[Marmot]

"Anything you tow that takes power to turn it will be costing you fuel,..."

I probably should have just wrote that to begin with instead of trying to lead him make the connection on his own.

Maybe there are other captains who don't understand that the boat is moving, not the water. If there is a power "recovery" device intended to extract energy from the passage of the vessel through the water, it is impossible for it to produce net energy. It will always cause drag and increase energy consumption far beyond what it returns. Energy is lost at every conversion, otherwise perpetual motion would be a fact of life.

Wind turbines create drag until the speed of the wind exceeds the relative wind created by the vessel's passage. Does the amount of time that net power is available exceed the cost of fuel to haul the weight and drag of the wind turbine around? Probably not, unless the thing is mounted on a little sailboat where alternative power sources are heavy, expensive, noisy, and smelly.

Dragging a generator around behind a sailboat is a trade-off. The power extracted slows the boat but the wind power to drive the boat is free. Slowing the boat is usually considered a fair trade for the noise and cost of running a diesel generator. The amount of power is very low but so is the power consumption of the little boats that use that system and those things normally just charge the house battery, not supply consumers directly.

The bottom line is there is no free lunch in the energy business. There are ways to utilize the waste heat from diesels. You can use it to run evaporators, generate steam for heating or to drive a generator. But given the cost of these applications and the operating pattern of most yachts, the equipment would probably rot before it paid for its own carriage. The only practical use for heat recovery I can see as worthwhile on a yacht is the evaporator.

I see the next decade of development ripe pickings for those who sell "hydrogen boosters" and energy recovery devices to the technically challenged. Real advances will be made and efficiency improved but it will be incremental, expensive, and for the most part difficult to justify for reasons other than a legitimate environmental concern or to meet increasingly stringent regulations.