Feathering Prop vibrates while sailing

[Vasco]

Hej Bavaria experts,

this is my first thread in this forum. I'm looking for help and a solution under water. The situation is as follows:

Our yacht is an B 40 Ocean (2001). I have installed a feathering propeller, SPW 4-blade VARIPROP GP 107 LH-SD ø18"(450mm) Pitch voraus 14", zurück 12"
The prop works good, speed, acceleration and Consumption is as expected. Nothing to mention.
But when I am starting to sail, stopping the engine, blocking the prop with the gear lever in "back" so that the prop will stop turning and the blades should turn by the current water "inline", the saildrive begins to vibrate and is affecting the complete drivetrain. This vibration goes with the complete hull and you can feel it while sitting in the cockpit.
It begins around 3 knots and can be seen at the engine/saildrive bearing in the motor compartment. Several trys with first gear in neutral than stopping the engine, or using the forward gear to block the prop from rotation have the same result.
I've done some videos and surveys     
https://www.skokie.de/ocean-40/propeller/
I got a 3 blade prop from SPW to test if the same behavior appears. It does also with the 3-blade prop.

I assume, since the prop is very close located to the keel (25cm), bigger turbulences (eddies) will force the prop-blades when they are inline with the current.
The trailing edge of the keel is also not perfect shaped in the upper zone close to the hull. Its not a small tip. It is wider and round shaped since an osmosis treatment was done last winter. So water flow will be not laminar when getting off the keel at he trailing edge, I assume.

My Question to Ocean40 / Ocean 38 owners is:

  - Do have installed a feathering propeller? Which brand?
  - What is your experience with this sort of prop?
  - Does anybody have similar vibration in the boat while sailing?

It does not come from the rudder, shure.

I will be glad to get good ideas.
Thanks
Jens

[IslandAlchemy]

Sounds like the prop isn't feathering.  Make sure that it's all running freely.  You need to be able to push the blades with a light fingertip pressure.

[Trundletruc]

It may be to do with the cutches in the sail drive. I was advised by the makers of our featuring prop to just turn off the engine with it still in gear?
Then listen by the side of the sail drive to see if you can hear anything going round inside. (Screwdriver to your ear resting on the casing, if you can get near it.)

[Vasco]

Thanks for the quick response

It was also my first idea, that the prop is not feathering. Two things are against it:
The 3 Blade Prop does it in the same way as the 4-Blade did which was mounted first. SPW double-checked the first prop in their factory after returning to them.
Second: When stopping the engine you can hear very good the still turning shafts and wheels inside the gearbox, driven by the prop. Than shifting in backwards stops all noises from this turning and after 20-30 seconds the vibes are starting.
See on Vimeo 01:10min and 01:40min
https://vimeo.com/436230496

Can it be, that the blades are turning very fast, with a small amplitude, around their neutral position when inline?
The hinge point of the blades are nearly in the middle of the blade, like a pre-balanced rudder.

[Yngmar]

Read your German article and listened to the videos and I think it's most likely the propeller blades fluttering slightly in their feathered position due to turbulence. This may well come from the rounded over trailing edge of your keel. See https://en.wikipedia.org/wiki/Vortex_shedding - that's why the original keel has a tapered edge, not a rounded one like your modified one. The "Keel-Connect" job where the yard laminated the top of the keel onto the hull surprised me and I think it's rather dubious

You should be able to confirm the blade flutter in an area with sufficient underwater visibility by way of a cheap action camera (€20-30 GoPro clone in waterproof housing) tied to a boat hook held over the side of the boat while sailing.

The simplest solution is probably to change to a folding propeller instead of the feathering one. Folding props won't be affected by turbulence (unless it's extreme). Or you can rebuild a new sharper trailing edge to the keel, which is easy to do with a bit of fiberglass and thickened epoxy, but you'll then need to prime and antifoul it again and it may not solve the issue if there are other sources of turbulence and the propeller is just very sensitive to those. The less resistance the prop offers to feathering (normally a good thing), the more sensitive it will be to fluttering. Perhaps this can be adjusted on yours?

We have the fixed 3-blade Volvo prop which spins continuously under sail but doesn't vibrate except sometimes when surfing down big waves at 8+ knots, which we aren't doing all too often.

[IslandAlchemy]

I agree with Yngmar.  You can hear the shaft stop when you put it in reverse, so it's not spinning, that's certain.

There has to be something creating turbulence under there, and most likely the back edge of the keel.

[Vasco]

Hi Yngmar and IslandAlchemy,
Your explanation sounds good and makes all the sense. Vortex shedding ist the keyword. It was my theory, that eddies or vortex forced the propeller when he is not spinning and bladesurfaces are perpendicular to the water flow. In some Videos it sounds really like flattering of the blades.
To make the flattering visible I tried already a Go Pro on a stick, but its just 2m horizontal in fast flowing water, which makes it not feasible to get the Cam into the right position. Next try will be a cam stick through the drain holes of the Cockpit. Cam from China is already ordered.

Thanks a lot for your opinion. I will keep this thread informed, about the progress
Jens

[Vasco]

Hello,
back again with good news:

Boat is in the water and we had already some tryout with the closed lateral plan from keel to the leg of the saildrive. It works.
There is now no fluttering on the prop, nor saildrive, nor the powertrain when turning the prop into sail mode.
We made a short video, and we tried to observe the circumstances under water with a UW cam. We went through the cockpit draining holes with a cam stick and came out of the hull 60com behind the prop. A Go Pro you cannot hold in 2m depth while sailing with 5kn speed. Already when our camstick came under the hull more than 10 cm, the stick began to vibrate. Nevertheless, the vortex shedding theory seems to be right.

If you want to see the video and the complete story, you can find it on our webpage 
https://www.skokie.de/ocean-40/strömung-kiel-saildrive/
https://vimeo.com/558991981

Thanks to all for help, thinking and recommendations