Prop vibration under sail only

[OrangeCrush]

I’m in the middle of a long cruise and I leave my Yanmar in neutral while sailing as per the manual. I’ve noticed a subtle vibration/wobble, but only under sail and when going above about 4 knots. There’s no perceptible movement in the stuffing box or prop shaft itself but I can feel it wobble slightly in the transmission, motor and exhaust. Above decks it’s almost imperceptible but when I’m tuned in enough I can feel it in the wheel. When motoring it disappears completely, unless I’m motors-sailing with good boat speed but low rpms. In other words, it vibrates when the prop is being pulled through the water but not when it’s driving the boat.

Last year I had all new stern gear put in, but this may or may not have existed beforehand without me noticing it. The new prop shaft was aligned at the coupling to a reported gap of three thousandths. The prop was balanced and is clean. The stuffing box with duramax packing is dripping about once a minute and staying cool. The transmission oil level is fine and the transmission spins quietly. The cutlass bearing has zero lateral play. All motor mounts are tight and look healthy.

It’s very subtle and I don’t think most people would notice it. I’m tempted to consider this a normal state of affairs, but I also hate the idea of doing motor damage while sailing. Has anyone else experienced this? Could it still be the alignment even if it stops under motor power? Should I consider locking the prop in reverse under sail to eliminate this effect? Thanks in advance

Jack

[Captndave1]

A good chance that alignment is causing this. Often times the angular alignment can be right on, but the lateral alignment could be off enough to cause a vibration.

[a64pilot]

It could be turbulent water flow off of the prop causing the rudder to shake maybe?

[Davo1404]

Originally Posted by OrangeCrush

I’m in the middle of a long cruise and I leave my Yanmar in neutral while sailing as per the manual. I’ve noticed a subtle vibration/wobble, but only under sail and when going above about 4 knots. There’s no perceptible movement in the stuffing box or prop shaft itself but I can feel it wobble slightly in the transmission, motor and exhaust. Above decks it’s almost imperceptible but when I’m tuned in enough I can feel it in the wheel. When motoring it disappears completely, unless I’m motors-sailing with good boat speed but low rpms. In other words, it vibrates when the prop is being pulled through the water but not when it’s driving the boat.

Last year I had all new stern gear put in, but this may or may not have existed beforehand without me noticing it. The new prop shaft was aligned at the coupling to a reported gap of three thousandths. The prop was balanced and is clean. The stuffing box with duramax packing is dripping about once a minute and staying cool. The transmission oil level is fine and the transmission spins quietly. The cutlass bearing has zero lateral play. All motor mounts are tight and look healthy.

It’s very subtle and I don’t think most people would notice it. I’m tempted to consider this a normal state of affairs, but I also hate the idea of doing motor damage while sailing. Has anyone else experienced this? Could it still be the alignment even if it stops under motor power? Should I consider locking the prop in reverse under sail to eliminate this effect? Thanks in advance

Jack

Is it a folding propeller?

[s/v Jedi]

How come the prop shaft doesn’t start spinning? With my feathering MaxProp the shaft spins and with the engine shut down I put it in reverse gear which stops the shaft turning, which makes the prop feather the blades and I can shift back to neutral. With a fixed prop it should turn and you need a shaft brake.

[Sailmonkey]

Your prop is passing through a rather large deadwood while spinning freely. This is putting uneven pressure on the blades, 2 will be impacted differently than the third. It’s this uneven flow that’s causing your odd movements. It won’t hurt anything, but it’s one of the reasons that we switched to a feathering prop. The barely perceptible vibration was driving me nuts.

[OrangeCrush]

Originally Posted by Davo1404

Is it a folding propeller?

No, it’s a 3 blade fixed prop. I should have specified that.

[OrangeCrush]

Originally Posted by s/v Jedi

How come the prop shaft doesn’t start spinning? With my feathering MaxProp the shaft spins and with the engine shut down I put it in reverse gear which stops the shaft turning, which makes the prop feather the blades and I can shift back to neutral. With a fixed prop it should turn and you need a shaft brake.

The shaft is spinning quite fast in neutral as I sail when I get the vibration. I could lock it in reverse and stop the spinning but it’s not what the manual says to do.

[OrangeCrush]

Originally Posted by Sailmonkey

Your prop is passing through a rather large deadwood while spinning freely. This is putting uneven pressure on the blades, 2 will be impacted differently than the third. It’s this uneven flow that’s causing your odd movements. It won’t hurt anything, but it’s one of the reasons that we switched to a feathering prop. The barely perceptible vibration was driving me nuts.

This explanation fits my instincts. I think if I tried to improve the alignment it would still happen because of the fluid dynamics under sail.

[tkeithlu]

Sailmonkey speaks with wisdom. Experiment: Clamp the shaft so the prop does not spin, and see if that stops it.

[OrangeCrush]

Originally Posted by tkeithlu

Sailmonkey speaks with wisdom. Experiment: Clamp the shaft so the prop does not spin, and see if that stops it.

I’m positive it would stop. The wobble is in sync with the speed of shaft rotation. I can try it to be sure.

So... would it better to lock it in reverse to stop the vibration or better to just ignore it let it vibrate slightly?

I’ll def consider a feathering prop for the future, but I’ve got at least a thousand more miles to sail this summer and I’d like to keep things functioning smoothly. Last year my cutlass bearing failed and it wasn’t fun, and I’m wondering if this vibration was a contributing factor.

[tkeithlu]

In theory, at least, a stopped prop should give you slightly less drag than a spinning prop, but I'll defer to those with more practical experience with the issue on sail boats - I skipper a stink pot.

[a64pilot]

Do not lock it by putting the tranny in reverse, if yours is the type that Yanmar says don’t, apparently transmission damage will result if you do.

If it’s the deadwood that is causing the prop to vibrate, it should be much more pronounced when motoring when there is a low pressure area in front of the prop which increase velocity of the water and the difference in induced flow between the dead wood and the unrestricted flow.

Being a pilot I too used to think a stopped prop would be less drag, because that’s easy to demonstrate in an airplane, but the opposite is true of a boat prop.
I believe the reason is simple, if you look at the average airplane prop, it looks like two thin sticks, its profile is a small fraction of the swept area if the prop is rotating, however look at a common three blade boat prop and it covers nearly 100% of the swept area.

[OrangeCrush]

Originally Posted by a64pilot

Do not lock it by putting the tranny in reverse, if yours is the type that Yanmar says don’t, apparently transmission damage will result if you do.

It’s my understanding that transmission damage would only actually occur if the prop were left in forward. In reverse, a visible metal latch moves into place and stops it from spinning forward. Last year when we had a leaky stern tube offshore, we had to lock it in reverse and sailed 350 miles home with no damage. I think that Yanmar says neutral because it’s the best overall choice, but that reverse would also be ok. I’m sure someone knows more about this...

On the other hand, shifting into reverse while already sailing fast does seem like it would result in a violent clunk of metal on metal as that latch engaged. I could come up into irons to reduce boat speed first.

But if people think that a subtle vibration is not likely to harm anything then I guess I prefer living with it for now.

[Cheechako]

I thought you said the prop wasn't moving? Anyway, Pretty normal if the prop is spinning. Put in reverse. I always did with my Yanmars. Never an issue.