If you pull up to a resupply ship and it is armed and you attack it... it will attack you back until such time that it sinks.
because of the nature of the supply ship spawning at the start of each patrol, if you sink it, then start a new patrol, the supply ship will be respawned.
this is the same reason you can sink the HMS hood 10 times during the war if you are lucky enough.
unfortunately this detracts from realism of course... but is is the best system we currently have.
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