Could one use boat propeller to drive el generator?

[double u]

Guys please don't get out the duel-pisols over this question!
The relative drag of freewheeling versus locked prop seems to depend on many things. While I know for sure, that on a helicopter the freewheeling prop has very much more resistance than a locked one (the procedure in case of engine failure in the air is to immediately put pitch to fine, airspeed should be 60-kmh or knots I don't remember. This I have from a professional pilot), a threeblader with say 50% swept surface area on a boat only in principle is the same thing as a 2blader on a helicopter.
Btw: a turbine driven helo has no mechanical connection of the rotor with the turbine, & as any resistance would slow the freewheeling rotor down I sincerely doubt that on a pisto-engined helo (is there such a thing&) in the case of an autorotation (thats what this procedure is caled in german, don't knkw the english term) the prop would remain linked to the engine, there would have to be a “cluth“ sort of mechanism. Perhaps a helo pilot could speak up here.
So to sum up: this could be either way, freewheeling or fixed having more resistance. I guess(!) that with the “right“ amount of resistance from whatever, gearbox friction, shaftgenerator, tight stuffing box, the freewheeling prop would have the greater drag.
The whole shaftgenerator question is on the way to being obsolete: I see saildrives becoming universal on most (european) production boats.

[double u]

Much more interesting if somebody has tried out
https://www.minnkotamotors.com
using an electric trolling to generate electricity. Would be a fraction of the cost of Watt & Sea or similar.

[izzyforreal]

https://www.yachtingmonthly.com/gear...let-spin-29526

[Pete7]

Originally Posted by YANDINA

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

Ann-Marie, have you read post no 13 in this thread?

It has been discussed before with evidence from some pretty eminent people at the Strathclyde University.

Try this, page 22 has the conclusion you are looking for:

https://pure.strath.ac.uk/ws/portalf...ints005670.pdf

Pete

[Pete7]

Originally Posted by double u

Much more interesting if somebody has tried out
https://www.minnkotamotors.com
using an electric trolling to generate electricity. Would be a fraction of the cost of Watt & Sea or similar.

Agreed, wonder if using an old electric outboard would work though. Previous discussions suggested that the prop is the wrong shape for generation. Also doesn't the power generated significantly with speed so at 6-7 knots then great, but womble along at 3-4 knots in not alot of wind and the the numbers are so great.

Anyway 10 minutes in the video is the Watt and Sea, oh and yes he is a barking mad Viking.

https://www.youtube.com/watch?v=TJBHr3_vpPc&t=1384s

[Hugh17]

My first priority would be to check with the transmission manufacturer and follow their recommendation. For example mine is supposed to be left in reverse while sailing. It’s not worth damaging a transmission to second guess the manufacturer in my opinion.

[StuM]

Originally Posted by gold

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...

This has been thrashed out many times here. Theory and empirical evidence both tell us you are mistaken.



I can't be bothered posting the relevant references yet again.

[YANDINA]

Apparently from the theory it depends on the relationship of the propeller pitch to the boat speed. If the pitch causes the propeller to rotate at the displacement per revolution the drag would be minimum, the blade would screw through the water with negligible disturbance. If locked at speeds less than that there would be drag and at rotation speed higher than that there would be losses.

[a64pilot]

Originally Posted by izzyforreal

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.

No, it’s not, a gyro free rotates, if you fly one you turn the rotor by hand to get it going a little to shorten the take off run.

All helicopters have free wheeling units or sprag clutches that disconnect the engine from the rotor to allow autorotation, if one fails and you have to autorotate, your going to die, a sprag clutch is simply an over running clutch, it’s completely automatic
A sprag clutch is identical to the starter clutch on a Japanese motorcycle if you have ever seen on of them
https://en.m.wikipedia.org/wiki/Sprag_clutch

[StuM]

Originally Posted by izzyforreal

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,

Incorrect. It is NEVER engaged with an engine.


You are apparently not familiar with gyrocopters:


https://www.skybrary.aero/index.php/Gyroplane
A gyroplane is an aircraft that achieves lift by means of a free spinning rotor.

...
Unlike a helicopter which has a powered rotor, the rotor of a gyroplane spins in flight due to the air loading on the rotor blades (aerofoils) as the aircraft moves forward. The free spinning rotor of a gyroplane does not require an anti-torque device, such as a tail rotor, as there is no torque load as would be associated with a powered rotor.

...
A fundamental difference between helicopters and gyroplanes is that in powered flight, a gyroplane rotor system operates in autorotation. In other words, the rotor spins freely as a result of air flowing up through the blades, rather than using engine power to turn the blades and draw air down from above.

[double u]

we are talking helo props in AUTOROTATION!!!

[Wotname]

Originally Posted by YANDINA

..........If locked at speeds less than that there would be drag and at rotation speed higher than that there would be losses.

Originally Posted by double u

........
So to sum up: this could be either way, freewheeling or fixed having more resistance. I guess(!) that with the “right“ amount of resistance from whatever, gearbox friction, shaftgenerator, tight stuffing box, the freewheeling prop would have the greater drag.
.....

Please, it is never either / or!

A locked fixed pitch prop on a sailboat always creates more drag than one that is left free to rotate. More drag means slower boat speed.

One can look at this in a dozen different ways but the simplest way is from a basic elementary (high school) physics problem.

Given that energy is a system is neither created or destroyed but simply converted from one for to another (the first law of thermodynamics, also known as Law of Conservation of Energy).

and

It takes energy to lock a prop

then

that energy must have been removed from some other part of the system!

This is not controversial, it is purely simple applied physics and rational logic.

So if nothing else is changed (i.e. wind speed, wave action, sail area etc), where does the energy that is needed to lock the prop come from, the only remaining available sources are of course, the momentum (mass times velocity) of the boat and the potential gravitational energy contained in the angle of heel while sailing.

Extracting the energy to lock the prop reduces the velocity of boat because clearly the mass of the boat hasn't changed and reduces the angle of heel by a very very very tiny amount. There are some other changes with respect to turbulence (heat) but this doesn't change the fundamental aspects of the issue. It takes energy to lock (and hold locked) the prop - this is why the boat must slow down when the prop is locked. The ever so tiny change in the angle of heel can be seen when doing a vector analysis of the forces required to lock the prop.

Note, the above refers only to a fixed pitch prop on a sailboat when the prop is locked or left free to rotate. It isn't applicable driving a generator or a helicopter or gyro copter or an airplane prop - these have very different factors to consider!

And if anyone wants debate further, try looking at an vector analysis of the forces in question, the answer remains the same.

[NotEntirelySure]

Surely its as simple as the prop will do what ever is the lowest drag. ie spin at a particular speed. not faster or slower than the minimum drag speed.

I'm pretty sure if the prop being stationary was the lowest drag speed (zero) it would just sit there at zero speed.

The drag energy though is turned into rotation and then one balances the other at minimum prop drag speed.

[Wotname]

Originally Posted by NotEntirelySure

Surely its as simple as the prop will do what ever is the lowest drag. ie spin at a particular speed. not faster or slower than the minimum drag speed.

I'm pretty sure if the prop being stationary was the lowest drag speed (zero) it would just sit there at zero speed.

The drag energy though is turned into rotation and then one balances the other at minimum prop drag speed.

EXACTLY and INDEED .

Left to it's own devices, the prop will do what it wants to do at the lowest level of energy required. To do anything more, intervention is required and the energy has to be converted from somewhere else in the system.

This shows up in real life (). If the prop is left unlocked (i.e. free to rotate), it will not rotate at very low boat speeds but as as the boat speed increase, it will start to rotate.

At low boat speeds, the prop remains stationary until the frictional forces in the prop / shaft system are overcome. As the boat speed increases, the moving water on the prop overcomes the the friction and the prop starts turning.

It takes an effort to lock the prop!

Guys, it isn't rocket science, stick to the basics and don't over think the problem!

[double u]

anybody ever held a wind generator by hand into the breeze? First hold it with blades kept from rotating - then let them rotate. Will be an eyeopener.