What's that strange noise?

[Dixon Hemphill]

Can somebody tell me what causes a sort of "thump, thump, thump" that seems to be coming from the stern of my CD28? I don't notice it when motoring but only when sailing at a pretty good clip. It may happen while motoring but the sound of the engine may prevent my hearing it. Frankly I'm too novice to determine if it's an engine knock because I don't really know how an engine knock is supposed to sound. My 2GM20fF Operational Manual says an abnormal sound could be a worn bearing or gear, loose bolt or nut or low lube oil level.
Frankly I have yet to check the last mentioned but had the engine serviced last July and have motored only about 30 hours (max) since then so I don't think the lube oil needs replentishing. Your comments will be greatly appreciated.

Dixon Hemphill

[jerryaxler]

Put the boat in reverse while sailing and see if that stops the noise.
It might just be a free wheeling prop.

[Dixon Hemphill]

I tried putting the engine in reverse as well as forward but the noise continued. It's almost as if someone is slapping the inside of the boat below the waterline at the stern.

I sailed Sunday with the former owner who says he had never heard that sound before. Would a worn bearing make a thumping sound rather than a grinding sound?

[Neil Gordon]

Maybe you picked something up that's lodged in the rudder and is slapping the hull as you sail. If the prop is really locked, I'm not seeing where engine/prop/cutlass bearing/etc. would be the source.

It wouldn't take much to give you a significant thumping noise. The inside of a sailboat has about the same accoustics as a guitar.

The rhythm of the noise isn't the same as the wave rhythm, is it? Same noise on either tack?

[Warren S]

What is the "frequency" of the slapping (approx. one each second? more)? Does the frequency change with speed through the water? Any additional water coming through the stuffing box? Is there any vibration transmitted through the tiller handle with the noise?

(20 questions here).

[Warren S]

I'm with Neil on this one. While a locked propeller with enough bearing play can still rattle, it's jut not that likley. You almost have to be dragging something. Well, there's always play in the rudder gudgeons... but I think you'd feel that in the tiller and arrest it somewhat by steering.

[Steve Kuhar]

Noises can be very difficult to diagnose, they do not always come from where you think they are coming from. If the previous owner has never heard the noise look for things that are different from the way they were when he sailed the boat. Storage in particular tends to create annoying and hard to find noises, a loose aspirin jar in the forepeak can sound like a bowling ball rolling around in the bilge. Just the right (or I guess wrong) combination of wind speed and halyard tension would cause a halyard slap in my boat that would make you think that the mast was coming down. The noise could also be exactly what it sounds like to you, water slapping against the hull, hopefully from the outside though, rather than the inside. Whatever you find out, let us know, we all like to hear the outcome of these little mysteries.

Steve Kuhar

[Ed Haley]

Aside from the obvious object that may be loose and rolling from side to side, two things come to mind that would cause a "thump" (as opposed to other sounds, such as a knock or rattle) could be:
1. loose steering cables that allow rudder to thump from side to side (could also be a loose rudder on the shaft)
2. Wavelets that come from behind and pound on the stern

Sounds on a boat are extremely deceptive when you hear them from various places on the boat. I once had a loose barbeque lid in the starboard cockpit locker that I swore sounded like blown engine valves.

If the sound comes from the steering mechanism, you should feel the thump in the tiller or wheel. Before you start sailing, make sure nothing can move around or roll in the lockers.

[Boyd]

The previous posters make very valid points about the mysteries of boat noises.

I had a rattle that only occurred when motoring till one day I happened to lean back on the swim ladder and it stopped. Now I keep a line wrapped on it. Silence is great.

I suggest you go below and put your hand on the prop shaft, hull inside various lockers, the rudder shaft, and bilge. Then feel each stanchion, mast, stay, halyard, and most importantly check the anchors. You are feeling for the thump. This has proved to be much more reliable for me than my bad ears.

I have a rhythmic thump thats caused by the anchor swinging side to side as it hangs on the pin. This only happens at certain combinations of wave and wind. Drove me nuts till I found it. Now if I can just think of a way to stop it.

Boyd
s/v Tern
CD 30 MkII
Fort Lauderdale, Fla.

[Dixon Hemphill]

You guys are really great! I had no idea I would receive so many logical answers so quickly. Now my task is to get back to the boat this week and start checking the many places to look and following the many suggestions to see if I come with the reason for the noise.

If and when I discover the culprit I will let you all know my finding.

[Tom in Cambria]

Another one you might consider is one that it took me a long time to find, but I only heard it at anchor. Still not impossible that it could happen underway. The traveller on the main and stay sail is locked in place by those sliding rubber sided "clamps". You pull the pin up and slide them where you want to lock the traveller. Over time either the rubber wears down or the holes for the pins get wallowed out a little. This gives the traveller just enough play to thump as the boat rolls from side to side. You can wedge something in there to fill the gap or you can tie the car to one side so it can't work back and forth. Probably under sail the pressure on the main sheet would hold it to one side or the other, but you never know.

[seadog6532]

Please check your oil...It's probably not the problem, but you should check it every time you use the boat. It only takes a second to do it and can save you thousands in repair costs. Never assume that it must be okay because it's not due for a change. I always do my own service but if I had someone else do it I would still check behind them to make sure it was done right.

I spent 28 years working in aviation and most of it in quality assurance. Even the best make a mistake sometimes and it is more likely to happen on a simple job like fluid servicing that you don't think too much about. A final note: After working 7 years as a quality inspector at an aircraft heavy maintenance facility I don't fly anymore.

[Neil Gordon]

Please check your oil...

Right... check the transmission, too. It's harder to get to (talking MD7A here) but it's comforting to know it's lubricated.

(How else can you assess the full difference between free spinning and locking the prop?

[Carter Brey]

I had something going "bump" while under sail in boisterous conditions-- when the wind would suddenly gust up and the boat would heel quickly, and usually only on port tack. It was more of a sharp knock.

I kept checking my shrouds and chainplates to make sure nothing was about to fail. It freaked me out.

I finally realized that it was the handle for the Whale cockpit-mounted manual bilge pump. It was rolling around in the portside cockpit cubby hole under the winch.

Sheesh.

Carter Brey
Sabre 28 Mk II #532 "Delphine"
City Island, NY

[Neil Gordon]

I kept checking my shrouds and chainplates to make sure nothing was about to fail.

No sound is more horrifying than the soft tinkle of half a cotter pin falling to the cabin top.