A single 20mm cannon from a PT boat probably not that dangerious... however...
http://en.wikipedia.org/wiki/Westland_Whirlwind_(fighter)
That thing was flying with 4 20mm cannon in late 1940-42. hehe a shame it hasnt made it into GWX however with only 116 I can see why. It was used in a few coastal patrols and the squadron operating them was credited several E-Boats.
Nasty wee beast - 4 20mm cannon could do a fair number on a Sub. Its slightly faster than a Hurrican, and considerably better armed compared to all other British Figthers of 1940-41
anywho... somewhat off topic...