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Old 10-09-2017, 08:46 AM   #1
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vibration at high rpm

My mainship 30 pilot has a prop vibration at high rpm's was told this was a clearance problem on this model does anyone know the fix
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Old 10-09-2017, 09:37 AM   #2
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If you have a three blade propeller you could change to a four blade as this commonly reduces the diameter by around 6 to 8% keeping the same/similar RPM, they also tend to be smoother in most cases,(although normally found on larger vessels)

You could talk to a propeller expert about reducing diameter and increasing pitch again vessel/engine considerations

Other fixes include having a longer prop shaft normally moving the prop away from the hull as it sloops upwards toward the stern, this obviously depends on the hull shape and location of stern bearings or P brackets in relation to the propeller as these distances are critical ( normally only around 1 1/2 times the shaft diameter) so it dosent normally work out with out major works
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Old 10-09-2017, 10:22 AM   #3
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Putting lead or concrete on the inside of the hull right above the prop would probably have a significant effect. Only a small dia area right above the prop would need the weight and I'm guessing it should be firmly affixed to the inside of the hull. Epoxy or perhaps 3M5200 comes to mind. May be nice to be removable but probably not. One could just put lead shot in a bag on the hull to see if it was going to work. Pices of steel plate 1 1/2 to 2" square laid in thick epoxy is another idea. Probably galvanized first.
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Old 10-09-2017, 10:43 AM   #4
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Could be nature of the beast. Prop noise. If you have a full keel ahead of the prop, it will put disturbed water into the prop and it will be noisy when at higher speeds. Different props (more blades) can help, but it is hit and miss and expensive to experiment.
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Old 10-09-2017, 07:17 PM   #5
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The propeller vibration issues with that model are well documented, and I believe there was a factory "fix." The traditional "rule of thumb" for tip clearance is around 15% of propeller diameter. Many boats with tunnels reduce this clearance because it increases thrust by creating a partial Kort effect making the boat a little faster. It also causes propeller driven vibration and sometimes "tunnel burn" where some bottom paint just won't stay on the bottom in the vicinity of the prop.

I found this is a quick search along with plenty of other sources:

BoatUS - Boat Reviews - Mainship Pilot™ 30

"In its first year of production the Pilot™ 30 encountered noise and vibration problems particularly on boats equipped with larger Yanmar engines. The cause seemed to be insufficient propeller tip clearance and was addressed and repaired under warranty by Mainship."

Propeller change by adding a blade and reducing diameter or prop shop modification of diameter and pitch are two methods. Many also suggest "keel fairing" where the back of the keel is modified to a taper instead of being "square" to "clean" waterflow to the prop. These have been mentioned above and are sound advice.

Concrete is an old school method of adding mass over the propeller to reduce propeller induced vibration. The modern concrete replacement is known as "Navy Tiles" which absorb vibration. I consider this a band aide as it does not address the root problem, tip clearance.

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Old 10-09-2017, 07:46 PM   #6
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Not so ...
The "problem" is vibration.
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Old 10-09-2017, 07:56 PM   #7
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Just curiuos is it a new condition, cutlass worn?
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Old 10-09-2017, 08:31 PM   #8
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As clunky as it is there is a massive amount of information available on the Yahoo Mainship site about this well known feature. The Yahoo site has been around since the late 90's and the search feature works pretty well.

https://groups.yahoo.com/neo/groups/mainship/info


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Old 10-09-2017, 09:31 PM   #9
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Quote:
Originally Posted by Nomad Willy View Post
Not so ...
The "problem" is vibration.
If the cause of the vibration is insufficient prop tip clearance, then I'd rather eliminate the cause than mitigate its effects. One is a cure, the other is a method to mask this effect.

No telling what wear this is vibration is also causing to other running gear components like cutlass bearings, rudder posts and props.
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Old 10-09-2017, 09:52 PM   #10
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Al,
If the problem is vibration from prop blade tips being too close to the hull right above the prop then if you can stop the vibration the close blade tips are no longer a problem.
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Old 10-09-2017, 11:08 PM   #11
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But you're not stopping it. You're trying to counterbalance the worst of it through the hull after it has already passed through portions of the hull. It's the remnants vibrating through the rest of the hull/components that still have an effect.

If you stop the source, you don't have to be worried about the rest.
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Old 10-10-2017, 05:21 AM   #12
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thanks all, just bought the boat in july been there since will address this next week when i haul out for the winter , again thanks
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Old 10-10-2017, 05:31 AM   #13
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The hassle is GRP construction is easy to create strong enough , but difficult to create stiff enough.

IF concrete stiffens the vibrating hull enough to be silent,, why not?

Its way cheaper than a 4 or 5 blade prop which may not solve the hull flexing .
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Old 10-10-2017, 12:05 PM   #14
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The Pilot 30 is a Downeast planing hull. Concrete may be an old school fix for a full displacement hull, but the additional weight so far aft would probably have a negative effect on trim both static and underway.

I've been in plenty of old Feadships and Burgers with concrete in the bilge. In my own humble opinion for this vessel concrete would not be a good solution.

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Quote:
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The hassle is GRP construction is easy to create strong enough , but difficult to create stiff enough.

IF concrete stiffens the vibrating hull enough to be silent,, why not?

Its way cheaper than a 4 or 5 blade prop which may not solve the hull flexing .
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Old 10-10-2017, 01:25 PM   #15
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Keys,
The weight I was talking about would only be for material over an area perhaps only about 1 sq ft. It's hard to express oneself accurately on forums. I get wordy enough as it is.

But re this problem prop clearence is golden.
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Old 10-10-2017, 01:58 PM   #16
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The logical start point is to have the propeller, shaft, bearings and alignment checked.
Having just bought the boat in July, you have no way of knowing what's gone on down there in the past.
Groundings, running over debris, even mishandling on shore may have occurred.
If all's well along the drive train, then it's time to look into some of the above recommendations.
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Old 02-10-2018, 05:44 AM   #17
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Called marlow, they wouldn't give me any information, perhaps they think i want something for free not the case i do believe a five bladed prop may be the solution i just want to get it right the first time
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Old 02-10-2018, 06:55 AM   #18
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Marlow probably doesn't really know anything since they were not around when the 30 was designed and built.
Listen to Keys he's been around Mainships and the owners groups for a very long time. This is a well documented problem with that model.
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Old 02-10-2018, 08:04 AM   #19
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does everyone responding have Gerr's book?

If the cutlass is not a mess, and the prop is balanced, what you have is the blades causing high then low pressure pulses against the hull. The closer the tips, the thinner the hull, the more the vib. The vib is the hull flexing.
He recommends 15% on 1000 prop rpm and 10% can work from 300 to 1800 rpm. But those are guides. He admits pushing the dia further has benefit for low speed handling on low speed craft, but will increase the tendency for noise.

As reported before, you can make up some loss of dia with more blades, Gerr also recommends a bit of skew.

Of course, you could stiffen the hull in that area. Doesn't necessarily need lead, concrete, etc. We are in the modern age, with cored composites that might add enough stiffness to keep the vib and noise down. Sounds highly experimental to me, depending on the complexity of the inside surfaces involved. Certainly lightweight methods could work, my earplugs don't contain lead.
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Old 02-11-2018, 09:57 AM   #20
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A good prop shop will let you try different props if their first recommendation doesn't work.
I do agree with checking everything else before assuming the prop needs to change.
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