Propeller vibration.

[Nemisis]

About 20 years ago I remounted the engine in our sailboat 5” lower, 10” further back and straight. The original had the prop offset to port and the boat would continuously want to turn to port.
I used the same approx 4” long 1” diameter shaft and rather than using the brass strut , I made a deeper skeg ( on center)and passed the log thru it. I also increased the prop tip clearance by an inch.

At this time I installed a new Flexofold folding prop.
As you can imagine I was looking forward to enjoying my new supper drive system.
However When I run the engine above 1900 rpm, I get a noticeable and noisy, vibration, that increases with RPM.
I changed back to our original Gory prop, and it was only a little better, but still there. BTW the original Gory did not have a noticeable vibration in the original setup.
The engine was aligned carefully according to Yanmar manual.
Anyone come across this sort of problem?
Thank you for any assistance!
Nemisis

[Love2 boat]

One of two things - bent blade on both props (seems unlikely) or bent shaft.

[Nemisis]

Originally Posted by Love2 boat

One of two things - bent blade on both props (seems unlikely) or bent shaft.

Thank you for your reply.
I have tried 2 separate shafts, both of which were checked for trueness at the machine shop.
I’m not aware of any prop strikes with the Gory, and the Flexofold was new.
Nemisis

[TrentePieds]

A possibility not yet mentioned is that the Centre of Gravity of the prop is NOT perfectly positioned on the axis (centreline) of the shaft. This will be the case if the Centres of Mass of the blade do not lie precisely the same distance from the axis of the shaft.

In such a case, the higher the RPM the more violent the vibration.

A firm of Propeller Adjusters can verify/rectify this matter for you.

TrentePieds

[Pauls]

In addition to tip clearance you also need front and back clearance on your prop.

Also, if your new installation creates a significant barrier to smooth water flow to the prop, as in a large blunt skeg in front of the prop, this can cause vibration as the prop blades transition from the smooth water flow to disturbed water flow behind the skeg.

[Greatest Lakes]

If the skeg is more than a few inches from the prop, there can be harmonic vibration causing you trouble.

[Paradox111]

I wonder if a Sigma drive would solve your issue. (It did for me)
https://youtu.be/tKvDYpJaaAY