In this you have a problem.
The best that can happen is a damaged propeller, (systems damage doesn't seem to happen to AI, by the way) which will slow him down to 15 knots until repaired. (Which can take anywhere from 5 to 130 minutes or so, or seconds with quick repair.)
If the mission is to have a "do not sink" order, then you have a problem, though, as it's pretty much random what systems are damaged. Particular directions help the probability of damaging a particular system, but...
Essentially, you'll have to make sure it's a major win (and that the player feels like it) instead of a minor win for sinking it.
There is a possibility still though. Set a trigger such: When ffg is >50% damaged, and speed is over 5 knots, run a script that reduces it to 5 knots. Make sure the trigger refires after a few seconds.
I'm not positive on how to make it fire on over 5 knots right now, though. But even if you have to use a "negate trigger".
Or you could have it set to 0 knots and force the FFG player to use the small outboards... I forget what they're called...
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