Hi All, i hope some one can help me eliminate some options, I have a 1987, 40ft Beneteau, about 8 months ago i noticed a vibration that appeared to be coming from the prop shaft end of the boat, when the motor was opperating between 1500 to 2200 revs the vibration was worse and over time became louder and louder, Last week i got the boat taken out of the water and had both the skeg bearing and what evver the bearing is, were the shaft goes through the hull, ( i know what its called i just cant remember at the moment) The guys that did the work suggested that i should get the shaft measured and straightened if nessesary, so it was sent off to an engineer and supposidly straightened, they also said that the engine was possibly not correctly aligned so they realigned the motor,(christ these guys know how to charge)anyway took the boat out on Sunday and low and behold the vibration is still there, all be it a hell of a lot less than what it was, around 1200 revs it is sweet, 1600 and i can hear and fell a slight noice, but give it full power 2400 revs and there is definatly a vibration, which will only get worse as time goes by, im thinking the prop could be out of ballace only because there isnt much else to left, I would love to hear any suggestions or thoughts as what else coulld be the problem. Happy sailing.
Ben 405

A 1987 boat!! She is getting long in the tooth so it may be time to go the whole hog.

Shaft out, check wear areas, straightness, balance, coupling and prop taper.

If too many things are wrong, get a new shaft.

When the shaft is right, fit the propellor, recheck balance and pitches of the prop blades.

Check attachment security of the P or A bracket, replace P/A bracket bearing and stern tube bearing. Refit shaft, prop and coupling. Replace engine mounts if necessary. Conduct an engine to prop shaft alignment accurate to .003". That is three thou of an inch, closer if you can get it there.

Mostly these alignments are in vain if your flexible engine mounts are soft.

If you still have vibration, rip the engine and gearbox out and start on them.

$10,000 or $20,000 later you might have gotten rid of the vibration.

Get a properly qualified marine engineer on the job. The forum is not necessarily the best place for advice on these kinds of problems.

A skilled and experienced marine engineer could probably pin point the problem by ear.

I would suggest next time you are slipped, remove shaft and prop and visit Porters in Parramatta. Have the prop tuned and balanced and the shaft straightened. Make up a wooden frame to transport the shaft and treat it carefully. Keep hammers away from the prop and shaft when removing.

The knocking niose is comming from under the cockpit close to the propeller, the stern gland was replaced but is leaking (didn't time the drips) and I think the cutless bearing was replaced. The knocking sound ( and the more I think of it it's a knock not a vibration)is metalic in sound and Like I said comming from under the cock pit.
Is it possible that the prop has been placed back on the shaft in a different position thus out of balance?
Keith

Hi guys, thanks for the great feed back and thoughts that you have sheared, i will go for a swim and see what i can, as i know ive been charged for new annodes that were put on the shaft, so ill check they are tight, if i can i might remove the prop and take away and get it ballanced, again thanks for the great advice and comments.
Ben

I'm having similar vibration problems.
After a week or two away I get vibration from the shaft and prop when I engage forward. It is similar, although not as bad, as when one blade of a ungeared folding prop doesn't come out.
I have a brand new Gori two bladed geared folding prop covered in prop speed. I've done a stethoscope (long screw driver on the gearbox and held to the ear) test on the gearbox and it sounds fine.
After a few goes it comes good and when I have been running for awhile there isn't a problem engaging forward.
I do have a bit of vibration from the shaft when underway but it always has, it is a pretty long shaft, about 8 feet, that is only supported by the stern bearing and gland so any vibration is accentuated. It isn't viable to change the shaft setup.
A Navy marine engineer felt it could be a bit of shaft drooping in the long unsupported section and it disappears once it has been spun up and self centres.
Has anybody else got a similar experience?

Ramona said...
I have the same thing with the folding prop and just engage neutral then put it into gear again with plenty of revs. Bit of crap in one side of the gears. One try usually clears it.

Sounds like you have separate engine and gear box controls. That is my preferred set up.

When docking, single lever controls can be disasterous if the gear control cable happens to fall off. Ouch and $$$$$.

I am not saying that flexible shaft couplings are bad. They are there to absorb minor differences in static alignment that will become greater and/or amplified in dynamics and under load.

What I am saying is that the closer the alignment, the less absorbtion work the coupling has to do which equates directly to lost horsepower.

Quote:-

Poly Flex flexible disc transmission couplings provides a damper between the gearbox and propeller shaft to isolate torsional vibration from the engine and gearbox. It also reduces the shock of forward and reverse gear changes. Our couplings work in conjunction with the engine mounts by providing the softest element in the shaft assembly to improve performance.

End quote.

They say nothing there about the coupling compensating for misalignment.

Quote from the Poly Flex installation instructions that came with my coupling:-

Step 2: Check alignment of propeller flange to gearbox flange by using a feeler gauge. Adjust engine to compensate for ANY misalignment. (Maximum industry standard for misalignment is .005" between flanges.)

End quote.

So I am not just giving an opinion here. This is standard marine engineering practice. The maximum allowable tolerance is .005", so an alignment to .003" is fairly average and only just tolerable.

My S&S 34 is 44 years old and the engine is solid mounted and I doubt it has ever been disturbed since the day it was launched. The gearbox to shaft coupling is metal to metal and the stern bearing to shaft clearance is less than .010". I can turn my propeller with one finger.

We used cigarette papers (.0015" thick) as feeler gauges on drive shaft alignments.

I hope this is of help and clarifies the situation. Flexible couplings and mounts are not designed to compensate for sloppy workmanship.

Prop would have a taper and a key, take real skill to get it arse about!

keithw said...
Why is the prop being to large a problem?

If too large for the engine then you would never reach the optimal torque curve where you get the most economical running, plus the engine would be effectively labouring and that surely aint good for longevity.

It would have effect on vibration if all else was in tune and balance with the universe.