Prop Problem with 34' Mainship Pilot Twin Yanmars

[Dan S]

Hello All. I am new to the forum and have a problem. Three years ago I had to replace the props on my twin engine 34' Mainship Pilot. I replaced them with Michigan Wheel Hy Torq MY-T4 18" diameter 25" pitch props. With these props I could only achieve about 3,000 rpm vs. the engine spec of 3,300 rpm. I had the pitch reduced to 22.5" and the rpm increased to 3,300. I just had the boat pulled for new bottom paint and it appears that the props burned holes in the bottom at the outside edge of the prop tunnels. There is very little space between the props and the tunnels, but the new props are the same diameter as the old ones. Has anyone seen this type of problem? Is my assumption that the prop is the root of the problem correct? Does anyone know the size, pitch and manufacturer of the OEM props? Any ideas on how to keep this from happening again would be really appreciated.

 

[Pascall]

It could possibly be cavitation of the ship's propellers.
What surprises me is that the ship's propellers are okay and show no damage from cavitation.

Greeting,

Pascal.

 

[DavidM]

Yes, probably cavitation. Do you feel any vibration at higher rpm’s?

If the props run smooth, I would leave them as is and fill the holes with high strength epoxy. Patching the holes every few years will be cheaper than new props. The epoxy might hold up better than the original polyester.

Whoever sold you those props should bear some blame. The original pitch of 18 x 25 is way over square and the pitch reduction to 22.5 was probably not done right.

David

 

[rgano]

I have heard of stainless steel strips being imbedded in the fiberglass at those locations. I would check with a profession prop shop about any ideas they might have about reducing prop tip cavitation because that is the culprit.

 

[Bacchus]

I have heard of stainless steel strips being imbedded in the fiberglass at those locations. I would check with a profession prop shop about any ideas they might have about reducing prop tip cavitation because that is the culprit.

Good advice
I know my MS OEM prop for my 2008 34HT has "cupping" and also runs in a prop tunnel. I dont see and evidence of the wear you are seeing.
I'm no expert on the subject of cupping but it might be something to consider/ discuss with a knowledgeable prop shop.

Here's a quick intro...

What is a “cupped” propeller and what benefits does it offer?

Propeller cupping provides a lot of benefits with relatively small changes to blade form. Learn everything you need to know from the pros at Michigan Wheel.

www.miwheel.com

 

[Dan S]

Thank you all for the great feedback. I should have joined the Trawler Forum years ago. Regarding vibration, I don't notice any difference between the old and new props, although with the vibration coming from two four cylinder engines it's hard to distinguish anything new or different. Regarding the props being over square, the tunnel is only about 19" side, so I wouldn't think there would be much alternative to the pitch. The props do have a 3 level of cupping (whatever that means). I need to spend a little more time with the article Bacchus referred to in order to get a better understanding. I am reaching out to the Michigan Wheel trouble shooting group to see if they have any thoughts. In the meantime I hope the epoxy repair holds and doesn't deteriorate too quickly and I will have the diver check it out at each cleaning. I agree epoxy is cheaper than new props, but I would like to understand the root of the problem. I do like the metal plate idea but need more research on the cause first. Thanks Y'all

 

[BB-marine]

The 18X25 was the original prop that Mainship installed but the diameter was to tall. It did not give enough clearance between the bottom of the hull/tunnel and blade tip. This causes loss of efficiency and cavitation. The 18" diameter needs to be cut down to a 17". I have a 2002 twin 4LHA-STP 240hp. I am running props that were 18 X 25" HyTorq and now are 17 X 25.5 cupped. Yes, not close to square but it is a design issue that can't be changed. The 1.5 to 1 gear ratio ZF would have to be reduced to load the engine correctly if a closer to square prop is to be used. The engines turn 3400 rpm 27.5 mph 17 X 25.5 cupped. I had the props scanned, balanced, pitched and cupped by Propulsion Engineering Airmarine Inc in Chicago Il. The 18" you purchased will not work but the props can be cut. I have them cut and just cruised with them for over 8000 miles and 900 hours. I just pulled them and having them scanned again 3 years of cruising and still look good but I'm sure they need rebalancing and truing.

 

[Comodave]

The tip clearance should be 15 to 20% of the prop diameter. This is measured between the tip of the prop blade and the hull. I suspect that is what is causing the damage to the hull. What is the clearance on the boat?

 

[Dan S]

Thank you so much BB-marine. This is exactly the info I needed. By any chance do you know how much cup was put into the prop by Propulsion Engineering Airmarine? By the way, you have a beautiful boat. I have seen the listing on line and someone is going to get a beautiful and well maintained boat. Perfect for another trip around the loop. Shout out to Comodave, if the prop diameter is reduced to 17" the clearance should be right about 15% of the diameter. Thanks for help

 

[BB-marine]

Thank you so much BB-marine. This is exactly the info I needed. By any chance do you know how much cup was put into the prop by Propulsion Engineering Airmarine? By the way, you have a beautiful boat. I have seen the listing on line and someone is going to get a beautiful and well maintained boat. Perfect for another trip around the loop. Shout out to Comodave, if the prop diameter is reduced to 17" the clearance should be right about 15% of the diameter. Thanks for help

The cup is .060.

 

[Dan S]

Thank you for the cup info. Planning to pull props in December when I do the heat exchangers.

 

[Snug Harbor]

I would say you have a tip clearance issue as others have noted.

Pick up Dave Gerr's book - The Propeller Handbook, The complete reference for choosing, installing, and understanding boat propellers. PROPELLER HANDBOOK

I kept it on my desk while working in engineering at a boatbuilder - it's a great resource and interesting reading. You'll learn more about props than you thought was possible to know. His other books are very worthwhile reads as well.

​

 

[Potomac boater]

I have a 30 Mainship pilot . The exact same problem occurs with my prop and tunnel every year. I haul each winter I have tried epoxy , glass and this year used jb weld. I doubt it’s any better but I will see next week at haul out. I was thinking of the stainless plate next year. It is not a real diy job as it need to be fitted to the tunnel and fixed with screws. I wouldn’t trust an adhesive on the plate alone. There are many Mainshippers in the same boat. Bottom line- the tunnel is to close to the leading edge of the prop in my opinion