34 main ship pilot cavitation

Recently purchased a 34’ main ship pilot 2001 w/ Yanmar 350 hp that has cavitation/ventilation around 1800 rpms and above that is intermittence. Took boat to reputable boat yard 70 plus years and was told it was the prop. Had prop redone and balanced and it helped a little so removed prop again and they added cup to it. Made it worst, removed prop the third time and it still has the problem has anyone been able to correct this problem. Also told a 5 blade prop might help but don’t want to buy a prop and it don’t solve the problem.
HELP

This topic has been discussed several times on this forum. Search on Pilot 34 cavitation and you should find them.


In summary:


Some do it and some don't. Fairing the deadwood works for some and not for others. Truing the prop generally doesn't help. A clean bottom helps.


No one has reported going to a 5 blade prop. It should theoretically help but will be less efficient.


Try asking on the Yahoo Mainship group.


David

Greetings,
Welcome aboard.

Thanks David for the feedback. With as many of these boats that were built surly somebody has come up with a solution
Doug

What is the diameter of the prop and how much clearance is there between the prop blade and the hull?

34 Mainship cavitation

The prop is a 25 x 28 . Distance from prop tip to Hull is 4 1/4 “. Prop had a cup of 2, changed to a cup of 4 made worst, changed again to no cup and did not help.
Doug

The clearance sounds ok. You need .15 of the prop diameter for minimum clearance which in your case is 3.75”. Maybe the faring of the keel as suggested before???

Comodave said:

The clearance sounds ok. You need .15 of the prop diameter for minimum clearance which in your case is 3.75”. Maybe the faring of the keel as suggested before???

Click to expand...

I had a slight cavitation on my Pilot 34 when the hull was a bit dirty and I was cruising at 2,800 rpm. A little annoying, but not bad.

So I faired the deadwood. Cost me $1,200 and didn't change the cavitation a bit.

The only solution I haven't read about, and I am pretty sure I have read everything about the Pilot 34 on this forum, boatdiesel and the Yahoo group, is a 5 blade prop.

The 5 blade prop can give you a smaller diameter so more tip clearance and the 5 blades will give you more but probably lighter hull to prop tip interactions.

So, be the first to try it!!!

David

The 5 blade may be the way to go. Right now you are at 17% tip clearance. 15% is the supposed minimum but 20% is ideal. You wouldn’t think that 3% would be that big of a deal but maybe it is.

34 Mainship pilot cavitation

Thanks Comodave for info.
Marine people and prop guy said clearance is O K. Also strut bearing and motor mounts good. Through about the fairing and it makes since with water running aside the keel then at the end of the keel a flat end. If that’s the problem l need to know what to do to the keel.
Thanks Doug

A flat end to the keel will cause the water to be turbulent as it passes by.
A bullet shape or a taper (chisel edge) will let the water flow be smooth as it passes by.
I faired the keel on my "ex", an old 34 Mainship. I gained 250 rpm at wot just by doing that.
Here's a pic of what I did. I only added to the keel with some wood fairing blocks I whittled up then glassed them in.
I must add that I did this in anticipation of repowering which I did a year after I did the fairing. After the new power (and changing from 2:1 to 1.5:1 gearing) I had vibration in the hull above the prop. I also anticipated that and the fix was to gusset the hull above the prop.
After that complete silence from vibration.

34mainship pilot cavitation

Thanks Leonard
I think the next time I pull the boat i will try the fairing. There has got to be a solution with as many of these boats that was built. Can’t believe people are just living with the noise.

Here is a pic of my fairing job that didn't work:


Before:



After:

OK time for a question, but I do not intend to steal the thread.
I know what the difference is between cavitation/ventilation and vibration.
But....how can one tell which is which from running the boat?
Can it be mis-diagnosed?
Anyone?

"I had vibration in the hull above the prop. I also anticipated that and the fix was to gusset the hull above the prop.After that complete silence from vibration."

Extra thickness is a very wise cure as its cheap and lasts forever.
Sadly the aroma of the new glass can sometimes persist , even with the preferred Epoxy resin.

Cavitation is the collapse of bubbles caused by the heavily loaded prop blades.

An auto supply stethoscope should let you track the noise from idle on up with slow throttle increases.

Winnjammer: You posted that the noise was intermittent. That does not fit the classic "prop noise" the boats are famous for. Also, with the engine at 1800rpm, that is below where most of these have the problem.

Can you go into more detail of what you hear? What boat speed are you getting at 1800? So the complaint comes and goes? Under what conditions?

Full keel boats do tend to get a lot of prop noise when approaching planing speed, like 12kts plus. The water going into the prop is not "clean", lots of variation in velocity, makes lots of vibes in the prop. Loud in cockpit right over prop. Nature of the beast. Fairing the deadwood can help, but does not get rid of the boundary layer flows set up by the keel sides.

Props can be had in heavier grade , if disturbed water is causing the prop to flex and sing , perhaps just a thicker HD prop would help?

I have been following this thread and others about vibration and cavitation on the Pilot models. I have been running our 2003 30' Pilot II with the 315 Yanmar for about 3 1/2 years now. As most know the 2003 and newer models have a tunnel the prop sits in. The clearance of my 5 blade 23 x 22 1/2 wheel is about 1 1/4" from the tunnel. I have no cavitation or vibration at any speed. She runs like a top and gets up on plane quickly. Very curious as to what
those in the know think about this. Any ideas? thanks

regards Holty

l.p. l.p.

Can you access the inner hull right above the prop? If so it would be a simple job to lay in a couple of layers of 1708 and some wood to strengthen the hull in that area, probably a couple of hours work and certainly cheaper than buying a new prop.