Regarding the explosion of the Pyro, my understanding is that the ship had sustained enough structural damage (hit points) and just needed another 1-2 88mm rounds on the right spots. Since they were not handy, they could be replaced with some 20mm hits.
The Pyro is definitly a "sensitive" ship as all ammo ships in real life. But it can sustain 2-3 torpedos, if hit on the wrong spot. Donīt forget the HMS Barham, an cruiser with thick hull, which blew in pieces just because a torpedo hit the ammo magazine.
By the way, enjoy your DG while the merchants are unarmed in GWX. Because you donīt want to engage an armed merchant later in the war. You *will* be forced to submerge.
Von
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