ship at long range, and this led to our Q-ships having to be armed with at least one 4-inch against his 4·1-inch gun. The famous Arnauld de la Périère, who, in spite of his semi-French ancestry, was the ablest German submarine captain in the Mediterranean, was especially devoted to this form of tactics. Most of the German submarines were double-hulled, the space between the outer and inner hulls being occupied by water ballast and oil fuel. The conning-tower was literally a superstructure imposed over the hull, and not an essential part of the ship. That is why, as we have already seen, the Q-ship could shell holes into the tower and yet the U-boat was not destroyed. Similarly, a shell would often pierce the outer hull and do no very serious damage other than causing a certain amount of oil to escape. Only those who have been in British and German submarines, and have seen a submarine under construction, realize what a strong craft she actually is.