Rattling/knocking noise

[adamf]

I went out on my SO37 and motored in the river for about 30 mins. Having started to sail using the Genoa I shut the throttle and put the gearbox into neutral. As I did this, a rattling/knocking noise started from the rear of the boat. I went below and checked the engine and gearbox and confirmed that this was NOT the cause. The noise was definitely coming from under the stern of the boat and not inside the boat. The next thing I did was to turn the engine off altogether. While still sailing, the noise was still evident with the engine turned off but with the gearbox in neutral. If I put the gearbox into reverse (locking the prop while sailing) then the noise goes. I have no problems with power and drive in both forward and reverse and so I am sure it is not the prop itself which is a fixed three blade type.

My thoughts are that it could be caused by the prop shaft anode. This was changed by the broker when I bought the boat back in late April but perhaps it has come loose on the shaft.
The “P” Bracket Cutless bearing was also replaced by the broker at the same time as the anodes were replaced.
I am interested to see if anyone else had similar experiences or any other ideas as to what it could be please. Any advice would be most welcome.

[Zanshin]

I think that you've nailed the cause of the noise on the head, I've had my anode on the shaft come loose before as well (but without the noise) and since it needs a tight connection in order to function you'll have to try to go over side and below with an Allen wrench sometime soon. I hope that your aren't in cold or murky waters...

[peterjb]

I agree with Zanshin. I've had exactly the same symptoms as yours and they were caused by the prop shaft anode becoming loose, then sliding down the shaft to rub on the p bracket/ cutlass bearing.

[dublin]

Fit a jubilee clip each side of the shaft anode to stop it moving around

[MalcolmP]

Yes this happens too often - there is also a slight risk that in reverse the anode may work upwards to the hull which potential to damage the hull. I left an anode loose for a little too long on a Beneteau and it did noticeably mark the P bracket - but did not actually damage it - the zinc is relatively soft

This was one of the reasons I developed the IonGuard anode system

Malcolm

[SteveT]

Hi, we had same problem - anode slipped down and rubbed on p-bracket. We've since fitted a wire tie above and below the anode. jubilee clip would work just as well.

Steve

[adamf]

Thanks everyone for responding.
I have limited choices for further investigation and I don’t want to use the boat until this is sorted. I can arrange for a diver to go down and take a look but this cannot be done in the Marina due to H&S restrictions. I was told that it has to be done in the river. I am not too keen as I think it is less safe than the marina and I am not so sure that visibility will be that good in the Medway. I am also trying to find a scrubbing post to use to try and reduce the cost of a haul out. There is one at a local yacht club but I was told that it could only be used by members:-(( I have a couple of other places to call and so fingers crossed.

To be on the safe side I am buying a new anode and if the current one is loose then I will tighten it using star washers (if there is the space) under the bolt heads and I will also plastic wire tie or jubilee clip either side as your suggested to stop it running up and down the shaft.

Thanks to everyone again. I have to say that this is a great site.

[adamf]

Hi everyone.
I just wanted to let you know that I had the boat lifted and sure enough the prop shaft anode had come loose and was sitting against the “P” bracket. It had actually almost fallen off!!
I removed it, cleaned it and the shaft and replaced it using loctite etc and plastic wire ties as previously advised. I hope that now lasts until the end of the season.