Wondering if this will work?
Not that there's already a shortage of things to do around here, but because I think most engineers got down on their knees to get their degrees, I'm always trying to figger out better ways to do things. Every boat has problem areas, and one place I hear about all the time is the raw water heat exchanger circuit. It's a bad concept IMO, and far, far more likely to give trouble than the freshwater part of the system.
Now given that (at least in this part of the world) we sail in damned cold water, water that is very reactive and tends to eat stuff, why are we sucking it aboard, running it through our trannys, engine and exhausts?
I'm wondering about taking the coolant from the freshwater circuit and running it through a section of looped hose epoxied against the inner hull. Glass is a reasonable thermal conductor and the water is maybe 8-9 degrees C, so the big question would be the surface area needed to conduct away heat to keep it below the boiling point.
Basically use the hull as a heat exchanger.
The thermal capacity of water is quite high, so I don't think you would need an outrageous length of hose.
I need to do some research on the surface area required, and the effects of heat on fibreglass. Anyone ever try something like this?