Vibratios Hunter Legend 37.5

[Gil]

Hey guys,

I have a hunter legend 37.5, 1991 for a few years now.

Since day one I'm experiencing very notable vibratos at various RPM.
Here is a partial list of work was done regarding:
1) Replaced all four engine mounts and filled the engine beds (fiberglas).
2) Replaced with new couplings and shaft.
3) Replaced cutlass bearings few times.
4) As the vibrations damaged the transmission, I also got new transmission.
5) Checked for alignment between engine (transmission), shaft, strut.
6) Replaced the 2 blade prop with a 3 blade one and back to a brand new 2 blade prop.
7) Made numerous tests with streamline zinc, change places, two zincs etc...

The engine is a Yanmar 3 cylinder (original engine).
When not in gear, there are no vibrations.

After all this work, to minimize vibration on gear, engine needs to run between 900 to 1,100 RPM and then over 2,000. Anywhere lower than 900 or in between 1,100 to 2,000 it vibrates so much the shaft bangs agains the channel.
In reverse I get same results.

Interesting fact, I could not find online any other Hunter legend with vibration problem...

 

[Ralph Johnstone]

3) Replaced cutlass bearings few times.

What is the distance between the aft end of the cutlass and the prop hub ?

5) Checked for alignment between engine (transmission), shaft, strut.

What diameter is the coupling and what was the figure for axial alignment when you were finished ? Sorry, had to ask this as some people figure eyeball is "close enough".

I see you're in Cabo San Lucas. Thinking outside the Twilight Zone here, any chance of removing the prop (no haulout required) and run the engine in gear at the dock to see if the shaft only is the cause of the bad vibes. The more data, the better the chance of finding the problem.

Let's see what fuel others can add to the fire.

 

[Gil]

Thanks for replying.
Of everything I already tried doing, never thought about taking the prop off and running it without...
I'll have to measure both distance and diameter to give you a good answer.
Someone already gave me an idea of adding a second strut.
Another this I was thinking was adding a Globe Drivesaver between the two flanges.

 

[Ralph Johnstone]

Someone already gave me an idea of adding a second strut.

If one properly placed strut doesn't do its job, two is doubtful at best. Unless the prop is flopping around and the "someone" thinks that's got to hold it in place. With all the work and observations you've reported so far, it's not loose, is it ? I knew it wasn't .

 

[Gil]

You guessed right. We're down here in Cabo
Prop is secure, I've replaced few... Could not dive under to measure the distance, but it is fairly long for a 1" shaft.

 

[Ralph Johnstone]

but it is fairly long for a 1" shaft.

Now there's there's an absolutely definite MAYBE to look at. There is a maximum length/diameter ratio which should be referred to, so this may a new point to start looking. As you've said above, you've replaced everything else down there so may as well look at this next.

If you're unable to dive under the boat, do you have a way of taking an underwater photos (within a foot or two) of the prop strut assembly ? From the photo, you can estimate the length between the two using the shaft dia. and go from there.

Keep us up to date.