Not to change the subject on freewheeling but..

[exoduse35]

I have wondered if it would be possible or worthwhile to set up a power take off system off the prop shaft to run an alternator. I realize that it would take some work and require both a clutch set up and gearing to step up the shaft speed. The up side however is that when on an extended cruise electricity is the key to comfort and is always at a premium so every bit helps. If you can drag a prop off the back why not use the same idea on a prop that you are either turning or dragging anyway? So give it some thought and tell me why I am an idiot I know I must be or everyone would have beaten me to the idea! Edd

 

[Loren Beach]

Turning the prop, making amps

I watched this boat being constructed in Portland, but do not know much more about it than is published on the web.
You might want to contact the owner about how well his electrical generation scheme is functioning now that he has used it for a while.
http://huberman.org/prestissimo/

Loren

 

[exoduse35]

Loren, I looked as you recommended. He is running an electric propulsion system and basically running it back wards to generate power. For those confused: a generator and motor are the same thing, IE if you spin a motor it will create electricity and if you energize a generator it will turn. The system he is using confirms to me that there is enough power there to drive an alternator if you can harness it. So the $64 question is why do people spend thousands on wind generators and solar ect. instead of simply adding a pulley to the prop shaft and using it to drive an alternator. It seems to me that a boat with a prop in the water is already paying a price in boat speed, and cluttering the stern up with a wind generator or dragging another prop will only compound the penalty. But on the other hand smarter people than me have pondered this problem and shied away from harnessing this penalty and putting it to a good use so I wonder why. Edd

 

[Mark F]

Hi Edd,

I have an electric inboard and have seen 4 amps (@48volts) being generated while sailing at 6+ knots. I think someone over on the Yahoo electricboats site referenced a setup like what you are talking about. If you were making long passages it could pay off.

 

[exoduse35]

Thanks Mark, As I cruise @ 7.5-8 knots and probably have more prop than you, it should be feasible to run a 40 amp alternator off the prop shaft which is what my A4 came with and is comparable to a noisy Airex unit. On a trip such as Cal. to Hawaii it could be done without the engine, solar, or a wind generator, and still have enough electricity to run the radio, electronics, and have ice when you get there! So why does nobody take advantage of it?

 

[Loren Beach]

Thanks Mark, As I cruise @ 7.5-8 knots and probably have more prop than you, it should be feasible to run a 40 amp alternator off the prop shaft which is what my A4 came with and is comparable to a noisy Airex unit. On a trip such as Cal. to Hawaii it could be done without the engine, solar, or a wind generator, and still have enough electricity to run the radio, electronics, and have ice when you get there! So why does nobody take advantage of it?

Edd, I sure don't know the whole answer... but do know a number of cruisers that have done ocean crossings in the last 20 years. Most of their time (on an annual basis), overall, was not spent at sea but rather at anchor or sometimes at a dock. So towing a water-generator would not have helped them much overall. They all used engine charging, solar cells, or a wind-powered alternator.
Note that there have always been great differences in ambient noise between different designs and makes of wind generation devices. I have been around some that were irritatingly loud and others that were much much quieter.

And then there is sailing efficiency. Towing a prop-generation setup will probably take a knot off your sailing speed on boats the size of ours. What with the majority of time being spent in light and medium airs, and the boat being loaded down with supplies already, I suspect that you want all the sailing speed you can muster.

Opinions rendered on the hour, deposit one cent...


Loren

 

[exoduse35]

Loren, that is exactly my point. You are already paying the penalty for the prop under the hull so why not mitigate it instead of adding to it with the wind generator or towing a second prop for generation? I realize that at anchor you are back to shore power or wind & solar, but it seems that instead of motoring to top off the batteries as many do it is much more efficient to just use the drag of the prop and save the fuel. A wind generator could be removed while under way to save the windage and any solar panels would not need to be constantly adjusted to catch the rays. Many years ago when we were racing VWs one of the tricks we used was to gut the generator because without charging we could pick up about 4hp. An alternator has less moving mass so draws even less power to run. Therefore it seems that the added drag of the alternator would be minimal drain on boat speed and certainly less than any other method while underway. I Know there must be a downside somewhere, I just don't see it and that bothers me! Edd

 

[tadslc]

Maybe I'm missing something but I can't imagine a free-wheeling prop (that is normally used for our engines) rotating fast enough while sailing to come close to providing the rpms necessary to run any sort of alternator / generator.

 

[exoduse35]

Tad. the only requirements are that the shaft turns and that there is resistance through the water. Then the rotation of the shaft can be geared up as long as you do not overcome that resistance. An alternator will function well at less than a thousand RPM. At a shaft speed of 50 rpm and 2 4to1 gear sets you can achieve 800 RPM. 50x4=200 200x4=800 so as Long as there is enough power behind the rotation of the shaft to overcome the drag of the alternator it will work! Edd

 

[Guy Stevens]

There is nothing new under the sun

This has been tried over the years, including specifically and specially rewinding an alternator, and replacing the rotor with permanent magnets.

The results have always not been worth the hassle. Either a take off of the existing prop, or a separate prop shaft have been used. The seperate prop shaft was the worst, more holes in the boat, always more drag to slow the boat down etc.... Also once you start getting deep into this you find out that the idea shape of a prop to be pulled through the water to generate electricity is not at all the shape of the props that we use on boats.

I have seen a few boats go this route after having been sold on it. The costs added up quickly for this free power. All of them later told me that it was the biggest waste of money they had spent in preparation for cruising.

A better idea would be to design a pancake generator like some folks are doing for home made wind generators and try that... Of course that raises the cost of entry into the experiment. I still would not expect it to work out well.

A passage of say 1800 miles, the difference in the duration of the passage would be about 3 days... So the question becomes is the generation system going to generate enough amperage to give you an additional 3 days of electricity for every 12 you are out? What is the cost of this active system versus the cost of a passive system such as additional solar panels.

This is the same issue we face as a country right now. To increase generation of power, or to become more efficient at using it. This is an easier equation on our boats. Such as do you already have a wind vane steering system, that saves 100's of amp hours a day over an autopilot. Do you have CF or led lights everywhere? Is your refigeration as well insulated as it can be? These are much more likely to give a good ROI than more power generation.

YMMV

Guy

 

[exoduse35]

Thanks Guy. I know there must be a down side, I am not smart enough to come up with an original brilliant idea. So since it looks good in theory, I wondered if it was tried, and why it wouldn't work.
As for saving power in the first place, of course it is easier to save it than create it! But like the world in general those with the power have the power! More is always tempting, especially when it looks like you may already be paying for it. Edd