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Old 26-11-2019, 04:22   #1
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Question Could one use boat propeller to drive el generator?

When my boat is under sail, my (fixed) propeller is rotating (I did read the fascinating thread somewhere here whether to put it into gear, reverse or leave in neutral - I leave mine in neutral). However, I was wondering whether I could not jig up an electric generator to it. Would it un-balance the shaft? Why is it not done? (like the dynamos in the old fashioned bikes)
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Old 26-11-2019, 05:21   #2
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Re: Could one use boat propeller to drive el generator?

"I was wondering whether I could not jig up an electric generator to it. Would it un-balance the shaft? Why is it not done? "

It could work in a heavy blow where the boat was near hull speed and you had excess energy to use.

A spinning prop absorbs massive energy , which is why multi engine propeller aircraft feather theirs.

To see the effect simply cut a piece of plywood the same diameter as your current prop rigged as a speed brake . When towed in the water the drag , and speed reduction will be ez to see.

Folks that do not care about the speed reduction can get alternators rewound to produce electric at very low RPM.
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Old 26-11-2019, 05:55   #3
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Re: Could one use boat propeller to drive el generator?

It sounds like y'all think this is a new idea; it is not.

Would be more widely implemented if the prop geometry optimising propulsion was anything like that for hydro generation.

https://www.google.com/search?q=site...tion+propellor
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Old 26-11-2019, 06:04   #4
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Re: Could one use boat propeller to drive el generator?

Quote:
Originally Posted by suiramor View Post
When my boat is under sail, my (fixed) propeller is rotating (I did read the fascinating thread somewhere here whether to put it into gear, reverse or leave in neutral - I leave mine in neutral). However, I was wondering whether I could not jig up an electric generator to it. Would it un-balance the shaft? Why is it not done? (like the dynamos in the old fashioned bikes)
you can do it with a standard alternator,just need to get spindle rpm to above 1200 rpm@ 4 knots
this requires a pulley on the prop shaft of about 30-35cm,with a 8cm pulley on the alternator.

ideally the prop shaft has an intermediate thrust bearing behind the input flange,with the alternator mounted to the hull or the pulley is mounted on the output flange of the gearbox and alternator is attached to the engine.

wiring as a normal self regulating alternator,with an on/ off switch and light on the field wire
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Old 26-11-2019, 07:22   #5
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Re: Could one use boat propeller to drive el generator?

Despite what you would think, the propeller will be less drag on your boat if it is locked from rotating when not in use. On our boat each prop had a disk brake applied with a spring and a hydraulic cylinder operated by an oil pressure line from the engine to release it.
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Old 26-11-2019, 09:35   #6
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Re: Could one use boat propeller to drive el generator?

A prop large enough to produce enough electricity in moderate conditions to make it worthwhile will create significant drag, maybe more than a knot of boat speed. Depends on how happy you'll be to go slow and not generate a lot of energy.
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Old 26-11-2019, 10:14   #7
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Re: Could one use boat propeller to drive el generator?

Yes this was at one time not uncommon. It is rearly seen now mainly, I think, due to changes in configuation. To work well you need a large prop so on a mod/heavy displacement mono with a large slow turning prop it can be quite practical. On a shallow flat bottom hull with a small prop it is unlikely to be possible or practical. In order to get the necessary 3:1+ pully ratio you need quite a bit of space under the shaft which you only get with a deep bilge. I have set one up on my boat (15" prop") and while it worked there where quite a few problems. Getting the large pully securly fixed on the shaft was an issue and as everything had to be in the builge under the floorboards there were reliability issues due to damp. I took it out again as it proved morehassal than it was worth.

The other option is a towed generator. These do look to be quite a good idea if your boat speed is high enough.

The down side as others have said is the drag but also that it only works when underway at 5kn+ so quite a bit of investment for limited use.
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Old 26-11-2019, 10:50   #8
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Re: Could one use boat propeller to drive el generator?

Re Towed Generators If you are only looking for a small amount of current a towed prop is cheap and practical. In Trade Wind sailing most 35foot plus boats can maintain 5 plus knots towing a 10 inch or so used Outboard Prop with a piece of pipe or broom handle thru it and good stiff -lay line less than 100ft to a small generator/motor, That part is easy Getting the prop back is not obvious because it is rotating still. So you need a good sturdy Plastic or metal Funnel
with its own retaining lanyard . You tie this to the generator mounting , When you want to Haul in you release the funnel and it slides down the revolving line wide end first and it stops at the Prop which then stops rotating. Now it is easy to haul it in coil it up and hang it on the "Pushpit".
If you leave it out at night Sharks will sometimes attack the prop.
Remember to put the funnel on when first rigging it up you can't do it later and the birds nest the prop will weave for you is a sight to behold: Mike Pope
PS. Figure the speed you can maintain and the RPM min. you need and then you may be able to select an old prop of about the right pitch
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Old 26-11-2019, 10:50   #9
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Re: Could one use boat propeller to drive el generator?

On my old boat, a Morgan Out Island 41, I had a wind generator that had an option to convert it to a towed generator. I never did it. Talked to some who used it that way on long cruises. The only drawback was the loss of speed
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Old 26-11-2019, 10:57   #10
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Re: Could one use boat propeller to drive el generator?

Unfortunately many transmissions are damaged from having the screw free wheel under sail. In these transmissions the input drive shaft is turning a hydraulic gear pump located in the forward end of the transmission. Even when the transmission is in natural the running engine turns this pump and this pump moves the fluid through the bearings inside the unit. When the engine is not running the pump stops, so if your transmission is turning as results of thrust from the screw (propeller) there are vital bearings not being lubrication. If you have this type of transmission you must secure the shaft from turning, so BEFORE you think itís ok make sure you contact a knowledgeable individual or you might find yourself with premature transmission wear.
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Old 26-11-2019, 11:48   #11
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Re: Could one use boat propeller to drive el generator?

People have done it ... more years ago. I think solar is the more popular option now.
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Old 26-11-2019, 11:59   #12
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Re: Could one use boat propeller to drive el generator?

"Despite what you would think, the propeller will be less drag on your boat if it is locked from rotating when not in use."


Common misunderstanding. May be required to stop turning related to the hydraulic tranny issue as above.


Spinning prop is less drag, otherwise there would be no "excess energy" to harvest by slowing it down with a shaft mounted alternator. It would not spin spontaneously when towed if it had less drag when stopped.
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Old 26-11-2019, 12:21   #13
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Re: Could one use boat propeller to drive el generator?

Quote:
Originally Posted by YANDINA View Post
Despite what you would think, the propeller will be less drag on your boat if it is locked from rotating when not in use.

Not this false statement again after the number of times it has been de-bunked here! Theory and plenty of empirical evidence both show you to be wrong. Last couple of times here:


Prop Freewheeling or dragged?


Free prop or locked prop?
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Old 26-11-2019, 13:19   #14
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Re: Could one use boat propeller to drive el generator?

Quote:
Originally Posted by suiramor View Post
When my boat is under sail, my (fixed) propeller is rotating (I did read the fascinating thread somewhere here whether to put it into gear, reverse or leave in neutral - I leave mine in neutral). However, I was wondering whether I could not jig up an electric generator to it. Would it un-balance the shaft? Why is it not done? (like the dynamos in the old fashioned bikes)
If you really need more electricity, then a folding prop (less drag) and a small solar setup would seem to make more sense. The dragging prop is usually not worth the small amount of energy produced versus speed loss.
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Old 26-11-2019, 14:11   #15
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Re: Could one use boat propeller to drive el generator?

Oceanvolt does it as a standard feeature

https://oceanvolt.com/solutions/hydro-generator/

Quote:
DOES HYDRO GENERATION CREATE DRAG?
Hydro generation creates drag of only 0.1 of a knot at boat speed of 7.0 knots - barely noticeable.
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