Just a quick point---there are 2 ways regen doesn’t yield great results thru a prop on a boat:

(1) power ~ speed^3 and

(2) efficiency losses.

And it’s really speed of the fluid thru the prop that counts.? So as a propulsor, the prop is pushing water at greater than boat speed.

As a generator, the speed thru the prop is less than boat speed.

In the second case, you have (1) prop efficiency (at best 70%, typ. 55%), (2) drive efficiency (maybe 90%), (3) e-m conversion eff (84-94%eff at best) and electrical and electrochemical losses.? These losses add up to yield at best 60% efficiency.

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So, let’s say that Eric’s boat can achieve 60% efficiency from battery thru the prop --- then we’d expect about 3kw mechanical power delivered to the water.? And as a generator, at 100watts delivered, we’d expect about 160watts mechanical from the water.

The ratio: 3000/160 (about 18) must be explainable as the ratio of the propeller speeds or water speed thru propeller---and that’s a cubic.? Solving for the speed ratio, we get 2.62 --- or the water speed thru his prop at 6kts (as propulsor vs generator) is 2.6x greater.

This makes sense---probably the water expelled by the prop as a propulsor is 2x the speed (or more) of the boat.

And we’d certainly expect the water speed thru a prop in regen to be 1.3x slower than the boat speed.

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Now this is simple calcs and ignores details, but its intent is to explain why the big difference and where the losses are so you might be able to tackle them.? You might also realize that swinging a gargantuan prop (the biggest loss source) isn’t an option and that the other efficiency losses just don’t offer much opportunity.

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But this could make you think: Can I, for example, add/drag outrigger elements that direct more water thru the prop---particularly at anchor?? Would a prop nozzle help or hurt?? A giant deployable cone underwater?

Ultimately, you need water speed.

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Sorry for the ramble.

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-MT

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