Eric,
Myles, your numbers look good. I have noticed that most prop manufacturers use a 50-55% "slip" factor in determining the proper pitch for displacement auxilliary sailboats, so your 65-70% prop efficiency may be a bit optimistic for many of our conversions.
One problem in tuning a prop to maximize regen will decrease it's drive efficiency. Since I'm personally more interested in drive efficiency to maximize my powered range, then I'm will to accept less regen. You have clearly demonstrated why regen is limited due to the different water and prop speeds, so adding 20% to regen efficiency could make a small difference, but if came at the cost of 20% drive efficiency, it looks like a losing proposition to me. Others may think differently...
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "Myles Twete" <matwete@...> wrote:
>
> Probably the best you could do is get the same prop and conversion
> efficiency as your system does in delivering electrical energy into boat
> kinetic energy. So if you happen to have a damn good prop that gets 65-70%
> propulsion efficiency and assuming the other electric-mechanical efficiency
> is a decent 80%, we see perhaps 50% energy loss in the electric-to-kinetic
> energy conversion.
>
> Reversing this, you'd similarly expect AT BEST, a 50% return energy
> efficiency of water energy delivered to the propeller in a generation mode
> to your battery bank. Now, as a drive, the prop sends out a water stream
> that is significantly faster than the free flow under the boat. Therefore,
> cruising at say, 5knots, the water flowing thru the prop might be 8knots.
> Since the power is proportional to the speed cubed, the prop driving 10knots
> of current will transfer substantially less power than if it is "driven" by
> the water with the 5knots of water stream current in regen (assuming
> efficiencies were the same both ways, which they aren't).
>
>
>
> So say that it takes 2kw from your batteries to cruise at 5kts.
>
> And say that the water shoots out thru the prop at nom. 10kts to do this.
>
> It's arguable that the best you could get in regen at 5kts with the same
> prop is about 2kw*(5/10)^1/3, or 125watts.
>
> This is optimistic depending on the prop and the boat.
>
> But with regen we can take the boat largely out of the equation because it's
> just a matter of stream flow and prop and prop loading. Someone might want
> to explore this for a few typical props used on your sailboats.
>
>
> -Myles Twete
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