Prop vibration under sail only

[a64pilot]

Originally Posted by OrangeCrush

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.

What kind of transmission do you have? It sounds as if you have a shaft brake.
Think parking pawl on an automatic transmission on a car, but I can’t see how your transmission would function in reverse with this pawl engaged?


Some transmissions like my KBW20 for example are built so that as the power of the engine is increased, the clutch packs are squeezed together further increasing friction and not allowing them to slip.
However if the force is fed backwards if anything it reduces the clutch pack friction so that slippage could occur, in my transmission for example if I were sailing with the engine off and the transmission in forward, the prop might could spin, which would be slipping the clutch, eventually this would of course wear out the clutch plates, so sailing engine off in forward gear is prohibited, but in my transmission if it’s in reverse and engine off any torque from the prop will act to increase clutch pack friction just as if the engine was running, so I’m allowed to have the gear in reverse while sailing.


However that is NOT true for all transmissions on Yanmar engines, it’s my understanding that the newer designed transmissions that utilize cones as opposed to clutch packs, that only sailing in neutral is allowed, sailing in any gear can cause premature transmission failure.

So what gear your allowed to sail in is different with different Yanmar transmissions, although I believe none of them will be hurt if left in neutral, so if your unsure, neutral would seem to be the most conservative response until you can find out for sure.

I would look at my specific manual for my transmission for guidance, and or call a shop that overhauls or services the transmissions, I would not go on what other sailors say, or even a mechanic as there seems to be some confusion on the subject.

[mikehmc]

Could be you are such a good sailor the boat is humming like they do when you get the balance just right. Yep, about 4 knots.

[OrangeCrush]

I just looked and I think I misremembered the “latch” I described. It’s just the cable mechanism on the side that moves, no shaft brake.

The model is KM35P, gear ratio 2.66


Attachment 1

[OrangeCrush]

Some issues uploading pics, here is the transmission info plate.

[OrangeCrush]

Originally Posted by 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.

No, the wobble occurs when the boat is sailing fast and the prop is spinning fast in neutral. I noticed it because the wet exhaust cylinder was rattling lightly against its mount. It does seem relatively benign just not ideal.

[a64pilot]

I believe that is a cone type of clutch transmission, I wouldn’t sail with it in gear, not until I found a manual that says you can, and I don’t think you will find that, if I’m not mistaken this type of transmission is specifically prohibited from sailing in gear, but I’ve been wrong before so find out for sure

[model 10]

It's a loose blade.

[Dave_S]

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.

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.

I'm not understanding how a "free to rotate" prop on boat or aircraft can have more drag than a stationary one. I get the projected area point, but I would have thought the props would naturally follow the path of least resistance.

[OrangeCrush]

Originally Posted by a64pilot

I believe that is a cone type of clutch transmission, I wouldn’t sail with it in gear, not until I found a manual that says you can, and I don’t think you will find that, if I’m not mistaken this type of transmission is specifically prohibited from sailing in gear, but I’ve been wrong before so find out for sure

Fair enough. Yeah, the engine manual clearly says neutral only or you void your warrantee. Last year offshore we had no choice, since letting the shaft spin would have made our leak get much worse. It didn’t do any transmission damage that I can perceive but who knows.

[a64pilot]

Originally Posted by Dave_S

I'm not understanding how a "free to rotate" prop on boat or aircraft can have more drag than a stationary one. I get the projected area point, but I would have thought the props would naturally follow the path of least resistance.

On an airplane it’s easily demonstrated, I used to Certify about 100 or so new turbine Ag aircraft per year. I would normally set the blades min pitch pretty flat as that was a big air brake which made for much better short field landings and or slowing down quickly and getting on the ground fast, something most Ag operators like.
What slows it down so much is with the flat pitch, it’s backdriving the power turbine, your drawing power from the airflow and that increases drag tremendously.
In a piston aircraft you have to stop the prop by shutting down the engine, but it’s real easy to see the glide distance increase and rate of descent increase with a stopped prop.
One theory is the power required to spin the motor over is coming from the potential energy of the aircraft at altitude, stop the prop and your no longer robbing that energy.

However a boat is completely different, I believe it’s due to the greatly increase surface area of a boat prop, when stopped it covers nearly the entire area, an airplane prop is like two sticks and only covers a small fraction of the circle.

But both are easily measured / demonstrated and have been done so many times.

This is where I stopped the prop on my little C-140. To get it started again push the nose over and speed up. Eventually the air will start turning the prop / motor
https://vimeo.com/268448424

[Montanan]

Reference article from yachting monthly.

Read more at: https://www.yachtingmonthly.com/gear...pMkd1ygYdfU.99

"wherever possible you should let your fixed three-blade prop spin. The drag when it was locked was nearly three times the drag when spinning, and the loss of speed was over half a knot at 4 knots, and extrapolated to be 0.75 knot at 6 knots."

"Many gearbox manufacturers have their own restrictions on whether you should lock or spin. Almost all of today’s hydraulic boxes can be safely left to spin in neutral but in the past, some hydraulically-operated boxes had their lubricating oil pumps driven by the input shaft from the engine, so if you left them in neutral under sail, with the engine stopped, they would suffer oil starvation.

On the other hand, some mechanical boxes actually warn you not to lock them in gear when sailing, as the load on the fixed prop can damage the internal components of the box, and even turn the engine over, so you should let them spin. As always, check the specific instructions for the gearbox in your boat before deciding."


Did you change to a different propeller when you had the work done in the last year? Is is a different propeller than you had before, in which case it might behave differently, e.g., spin at a different rate or induce different vibrations?

[s/v Jedi]

You have a mechanical transmission, I have it’s bigger brother on my 140hp Yanmar.

With a fixed prop, you need a shaft brake. With a folding prop, I switch it in reverse to stop the shaft, after which the prop feathers and I can shift back to neutral without the shaft spinning anymore.

When you do this, you move a brass or bronze clutch cone into a spinning cone gear. This absorbs the shock the same as with engine running as long as the shaft doesn’t spin faster than in idle engine speed.

[wsmurdoch]

About Yanmar marine clutches when sailing....
February 8, 2008 http://www.shaftlok.com/YANMAR_MSA08...al+Sailing.pdf
October 11, 2011 https://j109.org/docs/yanmar_technic...tqtb11-017.pdf

When sailing with my 1988 3HM35F / KBW10 in neutral with a two bladed propeller in an aperture, the better the alignment the less the motion of the engine. I have never had zero motion.

Bill

[Uncle Bob]

Originally Posted by 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.

You might want to do a little research on that.

[Uncle Bob]

Originally Posted by Ecos

It's a loose blade.

?? A loose blade on a fixed prop??