[Dead Mans Hand]

The U-boats were pretty darn succesful up until the Enigma machines started to be compromised at Blenchly Park. The first computer, ENIAC (I do believe was the name) was employed there to crack codes, not the most effective - but anyways yeah, until they then there was no warning for a U-Boat attack and yes that's a psychological war, not to mention the fact that if in a convoy of twenty ships four a sunk, that's only 75% of ships making it to the destination. Also before the US entered, the primary concern was we would stop sending supplies due to losses (and that was the German objective, not stopping every ship in the Atlantic, but making it damaging to us so that we'd stop shipping.)