The rake rule is a real surrender rule for me. The one you put in there is for players to abuse of the rule, or because they realized their mistake in joining the game. If you remove the option altogether, then it becomes an automover at worst and we can sink it.
In conclusion, maybe you should remove the option totally. You cannot surrender. Or else, we can sink it and we get the regular points or we do not sink it and get twice the points as a captured ship at the end. Hum, this is good too actually, but complex again.
PS: Normally, a country would sink its own ship instead of risking giving it to the other side. So surrendering is almost like sinking your own ship.
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