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My boat idea was a water to water heat exchanger, rather than saltwater through the engine. My Bayliner 5.7 liter engine has water to water heat exchange via conduits in the outdrive, and sits in a deep V fiberglass hull.
Methinks better to have something like a Subaru flat 6 engine, with its coolant circulating against the inner surface of an aluminum hull. Being an superb heat conductor, the aluminum would dissipate the heat well beyond the bounds of the heat exchanger jacket, making the entire hull one big heat exchanger. (The water around here is ~50 degrees year 'round.) That way, salt stays on the outside of the boat, coolant stays inside, and we hopefully have no corrosion problems. Electrolysis being what it is, this may be more complicated.
As for aircraft use, again using an aluminum skin as heat exchanger, the best route would be oil coolant, as used in BMW oilhead boxer motorcycle engines. (I have two. Oil is lubricant and coolant, and uses a small ram-air radiator in the nose of the fairing, accounting for ~60% of heat dissipation, the balance being from air cooling of the cylinders by the slipstream.) Cooling drag being a large portion of aerodynamic drag, use of the aircraft skin itself as heat exchanger could be a source of major aero improvement.
In that vein, and back to the original topic of this thread, a car skin would make a great radiator, except for the problems already noted with safety, etc.. An aluminum belly pan could serve this purpose and help aerodynamics as well. Subject to puncture, though.
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