Engine Vibration or prop?

[ontherocks83]

Last year I removed my 1984 Universal M25 with 690 hours on it (boat was new to me). I sand blasted/painted it, replaced the exhaust riser, impeller, coolant pump, engine mounts, Propeller shaft, prop shaft flange, put on a balmar 120 amp alt, upgraded the pulleys etc.

The engine runs great and I typically cruise between 2200 and 2500 rpm in the 6 knot range.

When I go to full power (approx 3000-3200 rpm) I get a bit of vibration through the boat. I thought at first it was the prop shaft alignment, but I was also wondering if it could be the engine vibrating through the mounts.

My reasoning is that when sailing and the trans is in neutral, we have been doing 7.5+ knots at times and if I go down and open the engine hatch, the prop shaft is spinning away and it is perfectly smooth and quiet.

Would it make sense that if the prop shaft is spinning while under sail and there is zero vibration that it wouldn't be a shaft/prop/alignment issue and more of just the engine transferring vibration through the mounts?

[kenbo]

Has the prop been either statically or dynamically balanced? When you installed new engine mounts did you size them correctly and is the rubber too soft, sometimes when the rubber is too soft on a flexible mount the alignment can go out due to torque rotation. Not having vibration when the prop is freewheeling means very little. At 3000 rpm your shaft is turning at 1500. I doubt freewheeling gets anywhere near that speed. Did you renew the cutless bearing as well?

Before you removed the engine did you run it up to the same rpm? How was the vibration then? What is the prop's tip clearance from the hull? Should be minimum 15% of the prop's diameter, 20% is better. The C30's running gear was originally sized for a high reving Atomic4, the switch to diesel later created a bit of a prop sizing problem.

Lot's to consider when chasing vibration.

[ontherocks83]

It's the original prop, but I wasn't able to run it up before I removed the engine. It sat on the hard for 7 years and I brought it straight home to work on over the winter. The cutless bearing is new.

[Stu Jackson]

I have an 86 M25 in my boat. Have you tried sailing with the transmission in reverse (NEVER fwd)?


https://www.c34.org/faq-pages/faq-tr...n-sailing.html


I do that to avoid wearing the cutlass bearing even further.

[Cat36Mahalo]

Ontherocks83 I’m interested in your new pullieys - is this the Balmar serpentine upgrade or other - interested cause that year of M25 shouldn’t have the mounting holes they require.

[Cadence]

Originally Posted by ontherocks83

Last year I removed my 1984 Universal M25 with 690 hours on it (boat was new to me). I sand blasted/painted it, replaced the exhaust riser, impeller, coolant pump, engine mounts, Propeller shaft, prop shaft flange, put on a balmar 120 amp alt, upgraded the pulleys etc.

The engine runs great and I typically cruise between 2200 and 2500 rpm in the 6 knot range.

When I go to full power (approx 3000-3200 rpm) I get a bit of vibration through the boat. I thought at first it was the prop shaft alignment, but I was also wondering if it could be the engine vibrating through the mounts.

My reasoning is that when sailing and the trans is in neutral, we have been doing 7.5+ knots at times and if I go down and open the engine hatch, the prop shaft is spinning away and it is perfectly smooth and quiet.

Would it make sense that if the prop shaft is spinning while under sail and there is zero vibration that it wouldn't be a shaft/prop/alignment issue and more of just the engine transferring vibration through the mounts?

Makes sense. By full power what do you mean? I wouldn't run at over about 75%.

[Pitchondesign]

There are so many possibilities. In my case, it was the injection pump causing vibration starting at about 2500rpm. Had the pump overhauled and the vibration stopped. Apparently, uneven fuel flow at higher rpm caused the engine to vibrate. I had never run into this before.

[AA3JY]

Shift zinc?

[dennismenace111]

2 or 3 blade prop? My old Yanmar acts similarly! I think it has alot to do with the 2 blade prop. When the blades turn vertical and are in disturbed water behibd the keel then rotate to cleaner water might have done vibration input!

Just my wild assed guess with absolutely no scientific basis what do ever!

That's my story and I'm sticking to it!

[Cheechako]

Some times that happens just because of the turbulence where the prop comes closest to the hull. And most boats have a sweet spot where everything is smooth and a spot where you get more vibration and turbulence.

[Searles]

Depending on the prop tip clearance ,at least 10pc of prop diameter and the amount of shaft outside the prop skeg to the face of the prop,max 1.5 dia I would bet on cavitation ,most yachts should keep shaft revs below 1200 . Just saying .⛵️��

[Nyanza Cav32]

I was just about to suggest cavitation, but see that I've been beaten to the punch. If your prop is bent (new shaft, so that would seem an unlikely cause) then you would be getting some vibration even at lower revs.

Most sail boat engine/prop setups are a compromise design, aiming for minimum drag under sail, often with less than optimum clearances from the hull or from skegs etc, and the pitch/design characteristics of the prop itself are usually aimed at providing reasonable drive at cruising revs, with highest pitch possible to reduce drag when not powered (I'm assuming yours is a fixed bladed prop). They are not designed to operate well at higher revs, cavitation is often a very real issue (I get a bit of it in my Cavalier 32 when pushing hard), but no one sits for long on max revs, so it isn't generally a major problem. Also, I see your boat is a Catalina 30 (is that right?), I guess your static LWL would be about 25'...so your hull speed (theoretical) is about 6.7 knots. When you're pushing along under engine power, the actual LWL will increase somewhat, but your theoretical hull speed still won't be much over 7 knots. When you try going above that speed, the drag increases dramatically, so the prop will be forced to bite/work even harder, which also helps promote cavitation. The boat will do it, but under protest. Do you get the sense that there are slight RPM fluctuations when you are experiencing the vibration(s) (like the prop is biting, slipping, biting etc)? If so, that's typical of cavitation. If you find that your engine is not quite able to achieve its max rated RPM at full throttle in forward gear, which is quite common in many sail boats (assuming the engine is ok)...that's a pointer that the prop is pitched up a bit higher than it ideally should be, so as to minimise its drag when under sail. If that's the case, some cavitation is likely to occur at very high revs. It isn't a problem just something to be aware of.

Your question regarding the prop turning freely without vibration or noticeable shaft movement when you're under sail, in neutral...that proves nothing really, if there were any wobbling/oscillations/vibrations in that situation, then you would have a significant problem, in which case you would experience vibrations under power even at low revs.

My apols at the wordiness of this response, hope it makes a bit of sense.

[dennismenace111]

Originally Posted by dennismenace111

2 or 3 blade prop? My old Yanmar acts similarly! I think it has alot to do with the 2 blade prop. When the blades turn vertical and are in disturbed water behibd the keel then rotate to cleaner water might have done vibration input!

Just my wild assed guess with absolutely no scientific basis what do ever!

That's my story and I'm sticking to it!

The other issue is harmonics in the engine itself. If youve ever seen an old radial aircraft engine you will note that they always have an odd number of cylinders..

Wikipedia has a great explanation...

https://en.wikipedia.org/wiki/Engine_balance