Prop Shaft Spinning when Sailing

[Mark Johnson]

If you get a feathering or folding prop, it solves the problem, and gives a substantially better daily run under sail. (WAY more than most folks realize). IMO, It is money well spent!

Most monohulls not set up for it, can't fit a folding type, so use the Max Prop, but if you could fit it, the Flex O Fold "folding" prop, is MORE efficient than a fixed prop, (when motoring in forward), AND it doesn't snag crab pot lines when folded!

Cheers,
M.

[rtbates]

Originally Posted by 44'cruisingcat

I can understand putting the motor in gear to stop the shaft from spinning, but why do people recommend reverse? Surely then if the prop did manage to actually turn, it would turn the motor backwards? Turning the motor backward, even just a little, could damage the rubber cooling pump impellors.

You could put a pulley on the shaft, and use it to spin a generator, then the noise of all those free amps being produced would be music to your ears.

I do NOT believe that you could ever get a diesel to turn over via the prop.., IF so you have other issues, like no compression!

[evm1024]

I think that the aspect ratio of the prop and propeller are very significant factors in this discussion. The drag (caused by fixed blades) to swept area (the area of the turning circle) ratio is very different between propellers and props. High aspect ratio propellers and low aspect props are oranges and apples.

A boat prop presents a large portion of the turning circle to the water stream. I'll call this the "felt" cross section.

I suspect that a turning boat prop in effect reduces the "felt" cross section. Water rather than hitting the the fixed blades head on is swept past the rotating blades at an angle to the boats motion (thus seeing a reduced cross section).

The test (outstanding by the way) done shows that for a given sized outboard the rotation prop presents less drag and thus allows for a faster motion.

The difference between fixed and freewheeling is non-linear so I would be cautious of giving a force difference. If the test were rerun and the force measured at a specific speed then we could compare the drag force. This would yield a set of curves (speed vs drag) for fixed and freewheeling that would be interesting. (surely this has been done in somewhere back in the '30s?)

My boat has a velvet drive that makes a low growl when freewheeling. You can see where the prior owners have clamped the prop shaft with vice grips. It is scarfed up.

We were at dock the other day and the current of the Columbia river was great enough to cause the prop to freewheel. The river is above flood stage. I ended up clamping the shaft as well so as to get a better nights sleep.

Who knows, someday I expect to put a zinc on the shaft. Drill a whole in the zinc and insert a shear pin in the whole in such a way to stop rotation but still break away if needed. (Oh, I think that I will patent that idea.... remember you saw it here)

Regards, Ethan

[bbbbbguy]

It may not answer your problem, but it will solve your problem. Buy a folding prop! Any parts that move cause wear... no matter what the speed or preasure. I believe that if the shaft is spinning, the seals are wearing, the stuffing is wearing, the plates are wearing, etc. The folding prop almost completely eliminates this, and it redices the drag below the WL. Just an opinion. Dave

[Astrid]

Just an opinion.

And a pretty good one at that. Even if a manufacturer says that allowing the prop and shaft to free wheel will not hurt the transmission, anytime you have something mechanical turning, you are getting a certain amount of wear on your transmission, stuffing box, cutlass bearings, etc. A feathering or folding prop will keep all of that to a minimum. There is really no good reason to accept mechanical wear if you can avoid it altogether.

[Target9000]

The only problem with the folding props is the $$$$

[zeehag]

only problem with the folding props is the small space into which they do not fit on boat ..LOL...

[SteveT]

I've tried 'locking' the shaft on my Yanmar by engaging reverse under sail and it does lock the shaft but ..... when you need to start the engine the gear lever is really stiff and feels jammed and I don't feel too happy with it so I am following the wisdom on here and going for freewheeling the prop in neutral. Of course that raises the issue of water lubrication to my shaft seal - does the flow through the cutlass bearings suffice?

[zeehag]

it will be ok. is how it was designed to work. yanmar is spozed to be in neutral under sail, as is my perkins with borg warner tranny. i like the noise made at 5 kts-- i can know how fast we are heading into rocks when i am off watch.....racers will be all over me on this one, but i am not that.

[sck5]

I have a 3gm30f and asked this question in the yanmar diesel maintenance course given at mack boring. The answer was "put it in reverse"

[zeehag]

did you ask the dealer / distributor? because they say leave it in neutral and warrantee is voided in reverse. but then i only have a 2 qm 20 h.
so many answers from yanmar so you do not know where to put it--i leave my yannie in neutral also, as that is where my dealer/distributor said to leave it. ......

[sck5]

I thought mack boring was a major north american distributor. But maybe I am wrong?

[Kapena]

Zeehag My Perkins says NO DAMAGE occurs if you leave in neutral but recommends placing in gear opposite to your direction of travel...ie; place in reverse. Mine is a Hurst trannie (RBW50).

[zeehag]

correct with perkins..my yannie man sed to leave in neutral, spin wont hiurt it, reverse may just hurt it. i leave both in neutral ...the formosa makes a noise at 5 kts, yannie doesnt.