Horrible Hum in Driveline. 1984 61My

[scottinsydney]

This afternoon I took the boat for a 20 mile run at 1100rpm. Boat had just been splashed yesterday from antifoul. No issues in departing. On arrival I brought back the throttles to idle and a loud hum resonated through the entire boat. Investigation shows noise coming from starboard engine side. Only in forward and between 650 and 750rpm. No problem changing gears. Gearbox pressure fine. No rough running. Revrrse fine. No noise in neutral rev up. . Hum goes away when neutral selected from forward and above 750rpm. Reoccurs precisely at 750pm and lower. I am wondering if a dry packing gland or worn shaft bearing or the yard antifouled the water tunnels in shaft bearings. Thanks in advance.

[Pete]

Antifouling in the water channels of the cutlass shaft bearings sounds like a prime candidate to check.

Pete

[SEVEN]

Did you have the shafts repacked? If so feel them for heat or wobble. If not I agree with the comment above. Hope it’s an easy fix.

[saltshaker]

If you have shaft zincs one may have come loose and slid down to the strut. They will get wedged in and make a sound like you are describing at low speed

[Tim Powell]

If you have shaft zincs one may have come loose and slid down to the strut. They will get wedged in and make a sound like you are describing at low speed

Grate reply Jack it is easy to do as simply as just not tightening the zinc.
Rattles and odd noises really really bother me. A rattle in my vehicle will worry me to death one good thing the older i get the less i hear.
FYI i purchase from one of the part houses a mechanic stethoscope works grate able to pinpoint noised about 10 bucks. A good investment for a rattle freek. They hang in the closet out of the way but accessible
After the BS now for the noise. I assume it was not there before going to the hill. What did they do any service work check transmission oil. Did they service packing on the shaft is it to tight if so feeling of it while underway if it is hot to tight. When i have a noise and isolate it to one side or the other i shut down the engine that
i feel is not making the noise helps isolate the problem. I get my trusty stethoscope and go to work it will pin point the noise quickly.

[SKYCHENEY]

I like the zinc idea as well; would make sense. But were the props touched? I had a set of props once that gave off a weird resonance at low rpms.

[Boatsb]

I like the zinc idea as well; would make sense. But were the props touched? I had a set of props once that gave off a weird resonance at low rpms.

could be the sharp edges and cup making the noise.

[scottinsydney]

Thanks. Anodes is interesting thought. Yes replaced. What are sharp edges a cup please?

[Cricket]

The ends of the propeller blades and or the cup on the ends of the blades, sometimes if they're too sharp they will sing. My guess is either the zincs or paint in the cutlass bearing.

[scottinsydney]

I emailed the slipway owner the earlier today and presented the points in the forum thread.He wrote back immediately and said that it was quite common for large boats that have been on the slip for an extended period of time to have the shaft bearings dry out slightly. Try it again in a few days time.

As this is the slipway that most large boats in our area use I'll take him at his word, and see what happens.

I was also presented photographs of the bearings taken before the splash, showing that the waterways were clear of antifouling. He said bearings are always checked when a boat is slipped for antifouling and again before it goes back in the water.

During my investigation of the gearbox coupling this morning I found two hoses resting on the 6" long output flange of the gearbox, the result of the stabiliser installation and probably my fault. I have moved these away. One was connected to a copper pipe. Perhaps that was the issue.