TORSIONAL VIBRATION in marine propeller drive line

[BBoat]

Bad vibration in 45" 22000 lb boat at 2600-2900 engine RPM
480 hp diesel engines 3300RPM max 2.00 to 1 reduction transmission vulkin coupling between engine and trans has been fine in other installs. We have tried five and six blade propellers. Six is better even thought the blade pulse with the water is exactly overlaping the 6 cylinder engine pulse. The drive line consists of 6 cyl engine Vulkin damper ZF trans flange yoke to 1710 dana spicer U joint 65" to second U joint properly phased to a bearing that takes propell er shaft thrust to the boats hull. Drive line is at 6.3 degrees ok by Dana. (CV joints do not fix the problem) From bearing to splined surfacing propeller is a 2" shaft 57" long.
I am looking for tube in tube drivline dampers or tricks in u-joint phasing that might offset the vibration. peaks show at 120,160 and 320 HZ. any ideas?

 

[rob768]

First, perform a torsional vibration calculation to identify where the problem lies and IF it really is a torsional vibration you are suffering from. Basically, you will NOT feel torsional vibrations, since they occur without significant outward reaction forces. If it are torsional vibrations, and you notice little difference between a 6 and a 7 bladed propeller, the excitation forces are probably engine-related, which means changing the rubber element of the Vulkan coupling may be a solutiuon. Don't go through a trial and error proces: perform that calculation (or have it done). The costs are low, relative to the damage or hinder you might prevail.
My guess is however, that you have a resonance problem with the engine or gearbox on its mounts. Just feel with stretched finger where the highest vibration occurs.

 

[BBoat]

Thank you Rob
We had acoustic testing Saturday and I hope to see the results perhaps today.

ZF has a large facality here and that is where the couplings came from so I will heed your advise an consult with them.

I played with the boat this weekend with six blade props.
The vibration moves around the boat as the engine RPM changes. From 2600-2900 it is bad, hatches buzz you can see the floor move. I have a high quality strobe and watched the drive lines for a long distance at the worst vibrations. They are smooth just a tiny movment at the thrust bearing can be felt but it is a fraction of the floor movment.

 

[rob768]

Acoustic testing is not very accurate with regards to the lower frequencies. Better is to perform some measurements with an accellerometer. In general, with vibration problems, use accelerometers.
The drive shaft is not a cardan shaft is it? If so, check the yokes (should be in the same plane) and the in and out angles (should be identical)

good luck.