Worried about torpedo impact sensitivity

[Barkhorn1x]

Starting in mid-1943, the gradual introduction of the Mark XVIII electric torpedo into the theater brought substantial relief from the persistent torpedo shortages of the early war years. Although slower than the Mark XIV by 10 to 15 knots and somewhat limited in range, the Mark XVIII left no tell-tale wake that could give away a submarine's position, and it was much easier to manufacture in quantity. By the middle of 1944, when all their teething problems had been solved, Mark XVIII torpedoes constituted three-quarters of the standard patrol load-out.

From here:

http://www.chinfo.navy.mil/navpalib/...12/rising.html