saildrive noise

[phildy]

I have just fitted a new yanmar 3YM to my existing SD 20 saildrive and notice that I get a disturbing rattle and grinding noise when motoring into steep choppy seas. This didnt occur with the original 3GM engine but that may be a red herring. I was told that this was probably a cavitation problem and had the prop repitched to accomodate the increased torque of the new engine but the problem persists. At normal cruising revs this is not a problem but when you need to punch into a steep sea it most certainly is. The noise,to my inexpert ear, sounds definitey mechanical rather than cavitation. Any thoughts would be appreciated

[Wanderlust]

I would suspect a soft bush in the propeller, allowing it to move forwards enough under power to contact the saildrive.

I had this happen recently with a new propeller. With low revs there was no sound, but as more power was applied a scraping/grinding noise could be heard.

A propeller repair shop said it looked fine, but I asked for it to be rebushed and that fixed the problem.

[phildy]

Thanks Steve for your reply. My propeller hub contains a rubber shock absorbing bush that is pressed fitted inside the hub housing. Does this sound consistent with your analysis? Where exactly was the chafe point as I could see no sign of external wear on the hub housing when I removed the prop. Had the prop back with the manufactures who did not replace the bush. Naturally do not to unnecessarily start dismantling the sail drive if that is not the problem

[Wanderlust]

Yes that rubber bush is what I had replaced. It was a new propeller and the noise developed after maybe 20 hours of motoring. No noise on low power, neutral or reverse.

With the boat hauled, I saw there was plenty of clearance between prop and saildrive housing. Like you I had thought it was internal in the saildrive so wasted time and money getting that checked. With no fault found, I had a closer look and found the antifouling had been worn away on the rim of the propeller hub.

The propeller repairer said that the bush looked perfect and was unlikely to be the cause of the problem. Fortunately I ignored that advice and had it replaced. He was able to offset it a little to give a little extra clearance.

In your case, perhaps the extra power causes the bush to distort just enough for contact to happen.

[phildy]

Many thanks for your input Steve. Sounds like you almost certainly have the answer. Like you I was on the verge of pulling the saildrive apart as I had thought the prop could be eliminated as a cause. I have no recollection of wear on the leading edge of the prop hub where it meets the drive but will certainly look more closely. As you appreciate this is not a cheap exercise as it involves slipping the the boat each time which becomes a bit of a heartbreaker when you discover the problem has not being solved. Once again the diagnosis of the 'experts' is not definitive as there is no substitute for personal experience which forums like this provide. I will take your advice. PS the grinding rattle is a most disconcerting noise that suggests things are about to disintergrate and forces you to throttle back when you most need the power