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Old 05-12-2019, 22:31   #31
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Re: Could one use boat propeller to drive el generator?

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Keep in mind that the propeller shaft is kept in position with a THRUST bearing, usually in the transmission, that the shaft pushes on to move the boat.

When rotating while the transmission is in neutral you will be PULLING on the bearings which are not intended for this mode and may be subject to excessive wear.

WARNING, IN particular, when the engine is not driving the transmission there is no forced lubrication which can result in excessive deterioration of the rotating parts.
When rotating while the transmission is in neutral you will be PULLING on the bearings which are not intended for this mode and may be subject to excessive wear."


This of course would be far greater than using reverse , or not."The load imposed by a slowly spinning prop would be insignificant.

WARNING, IN particular, when the engine is not driving the transmission there is no forced lubrication which can result in excessive deterioration of the rotating parts.[/QUOTE]


True for a HYDRAULIC gearbox however not so for the many and varied mechanical gearboxes out there.
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Old 06-12-2019, 04:21   #32
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Re: Could one use boat propeller to drive el generator?

There are thrust bearings for reverse gear as well. Force to drive an alternator is much less than to drive the whole boat in reverse.

The flip down outboard solution is best. More readily optimized to serve its purpose.

I have occasionally pondered trying to modify a small dead outboard to do so, pull off power head and replace it with an alternator. Probably not much friction loss in the lower gearbox, but I don't know how much power it might generate. My old Cape Dory 26 had a permanently immersed prop and it would freewheel all the time. A bit noisy. Lots of drag.
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Old 06-12-2019, 12:14   #33
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Re: Could one use boat propeller to drive el generator?

Thank you all. Very interesting comments. I recall seeing somewhere (Beneteau?) recommendation to put the gear in reverse when under sail. (I don't care much about speed anyway). Hopefully that will address potential issues with lubrication and stress and the gearbox. I will explore external prop for power generation - not much sunshine where I sail. Thanks again!
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Old 06-12-2019, 21:11   #34
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Re: Could one use boat propeller to drive el generator?

...& as so often in the forum I am flabbergasted by the big discrepancy between the dogmatic opinions of know-nothings & the gingerly voiced opinions of experienced people who know what they are talking about...(I am refering among other things to the "missing thrust bearing for reverse"...& the need for great water tankage because of the passage duration under shaft generator)...
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Old 08-12-2019, 16:47   #35
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Re: Could one use boat propeller to drive el generator?

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...& as so often in the forum I am flabbergasted by the big discrepancy between the dogmatic opinions of know-nothings & the gingerly voiced opinions of experienced people who know what they are talking about...(I am refering among other things to the "missing thrust bearing for reverse"...& the need for great water tankage because of the passage duration under shaft generator)...
You are utterly correct of course, I can only speak for myself but I for one endeavour to be as polite as possible, (I know, it doesn't always work ) and allude to things like the reverse thrust bearing.
Perhaps I should be more to the point and call out the male bovine excrement when it is flung. , particularly by a marine service provider.!!
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Old 09-12-2019, 07:44   #36
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Re: Could one use boat propeller to drive el generator?

On our boat, the main thrust bearings are on the propeller shafts with a flexible coupling to the transmission so no thrust is transmitted to the transmission. The reverse thrust or tension appears to be provided by the ball bearing but it is not as substantial as the thrust bearing, presumably because it only gets used 0.1% of the time. A disk brake with spring loaded pads is released by an hydraulic cylinder connected to the transmission oil pump. The oil pump does not run if the shaft were to free wheel.
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Old 09-12-2019, 08:38   #37
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Re: Could one use boat propeller to drive el generator?

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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.
This is not the first time I've heard this, but it is not correct. A fixed prop is like dragging a bucket through the water.
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Old 09-12-2019, 08:52   #38
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Re: Could one use boat propeller to drive el generator?

Perhaps but dragging a rotating prop is like dragging 6 buckets behind you.

For example, take the free-wheeling propeller blades on a gyrocopter. They provided more lift and resistance by rotating and wouldn't work at all if locked.
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Old 09-12-2019, 09:07   #39
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Re: Could one use boat propeller to drive el generator?

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Perhaps but dragging a rotating prop is like dragging 6 buckets behind you.

For example, take the free-wheeling propeller blades on a gyrocopter. They provided more lift and resistance by rotating and wouldn't work at all if locked.
I'm sorry, I want to remain polite, but you are completely wrong. There is a MAJOR difference between the gyrocopter theory and the free wheeling sailboat prop, and here it is: The gyrocopter prop IS ENGAGED with the engine at all times, therefore, the drag of the pistons, etc. within the engine is exerted against the propeller, which in turn is slowing the speed of rotation of the prop and slowing the rate of descent of the craft. The sailboat prop IS NOT engaged with the engine.
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Old 09-12-2019, 09:09   #40
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Re: Could one use boat propeller to drive el generator?

As we were circumnavigating, we became good friends with an Austrian couple on a Nijad 39. René, the husband of the couple, was a terrific engineer and added a pulley onto his prop shaft and a small alternator mounted off to the side. It was a beautiful installation. René claimed that the prop shaft-driven alternator produced enough electricity to power his autopilot.

As for how much speed they lost, I really don’t think it was that significant. We both left the Fernando de Naronha (off the coast of Brazil) islands at the same time and they arrived in Tabago about 18 hours after we did, a passage of almost 2,000 miles.

Fair winds and calm seas.
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Old 09-12-2019, 09:15   #41
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Re: Could one use boat propeller to drive el generator?

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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.
No, no, no. Naval architects disproved this long ago.
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Old 09-12-2019, 10:07   #42
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Re: Could one use boat propeller to drive el generator?

If you want to prove that locked prop has more drag than freewheeled you can try with the outboard on your towed dinghy. Whilst towing, Leave the engine down with the outboard not in gear and it just spins the prop. Put it in gear and the outboard pops up.

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Old 09-12-2019, 10:13   #43
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Re: Could one use boat propeller to drive el generator?

I've had two purpose built hydro generators I suggest you look at DuoGen, which also has a wind propeller which is not very efficient, but works at anchor. I had a lot of solar but cloudy days or shading by sails interfere with it. The other concern with an outboard type arrangement is hitting debris in the water. DuoGen just lifts up, but also has a "fusible" link so there is no damage.

I first had a SwiTec, outboard style vertical shaft which was nearly impossible to raise and lower under way and eventually the shaft broke off after hitting something. Plus, their support was abyssmal. I do not recommend it.

The water generation side of DuoGen got me across the Pacific and powered through the night running the autopilot and refrigerator at 6-7 kt. You have a prop and alternator optimized for the purpose. Saved me a lot of generator fuel. The only drawback was the controller which burned out but was easily replaced. I also added a cheap Amazon/china ammeter which bluetoothed to my phone. You need to keep an eye on it because seaweed will collect and disable it.
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Old 09-12-2019, 11:43   #44
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Re: Could one use boat propeller to drive el generator?

As others have said, the prop creates much more drag when rotating that if lockin in gear, so this would be a very costly way to produce power...
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Old 09-12-2019, 12:10   #45
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Re: Could one use boat propeller to drive el generator?

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I'm sorry, I want to remain polite, but you are completely wrong. There is a MAJOR difference between the gyrocopter theory and the free wheeling sailboat prop, and here it is: The gyrocopter prop IS ENGAGED with the engine at all times, therefore, the drag of the pistons, etc. within the engine is exerted against the propeller, which in turn is slowing the speed of rotation of the prop and slowing the rate of descent of the craft. The sailboat prop IS NOT engaged with the engine.
Gyrocopter Definition
An autogyro (from Greek αὐτός and γύρος, "self-turning"), also known as a gyroplane or gyrocopter, is a type of rotorcraft that uses an unpowered rotor in free autorotation to develop lift. Forward thrust is provided independently, by an engine-driven propeller. While similar to ahelicopter rotor in appearance, the autogyro's rotor must have air flowing across the rotor disc to generate rotation, and the air flows upwards through the rotor disc rather than down.

In a gyrocopter the blades have to be rotating before it can use them as an airfoil to take off. Wind rushing past rotates the free-wheeling blades and increases rotation speed and drag until it can lift the gyrocopter. There is no drag or lift if the blade is not rotating.
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