I can get harmonic vibration of my cockpit lazarette covers at just the right RPM, but it is the same whether underway or at the dock.

I'm trying to figure what is different about moving through the water that would account for your observations and coming up blank at this time.

My next step would be to replicate the sound, then with someone competent at the helm, snoop around the boat until I identified the source of the noise.

A friend of mine has a 43ft powerboat with twin engines and one of the twins does this exact same thing. I thought it might be the cutlass bearing.

I get this kind of noise when I'm sailing above 5 Kts SOG with the transmission in neutral and the motor off. It stops if I put it in reverse. I don't get the noise when I'm motoring.

Do you get any noise when your sailing with the motor off? If so, it may be the transmission.

You should always keep your transmission in reverse when sailing to prevent the prop from spinning, which can damage the transmission, and is noisy below. Your noise is likely coming from some slack in the cutless bearing or perhaps from the packing gland, either of which explains why it only occurs underway. Other possibilities are a dry bearing, but this would occur all the time. The forces on the shaft/prop are quite different if tied to the dock. We had this and eventually decided there was no easy fix. Ours is the dripless gland, and it was properly lubricated. We suspect the bearing, but there's not enough annoyance (who motors at 1200 rpm?) to justify the fix. Deising is right - use a rubber tube, and hold one end to various parts of the boat while the noise is happening, and listen to the other end of the hose. This helps isolate the source of the noise (and is way easier than trying to get your ear anywhere close to the source!) Good luck!

Two possibilities that have not been mentioned for causing a high pitched sound.

First, if you have a PYI dripless shaft seal, under some conditions a little carbon can transfer to the stainless steel sealing surface, causing noise. I recently replaced my old seal, and in talking to the people at PYI they mentioned that this sometimes happens, and that sanding the face of the seal with very fine sand paper will stop the problem.

Second. I recently read (I cannot remember where) about a person that had a problem with his prop whistling after he had it repaired. He took it back, and the prop shop said that the prop edges were too sharp and that this sometimes causes noise. They dulled the edges and the noise went away.

Good luck in solving your mystery.

Gary Teeter
1989 C36 Hull 966 "AnnieG"
Everett, WA

I had the same problem.
It turned out to be the PYI PSS dripless stuffing box.
And easy check is to burp the stuffing box ( pull the puck back and let a little water in ) and see if the noise goes away. What is happening is the water is sucking out of the log and the stuffing box is runing dry and also gets quite hot!
I phoned PYI and they sent me a new carbon puck with a vent barb on it, to which I installed a vent line which I ran up through the port cockpit locker.
End of probelm.
Cheers Wilf

John, it could still be your stuffing box. Check while powering that there is a slow drip from the box, a drop or two per minute, to lubricate the shaft. If the stuffing is too tight or dry, it can cause a squeal at certain shaft speeds, and will score the shaft. The cutless bearing can only be checked while the boat is hauled. Grab the shaft near the strut and rock it back and forth and up and down. There shoud be minimal play, less than a millimeter or so. If more, the bearing should be replaced. A loose bearing will usually cause a thump or hum while powering, since the shaft can rock back and forth, and is hardly ever perfectly straight and balanced. If the noise you are hearing is not changing as you power along, that is, not getting worse or louder, then their is likely no reason to worry, and you can check it at your next haulout. If it's merely annoying (like ours) at one speed, speed up or slow down slightly.

we have Catalina 36 Mark II 2000 - last year had noise at end of season we had the cutlass bearing changed. First out this year no noise - the noise has suddenly re-appeared ... does anyone else have this problem. It sounds like
a banging under boat...

Pat,

This sounds rather familiar. Could you give a little more detail i.e. what is the damper plate. Is there an easy fix?

[QUOTE=bunny;6102]we have Catalina 36 Mark II 2000 - last year had noise at end of season we had the cutlass bearing changed. First out this year no noise - the noise has suddenly re-appeared ... does anyone else have this problem. It sounds like
a banging under boat...[/QUOTE]

This could very well be shaft whip hitting the stern tube/shaft log. If the mounts have sagged the shaft gets real close to the stern tube/shaft log and if there are any alignment issues or prop issues you'll develop shaft whip and it will hit the inside of the fiberglass shaft log. Shaft whip is like a jump rope where once it gets going it is hard to stop. When hauled out the shaft should pass through the cutlass and stern tube perfectly centered.

[quote=windward1;6104]Pat,

This sounds rather familiar. Could you give a little more detail i.e. what is the damper plate. Is there an easy fix?[/quote]

Damper plate is at the back of the engine, it is the interface between engine and transmission. Not real easy to replace, have to take the trans off. There are aftermarket companies that make damper plates that use something other than springs, I believe that PYI makes one type. Supposedly quieter than the spring type.

Our last boat (M3-20) and our current boat both had the damper plate rattle at idle RPM's and it progressively goes away as the RPM's go up to cruising speed. I have no plans to do anything about it unless it starts getting worse. I am told that if a spring falls out you will hear it grinding away in there and it will be clear there is a big problem and it's time for a new damper plate.

Maybe you got a bad cutlass bearing and it has died? If it was really loose you likely could move the shaft in the bearing even with the boat in the water. I check mine all the time whenever I go for a swim I put the mask and snorkel on and check the rudder and running gear and have a look at the thru hulls and so forth.

Thanks for the replies. I have about 950 hours on the engine. The noise in gear at idle is minimal; at about 1200 rpm there is a lot of rattling; over 1600 it is pretty quite. The cutless bearing seems to be very tight when I check it on the hard. I just keep the rpm at a quieter speed.

I have what I believe is damper plate noise, but it is only at idle. At any speed just above idle, it goes away. If yours appears at 1200rpm I would guess it is some other vibration. Something is out of balance.