Free prop or locked prop?

[Kim Klaka]

Originally Posted by Dockhead

Actually it has been definitely proven in several tests that a spinning prop produces much less resistance than a locked one.

So I think there is a consensus -- lock it only if your transmission won't allow you to leave it spinning (some hydraulic ones for example). The idea that a locked prop produces less resistance is an urban myth which has been definitely busted.

But ultimately, yes, of course, folding or feathering is the only really decent solution, if you actually sail.

Whilst for the configuration of the boat in the original post (saildrive on Jeanneau) there is definitely less drag for a free-wheeling prop, there are a few occasions when that is not the case. However, my reason for posting is not about the prop drag but the gearbox. I had understood that a free-wheeling prop can damage gearbox bearings unless there is a mechanism for circulating the gearbox oil without the motor running. We are now told for this (saildrive) installation there is a risk of the cones glazing if you don't let the prop free-wheel. I am concerned about these two opposing factors, not least because a folding prop doesn't usually rotate in neutral. I am a naval architect not a marine engineer, so can someone explain in simple terms what is going on inside the gearbox, and how you can avoid both glazing (shaft not rotating) and bearing damage ( free-wheeling shaft) for a gearbox attached to a folding prop.

[four winds]

No one is suggesting to ignore the manual recommendation. That is related to the needs of the transmission.

Whether a fixed or freewheeling prop results in less drag is an additional discussion.

Luckily for me the transmission's needs and the lesser drag of a free prop are complimentary.

Can't afford a feathering or folding prop, nor do I wish to add the additional complexity or possible maintenance issues.

[StuM]

The descriptions that I have seen all refer to the locked prop causing "chatter" or "vibration" of the cones causing wear and eventually resulting slippage.

[four winds]

Originally Posted by Kim Klaka

Whilst for the configuration of the boat in the original post (saildrive on Jeanneau) there is definitely less drag for a free-wheeling prop, there are a few occasions when that is not the case. However, my reason for posting is not about the prop drag but the gearbox. I had understood that a free-wheeling prop can damage gearbox bearings unless there is a mechanism for circulating the gearbox oil without the motor running. We are now told for this (saildrive) installation there is a risk of the cones glazing if you don't let the prop free-wheel. I am concerned about these two opposing factors, not least because a folding prop doesn't usually rotate in neutral. I am a naval architect not a marine engineer, so can someone explain in simple terms what is going on inside the gearbox, and how you can avoid both glazing (shaft not rotating) and bearing damage ( free-wheeling shaft) for a gearbox attached to a folding prop.

Not an engineer. But.....

The opposing factors you mention relate to two different transmission designs.

Those transmissions that require the engine to be running for lubrication have an engine driven hydraulic pump that lubricates the internals and provides hydraulic pressure to engage a "clutch pack" of friction discs to transfer power to the output shaft. And they use a light weight hydraulic fluid like automatic transmissions. No engine running, no lube, free spinning bad.

Those transmissions with cone clutches have no driven pump. Helical groves on the main shaft force the cones into the drive gears under pressure to transfer power. They are lubricated by splash feeding from the rotating gears in the oil bath below. Even engine off, the freewheeling prop turns the output gears and splash feeds for lubrication. No harm no foul. Running this type locked in gear engages the cones via the helical cut shaft, but does not provide sufficient pressure to ensure the cones don't slip, like engine power will. So it slips eventually and wears the oil grooves on the cone faces.

So basically, if you have atf type oil and a clutch pack keep it locked.

If you have cone clutches and engine or gear oil, let it spin.

[transmitterdan]

Originally Posted by jrbogie

you keep repeating yourself with your prowess at googling even though it came back to bite you with your area claim that airplane propellers have less exposed area than do boat props. it's on the internet so it must be true. so allow me to repeat myself as well. with all that i've read and experienced there is no consensus on the issue.

One reason that an airplane propeller "free spinning" cause drag is because the propeller is not free spinning. It is turning the engine over and doing lots of work compressing air and generating heat. This energy had to come from somewhere. The only source of energy is the kinetic energy stored in the 1/2MV^2 of the aircraft. The M can't change so V decreases as a result. The law of conservation of energy must be satisfied. If you could somehow decouple the prop from the engine then it would spin a lot faster and cause even less drag.

Conversely a boat propeller is almost totally free spinning except for a tiny amount of friction loss. Compared to turning over a piston engine almost no work is being done by the spinning propeller thus not much heat generated.

A locked airplane propeller offers relatively little resistance compared to a locked boat propeller due to the size and shape of the blades plus the relatively high density of the water.

There is absolutely full consensus on this issue. A locked boat propeller causes more drag in water than a free spinning propeller and that causes more drag than a feathering prop. The physics explanation is not hard to comprehend. And test results prove it as well.

[HopCar]

"If you could somehow decouple the prop from the engine then it would spin a lot faster and cause even less drag."

That's not what happens. I had a powered hang glider. The engine was connected to the prop through a centrifical clutch. When I stopped the engine, which I did every flight, the prop was free to spin on ball bearings. It was not turning the engine.

A friend of mine invented a prop brake for our aircraft. My glider was fitted with a very sensitive variometer, vertical speed gauge.

If I was gliding with my prop free wheeling and then stopped the prop, I could feel and see my air speed increase. When I slowed back to minimum sink speed, my sink rate would drop by at least 200 feet per minute.

[transmitterdan]

Originally Posted by HopCar

"If you could somehow decouple the prop from the engine then it would spin a lot faster and cause even less drag."

That's not what happens. I had a powered hang glider. The engine was connected to the prop through a centrifical clutch. When I stopped the engine, which I did every flight, the prop was free to spin on ball bearings. It was not turning the engine.

A friend of mine invented a prop brake for our aircraft. My glider was fitted with a very sensitive variometer, vertical speed gauge.

If I was gliding with my prop free wheeling and then stopped the prop, I could feel and see my air speed increase. When I slowed back to minimum sink speed, my sink rate would drop by at least 200 feet per minute.

I don't understand then. If you stop the prop then drag decreases. That is not inconsistent with what I said. For clarity in the case of an airplane:

Prop locked to engine - highest heat lost and highest drag

Prop uncoupled from engine and free to spin - less heat loss and less drag

Prop stopped - lowest heat loss and least drag

So your experience matches what would be expected and explained by physics.

With a boat prop the prop itself causes more turbulence when locked and thus more heat loss and more drag than when allowed to spin freely.

[El Pinguino]

Re the comparisons between flying and sailing...

When you are sailing the wind is 'driving' your boat.

When you are flying and your engine stops nothing is 'driving' your plane.

Maybe the correct comparison would be aeroplane v. a yacht with no sails up that has just stopped the engine...... now the boat prop can be stopped ( in gear ) or spinning ( out of gear ).

In my experience in the day job it was accepted wisdom that 'slow ahead' which would see the prop turning slower than the speed of the water passing over the blades would stop the ship faster than a non turning prop...so there was more drag from the turning prop.
That ties in with the aircraft result.

Hope this makes sense.

[HopCar]

"When you are flying and your engine stops nothing is 'driving' your plane."

This is not correct. Gravity is "driving" the airplane.

I know from my own experience that a stopped airplane prop causes less drag than a free wheeling prop. I used to believe that would hold true for a boat prop.

After seeing Maine Ships test a few years ago, I'm convinced that a free wheeling boat prop causes less drag.

I don't know why this is, but like A64, I think it has to do with the cord (width) of the blades.

[four winds]

Props are designed to "screw" through the water. They are even called screws by some.

Take a common wood screw. Which would be easier, pushing it straight into a piece of wood or twisting it in?

Why anyone would think pushing a prop (screw) straight through the water is easier than twisting it through the water is hard to understand.

Except for maybe pilots who have an experience that misleads them to make a correlation to boating that is incorrect.

Apparently Maine Sail and others have data that proves a freewheeling prop creates less drag. I typically don't read posted links used to support a member's position, or post them to support mine. That's not a conversation on a topic.

But surely Main Sail or someone collecting the data might offer a reason as to why the conclusion drawn from the data is correct at least to their understanding.

[transmitterdan]

The reason is already explained. Drag is caused by the conversion of kinetic energy into heat. A fixed boat prop dragging through the water has to move a lot of water out of its way causing turbulence which requires energy (heat) and the only place to get it is by reducing boat speed. Letting the prop turn freely results in less turbulence and creates minimal heat within the bearings. So total energy wasted by the prop is less thus higher boat speed. It really isn't a complicated mystery.

The wood screw analogy is sort of ok except wood is a solid not liquid. Imagine you could somehow push hard on a screw but at the same time it was free to spin. If you pushed hard enough the screw would turn and screw itself into the wood. Now imagine the same experiment but you prevent the screw from turning. The energy required to push it in would be enormously higher.

[double u]

prop-area (% from full circle) & pitch has to come into the equation somehow too, as helicopterpilots "autorotate" if the engine stops, to keep the machine from falling down, & not stop the prop (which would mean instant plummeting!) - so in this case the freely rotating prop clearly has a tremendously higher resistance than a fixed one

[transmitterdan]

A helicopter blade (foil) has stored energy which can be used to create lift as the pilot adjusts the pitch of the blade. So as she adjusts the pitch to increase the rate of fall the "resistance" decreases and the blade turns faster converting some of the stored energy of the helicopter into kinetic energy in the blades. Then as she gets close to the ground, if the mental math, is right there is just enough energy left in the blades to re-pitch them to create enough lift to survive the landing. Interestingly, the process is not much different that adjusting sail trim to get the foil shape right to create lift and pull a boat forward.

A small blade in water cannot store enough energy to achieve the same effect except perhaps on a much smaller scale. The blade would have to autorotate at very high speeds which it will never achieve due to its shape and the density of water.

[four winds]

transmitterdan, thank you for your feedback. Makes sense to me. Your review of the wood screw analogy is as I visualise it.

And as to why there is less turbulence around a rotating blade goes back to my earlier comments about the apparent direction of travel through the water. A resultant vector of the forward motion and the lateral motion from spinning. Thus presenting a more shallow pitch to the flow. And effectively opening up the area between the blades providing less resistence to flow.

Picture a fully pitched variable pitch prop locked and not spinning. Then reduce pitch by some percentage and consider the change in drag and turbulence. This is what a standard prop is effectively doing when allowed to spin.

Thanks to all for your toleration of me in this thread. I'm a pain in the ass when curiosity gets the better of me.

[Lanevdl]

Originally Posted by Dockhead

Actually it has been definitely proven in several tests that a spinning prop produces much less resistance than a locked one.

So I think there is a consensus -- lock it only if your transmission won't allow you to leave it spinning (some hydraulic ones for example). The idea that a locked prop produces less resistance is an urban myth which has been definitely busted.

But ultimately, yes, of course, folding or feathering is the only really decent solution, if you actually sail.

Dockhead - Where has this 'Urban Myth' been busted? Spinning blades on a gyrocopter offer a LOT more resistance to the air than fixed blades which would cause it to plummet to the earth. Actually I'm pretty sure helicopters also need the blades to spin to offer enough resistance to the air to get off the ground. My prop people say a fixed prop is definately the way to go.