1. Three L and nine H-class.
2. They had never achieved their designed diving depth of 500 feet and in spite of strengthening had had to have it reduced to 300 feet. Their engines had proved unreliable at high power and had had to be de-rated to 80% power reducing their surface speed from 18 to 16 knots. The design embodied oil fuel tanks external to the pressure
hull and these tended to leak. The tanks were originally riveted but were all replaced with welded joints which largely solved the problem. Nevertheless these submarines, although considered fully operational, were not popular and were thought to be too large and unhandy especially for home waters and the Mediterranean.
3. Their engines originally gave 22~ knots but these submarines too had to be de-rated and their speed fell to little more than 18 knots. Their designed diving depth was 300 feet but they failed to achieve this and it had to be reduced to 200 feet. They were therefore not fast enough to keep up with the fleet and could only be used as large and
unhandy patrol submarines.
4. The first of these, Porpoise, was similar to the P-class but the rest were a complete re-design of the same size but with internal oil fuel tanks.
5. See Appendix 1.
6. The O, P and R-classes also had two stern tubes and the new T-class had, in addition to the six internal tubes forward, another four external bow tubes making a salvo of ten torpedoes altogether.
7. H32 was the only one of the class to have an asdic set. This was the Type 113 that had the dome on top of the pressure hull and could only be used submerged. In the majority of new construction submarines, a type 118 was fitted. This had a retractable dome in the keel that allowed the set to be used both on the surface and submerged
but not with the submarine lying on the bottom. The latest T and U-class submarines had a new fixed set, the type 129, which was mounted at the forward end of the keel and this could also be used when bottomed.
8. There were two periscopes, one with a small diameter top for attacking and the other, rather larger in diameter, for lookout. The attack periscope was monocular and did not magnify. The lookout periscope was, in new construction submarines, binocular and also bifocal (that is it could be switched from no magnification to the magnification of a pair of binoculars). They also had a ‘sky search’ which allowed elevation and included a device to measure the angle sub-
tended by the masthead to the waterline of a target from which its range could be found. The length of the periscopes, which dictated the depth of the submarine at periscope depth varied from 40 feet in the large submarines to 25 feet in the H-class. Large submarines could therefore keep periscope watch in rougher weather than small submarines.
9. Using asdic actively by ‘pinging’ gave away the submarine’s position and this was why it was seldom used. If an enemy submarine was seen to submerge, ‘pinging’ could be resorted to so as to try to locate it but the chance of hitting it with torpedoes was remote because the bearing was not very accurate and there was no way to find the depth.
10. This was because of the ‘water noises’, which were particularly bad when on the surface and because of the noise of the submarine’s own diesel engines. Nevertheless submarines normally kept asdic watch on the surface at night.
11. The submarine had to be manoeuvred so that the enemy passed at a distance between 500 and 5000 yards. The torpedoes ran straight out of the tubes and so the submarine had to be pointed in the direction they were to go. The torpedoes had to be fired at the exact moment the sights came on and there was no chance of a second shot. With internal torpedo tubes of new construction submarines the torpedoes could also be set to turn ninety degrees left or right on firing, but this was a complicated and inaccurate method only used if a straight shot could not be made.
12. More for the larger faster submarines, less for the smaller slower ones.