Possible Propeller Issues

[Lawrencemd]

I have a 28' trawler style boat with and inboard engine. The propeller is a 20" composite with adjustable pitch. I've actually never seen a prop like it.. Anyway, the issue we are experiencing, is that at a higher than normal rpm, like around 2200 rpm, we are getting a bad shudder in the prop and shaft. Up until that rpm it spins perfectly. We normally don't run the boat at that RPM anyway, as it gets along at about 6-7 knots at around 1800 rpm. But when we need to, we get this shudder. We have checked the jack shaft and steady bearing and both are in excellent shape with plenty of grease. The only thing we can think of is that the prop is wrong or adjusted incorrectly. The prop was originally setup for a small diesel but we have since replaced that engine with a 250 6 cylinder chev. We are entering the thought of replacing the prop with a 4 blade non adjustable but are thinking maybe an adjustment to the pitch may solve the issue. Pics are attached.

[MikeJohns]

Lawrence
I'm sorry but there is no simple advice for you.

The prop is too far from the bearing there shouldn't be that much bare shaft.
The shuddering is most probably shaft 'whirling'. But with these sorts of problems we need a lot of observations to nail down the cause.

I'd recommend you borrow a copy of The propeller handbook by Dave Gerr and give it a good read.

[daiquiri]

Anything can be happening down there Mike - whipping, pressure-induced vibration, cavitation-induced vibration, torsional vibration, a play between the shaft and the sleeve etc. More info would be absolutely necessary, like:
- shaft diameter and material
- propeller pitch and weight
- distance between the prop and the bearing (definitely looks too big)
- distance between the prop and the rudder (definitely looks too short)
- distance fro the tip to the hull (looks too small)
- is there a wear in the shaft bearing
- speed at which the vibration happens.
Only with these can we try to figure out what happens.
Cheers

[philSweet]

And reduction. Gearbox? Jack shaft reduction? what RPM are those, motor or shaft?

[CDK]

Variable pitch props can easily show slight differences in pitch between the blades, causing the prop to wiggle.
In this case there is a very long shaft inside and a rather large distance between prop and bearing. Apparently around 2200 rpm resonance sets in, with the largest amplitude between the bearing and the stuffing box. The large mass of the prop's hub amplifies this.

A precise readjustment of the blades should correct it.

[baeckmo]

.........and the propeller is too close to the rudder. There will be pressure pulses setting up vibrations. Just for a start, move the shaft forward closer to the bearing.

[Frosty]

Originally Posted by baeckmo

.........and the propeller is too close to the rudder. There will be pressure pulses setting up vibrations. Just for a start, move the shaft forward closer to the bearing.

If he put the anode on the shaft as he should have it would look better.

Hes got an inch or so to play with, that might be enough to back it off from the rudder.

If all three blades are pounding the rudder it would be a much higher frequency than just one blade being out of pitch.

[Lawrencemd]

Thanks for all the suggestions. I think our first step will be to pull that propeller away from the rudder, closer to the boat either by shortening the main shaft or having the jack shaft shortened. The jack shaft (with 2 universals) would be easier, as it would simply pop out, be shortened by three inches, reinstalled and the main shaft could be pushed in about 3 inches.

[daiquiri]

Originally Posted by Lawrencemd

Thanks for all the suggestions. I think our first step will be to pull that propeller away from the rudder, closer to the boat either by shortening the main shaft or having the jack shaft shortened. The jack shaft (with 2 universals) would be easier, as it would simply pop out, be shortened by three inches, reinstalled and the main shaft could be pushed in about 3 inches.

The prop tips also appear to be too close to the hull, and will get even closer by shortening the shaft. So it is imho possible that you'll have to opt for a smaller diameter if the vibrations continue, or if signs of cavitation-induced erosion appear on the hull above the prop.

[whitepointer23]

7 knots would be about your top speed looking at the pics , 2200 rpm is probably overloading the blades and cavitating.