On my engine, if you leave it with the engine trimmed right in, it says that no water will be trapped in the engine and it will be frost-proof. Quite a lot of water runs out of the middle of the prop when you tilt it down and trim it right in.
The book for mine also says that if leaving the boat in the water, you should leave the engine down with the leg in the water in freezing conditions. I'm not sure whether this lets all the water drain down to sea level, or whether the idea is that convection will carry the colder (nearly freezing) water from the engine, down to the sea, and replace it with slightly warmer water thus preventing freezing...
A Boat is a hole in the water, surrounded by fibreglass, into which you throw money...
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