Randomizer

Just opinion...

Leaving aside the Allied technology that hamstrung the U-Boat arm, codebreaking, centimetric radar, airborne surface search radar and blah, blah, blah I think that a root cause of why the much smaller American submarine service succeeded where the KM failed was largely geographical.

All Japanese shipping bringing raw materials to the Homeland needed to pass through one or more maritime choke points. These included the Luzon Straights, Makassar Straights, Taiwan Straights, the entrances to the South China Sea and Yellow Sea to identify just a few.

Once COMSUBPAC started patrolling these areas (and others) the losses to Japanese shipping quickly assumed catastophic proportions particularly since this also corresponded to the final fixes for the Mk14 torpedo.

Convoys operating in the North Atlantic had room to manouever so the U-Boats needed to actively hunt and rely on HF radio instructions from BdU and with the Allies reading Enigma, open ocean operations were doomed to fail. Also Allied ASW was sophisticated enough from the start that U-Boats could not dominate such Allied choke points like the North and South Channels to the Irish Sea for any length of time.

Radar equipped Fleet Boats could park themselves for weeks near a choke point and their prey would come to them with far greater frequency than in the Atlantic. Without tankers, supply ships or an HF radio leash to worry about American boats had a degree of autonomy that only the Type IX's on distant stations had. Evidence that this is so lies in the success of many Type IX aces who did so well often stalking choke points far from home. Also a purely offensive IJN mindset which inhibited the creation of an effective escort force and poor technological base prevented Imperial ASW forces from even coming close to decimating the American submarine forces during the torpedo crisis and while the USN was still searching for an effective anti-shipping doctrine.

I'll shut up now.