I agree with Jochen, especially regarding self-repair. Actually, many repairs were conducted while underway, depending on how significant the damage. Also, the American PT boats were twin-engine, so damage models should reflect the possibility of reduced speed from single engine operations.
Finally, while they were built from wood (the American ones, at any rate; not necessarily true of German E-boats, as I recall), they were able to absorb a surprising amount of damage. True, a bomb or naval-calibre cannon shell would likely destroy one completely, unless it failed to detonate (which is a possibility; contact fuses require significant resistance to arm and detonate, which plywood does not offer). However, machinegun and aircraft-calibre cannon would often to no more than punch some holes in a PT boat. So modeling should include the provision for minor flooding, which could be repaired over time (if not too rapid).
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Sabre, a.k.a. Rojo
(S-2, The Buccaneers)