Re: [electricboats] Time to re-power

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.

 

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.

 

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.

 

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.

 

Sorry for the ramble.

 

-MT

 

 

From: electricboats@groups.io [mailto:electricboats@groups.io] On Behalf Of Eric via groups.io
Sent: Friday, May 22, 2020 1:56 PM
To: electricboats@groups.io
Subject: Re: [electricboats] Time to re-power

 

And you asked about regen.  Here's something that I wrote back in 2011:

"Regen is a funny thing.  I have yet to hear any substantiated report of anyone hydro-generating more than 100W at 6 knots.  That's including regular cruising sailors with towed systems.   It's really a limitation of physics.  If you can sail consistantly at speeds greater than about 8kts (large racing monohulls, big catamarans, etc.) then regen becomes a reasonable power source.  But for boats like mine, under the best of conditions, regen might suffice to replenish house loads, or maybe not.  Regen will not put any significant charge into a traction battery bank on passages of less than a week.  It just that at 6kts, we're at the really flat part of the logorithmic power curve.   As it is, I have seen peak regen of around 1.4A (70W) while sailing around 6kts but the 5 minute average was probably closer to .7A (35W).   So any regen power that I get is just a gift, certainly nothing that I would count on.  If I wanted to optimize my regen, I believe that a towed log is most effective, but that would still not generate any significant power at the limited speed of my 30' ketch."  

These limitations still appear to be true, even with the dedicated, $10k, technologically advanced, Watt & Sea hydrogenerators.  If they can't generate 100W at 6kts with a specialized and optimized hydrogenration system, the power collected by for your re-purposed drive system will be less.

Fair winds and following seas,
Eric
1964 Cheoy Lee Bermuda 30, 5.5kW drive, 8kWh LiFePO4 battery bank
Marina del Rey, Ca