Repower on Allied Princess - Vibration/Cavitation

[Jam9ed]

Hi All, New to the forum here; Getting ready to make first crossing from Bahamas to NC in a couple weeks and we're checking out the boat. Boat is an Allied Princess; '72 year model; 36' with 8-9" width keel. We just repowered upgrading from the original 25hp Westerbeak to a new Yanmar 39HP. With the added horsepower, we increased the three-blade propeller's pitch. The new engine seems to really push the boat well (much faster than the old engine) but we get a signficant amount of vibration - perhaps caused by cavitation? Has anyone experienced this on their re-powers? Recommended solutions? Many thanks!

[Sailmonkey]

With the wide deadwood you’re going to have a significant vibration as one of the three blades is behind the deadwood.

There really is no cure, but it’s a sailboat, so shouldn’t be motored hard like a trawler anyway.

[a64pilot]

Are you making hull speed? What kind of prop?
Diameter and pitch?

[DaleM]

It may be out of alignment. Our boat had constant vibration and a friend said, "Did you check the alignment?". I finally checked and I think they didn't aligned the engine after setting the boat in the water after a repower. Aligned it and most of the vibration went away. I can still feel the rudder move due to prop wash. It has a large three blade prop also. Dale

[roland stockham]

I can think of two issues. It may be alignment due to the engine 'settling' on the beds, compression of mounts etc. All these can make the engine move a little. Also if the engine was fitted ashore the boat can change shape enough when relaunched to nock out the alignment. The other possible issue is the prop. You mention that it has been re-piched. Given that the engine hp was increased by almost 50% that is a major mod. Could be that the prop geometry is now off but also I would assume you are now putting a lot of power through a reletively small but high pitch propeller. Combine that with a large deadwood and cavitation/turbulence issues may be expected. If that is the case it is significant, cavitation can damage the prop and turbulence may accelerate cutlass bearing wear. I would expect to see a prop diameter around 14-15" for that size engine.

[boeing1]

where you said significant vibration, as other stated I would recheck engine /prop shaft alignment. .

[jewt]

Is the prop in aperture or open water? If it is in aperture you wheel could be too large. May need to cut it down and/or repitch.

jewt
S/V Bifrost
'89 Morgan 44

[meridian28]

What hull number are you? I had #28 for 17 years..

[meridian28]

There is an Allied website

[Pauls]

Cavitation occurs when too much horsepower is applied to a prop with too small a blade area. Buy a copy of Dave Gerr's "Propeller Handbook". It has the formula for calculating what blade area you need for your engine. This is a simple calculation, very basic math. Cavitation can ruin a prop pretty quickly.

[a64pilot]

I seriously doubt it’s cavitation. I don’t think it’s reasonable to cavitate a sailboat prop, as you say, and your correct cavitation will ruin a prop pretty quick.
I’m more prone to the deadwood theory myself.