Sealane Interdiction

SLI is the least popular doctrine, with Germany as its sole worshiper. It emphasises convoy raiding with U-boats, basically weakening the naval might of an enemy you're never going to surpass. It is the shortest tree out of the three, ending in 1942 and gaining most of its bonuses by 1940, when it even outshines the other two doctrines for a brief period. SLI offers the best morale, the best SS positioning, and the best convoy raiding efficiency.

A quick note about raiding and U-boats: Raiding won't make Britain run out of resources because of the pre-war buildup. It's more than capable of stopping supply convoys though, thereby hampering their ability to wage land wars overseas. DD seriously crippled the U-boat's sea attack rating, introduced sonar and ASW upgrades for ships, and added the U-boatkiller CVL into the mix. U-boats are still the best option for convoy raiding, but they will not destroy the enemy fleet like in HoI2. You should abandon this tree if you're going for naval supremacy.