my assumption would be that PT boats were made so powerful in GWX because they lack the ability in SH3 to fire torpedoes.
In real life the PT Boat would have rushed in on you and launched a salvo of 2 torpedoes... which by all accounts probably would have obliterated your u-boat.
Also... was he firing at you with every gun on deck? if so he was likely hitting you with the 20mm Oerlikon Cannon. if later in the war it may have been a 2 barrel 40mm cannon.
look here @ 20mm
look here @ 40mm
(between 1:08 and 4:00)
given the above is a vehicle mounted example, however these ARE 20 and 40 mm projectiles, and both were mounted on PT boats in various examples.
I can promise you that either of these rounds would poke a considerable hole in a submarine.
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