Originally Posted by Medicine Laker
How cold does it really need to get before this is a concern? Just because it gets down to 32 for an hour overnight should not be a problem, but where is the line?
Yea, I've thought about this question a few times. My rationale: If the boat is on the water, the lake will act as a 'heat island' and not get as cold as the air over land. However, it'll also be at the low point, so offset some by the cooler air gravitating toward the low spot. When temps dip slightly below freezing - especially if was a (reasonably) warm day - I have no concerns. Beyond that, I've occasionally taken my chances.
If you are concerned, you may want to try a small light bulb. An incandescent light bulb is basically an electric heater that emits a small bit of light energy. Placing a well-protected light (i.e. shop light, or similar) under the engine will certainly keep it warm enough on most spring nights in most locations. Notice the wiggle words, such that I didn't give a definitive answer? ;-)