Pros and Cons of feathering props

[Stumble]

Originally Posted by a64pilot

I put an autoprop on my IP. I expected to gain some in light winds, cause everyone says you will. Now an IP is no light wind performer or race boat by any means
I didn't get squat in light winds, but as Jim said I get a lot in higher wind speeds and if you think about it for a second it makes sense, prop drag at 3 kts is very little so even if you eliminated it, its not going to make much difference, but at 7 kts its a lot, and there the difference shows.

However almost every feathering prop has no blade shape, they are flat plates, and therefore inefficient when motoring, the shape on a fixed prop is there for a reason, it works. So be aware of that.
Also I believe the prop walk is a function of pitch more than anything, a high prop pitch will gives lots of walk, and a flatter pitch won't. My autoprop has almost zero prop walk, but at very low speeds, its pitched very flat too. So if you can set your reverse pitch and set it flatter you will I believe reduce prop walk and likely reverse thrust too

It's more complicated than no shape being more efficient. Fixed props also have to be designed to operate in reverse and that can hamper the design, where a feathing prop only sees water flow in one direction over the blades.

In our case the autostream increased our fuel efficiency and increased speed at cruising RPM's. We did loose about .25kn from top end boat speed.

[Elie]

I switched from a 2 blade fixed prop to a Brunton autoprop. Major improvement , in particular while motor sailing, and in big head winds and waves. The autoprop adhst its pitch in function of boat speed, vs engine revolutions. In heavy waves and headwind the engine will turn fast, but since the hull speed is low, so is the pitch, as a result more power to the prop. On the contrary, when the sea is calm, the current helping, then the pitch will increase seriously and so the speed and the fuel economy. Very clever British gizmo. It would be perfect if the blades were made of plastic like the Kiwiprop...

[Jim Cate]

It would be perfect if the blades were made of plastic like the Kiwiprop...

Might be "perfect", but it wouldn't work! IIRC, the Autoprop's clever design works in part due to the mass of the heavy bronze blades.

Jim

[wannacat]

I've had a Max Prop on my boat for over 20 years and would never switch back. Once you get the right pitch tuned in it is way faster than my original fixed 2 blade when motoring and virtually no prop walk!!! Don't think some of the other threads ever fine tuned their variables or they would have seen similar results. I wouldn't race without it either!!! Good luck with yours, I think you made the right move!!!

[roberthowell]

did not see it mentioned about placing the trany in gear to stop the spin of the prop with my hurth it requires being in reverse to stop the spin because the prop will continue to spin if the boat is under sail when the engine is retired.

[Elie]

Originally Posted by Jim Cate

Might be "perfect", but it wouldn't work! IIRC, the Autoprop's clever design works in part due to the mass of the heavy bronze blades.

Jim

That true: centrifuge force action is part of the force required to move the pitch of this type of prop. The laminar flow of water is the other force that adjust the pitch. In plastic-in fact injection molding of carbon fiber shaped blades- this can be aleviated by inserting weighted blade tips, for a total weight much lesser than a those massives bronze blades where mass distribution, close to the shaft is much less efficient. By the way, Kiwi props with its molded light blades avoid much of the effects of the heavier bronze blades, including vibrations, and bearing erosion.

[a64pilot]

Originally Posted by Jim Cate

Might be "perfect", but it wouldn't work! IIRC, the Autoprop's clever design works in part due to the mass of the heavy bronze blades.

Jim

Just having one and trying to figure out the principle. I am pretty sure that centripetal force drives the thing to low pitch and water flow drives it to high pitch. So its a the faster you go, the faster you go kind of thing.
It of course pretty quickly balances the forces of course or it would surge in RPM and it doesn't.
There were just after WWII aircraft propellers that did the same thing or similar, but are no longer made

Aeromatic Propellers: Introduction

[Suijin]

Originally Posted by Gamechanger

My hope is to gain about 1 knot of speed and virtually eliminate prop walk. Am I fooling myself??

Prop walk is primarily a function of the angle of the prop shaft relative to "level". The greater the angle, the greater the prop walk. My Maxprop has considerable prop walk in reverse, but also increase efficiency since the blades rotate 180 degrees.

[Stumble]

Originally Posted by Suijin

Prop walk is primarily a function of the angle of the prop shaft relative to "level". The greater the angle, the greater the prop walk. My Maxprop has considerable prop walk in reverse, but also increase efficiency since the blades rotate 180 degrees.

Not really. Prop walk is effected by a lot of things, but blade design is absolutly one of them. I think it was sail magazine that did a side by side comparison of about a dozen props on the same boat, and the prop walk differences were substantial between the different props. The Max was the worst of all designs if I remember correctly.

[a64pilot]

I am sort of convinced that a lot of prop walk is due to pitch, a high pitch is sort of like a paddle wheel and will therefore walk more than a flat pitched prop, my theory.
It is the only thing I can come up with why my boat that was way over pitched with a three blade fixed had such huge prop walk, but almost zero with an Autoprop. For those that have not used an Autoprop, from a standing start, your not winning any drag races, in fact it almost feels like your prop has fallen off, it takes lots more RPM than normal when the boat isn't moving to get going, but contrast that with once its going, it will go much faster at idle or partial throttle than normal. It feels like a slipping clutch in a car does.
Its this low initial flat pitch that eliminates the prop walk, I think.

[a64pilot]

Originally Posted by Suijin

Prop walk is primarily a function of the angle of the prop shaft relative to "level".

That is the definition of "P" factor in an aircraft, I don't know though how much that translates to a boat

[Stumble]

I finally broke down and found the article. Folding and feathering propeller test

It I should the most definitive folding/feathering prop test I know and well worth a read.

[DaveSIM]

I have a feathering max prop on my boat. To stop prop walk you need to have the prop feathered when you go to sail mode. To do this I use the engine to power the boat above 4-5knots in forward gear and then stop the engine while the transmission is in forward. Once the engine is stopped I move the trans to neutral. I have never had the prop walk when I do this. If you don't do it or have the speed under 3.5knots the prop will walk. Good luck, DaveSIM

[daletournier]

My last boat had a kiwi prop, no noticeable prop walk. My new boat has a max prop, considerable prop walk, if I give it a handful of throttle its like a stern thruster, don't know if this is good or bad yet.

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[quandary]

I have a gori 3 blade with "overdrive". Seems to take a hammering wear wise if doing a lot of forward reverse stuff. I think it might be better to fit a fixed prop if you are doing a lot of boat handling practice.