Here is interesting read:
http://uboat.net/allies/technical/torpedo_problems.htm
"The failure to note the defect earlier was due to the BO's reluctance to test the torpedoes in peacetime. Fish cost $10,000 a piece, and it was considered prodigious to expend them on target practice. In comparison, a similar German torpedo cost RM 25,000-but the Germans never shied away from such "peacetime expenditures."
So, maybe a German Uboat captain would waste two torpedoes on a small target. But, our point is that in the event that one did actually detonate the explosion of one torpedo would most likely destroy such a small craft. Most of the articles talk about Murphey's Law in the terms that the detonators never went off or the torpedo would get stuck in the mud. I've never read an article about the design of the torpedo having problems with the actual explosion once it went off.