What's that strange noise?

[The Patriot]

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.

In earlier (and more mischievous) times, we'd drop by the competition boats before a race and quietly drop a few cotter pins on the deck. This seemed to have the intended effect and gave up quite a few good starts.

[Dixon Hemphill]

Here's what the former owner had to say about the comments you all sent me. Can someone come up with a reason for the noise?

Hi Dixon,

I read through the replies and have several thoughts.

First, it would probably help the readers help you if you mentioned that the prop is spinning while sailing and the engine is shut down. And you might ask whether the transmission you have would keep the prop from turning when the engine is shut down -- in other words, is there a prop brake built into the transmission. The engine manual you have may speak to that but maybe someone on the discussion board will know offhand.

Second, it might be helpful to tell them that you're not getting any vibration through the tiller which you (and I) assume means that the rudder is not loose. (Contrary to one commenter's speculation, there are no cables to the rudder on the CD281. The tiller is attached directly to the rudder post.)

The sound is definitely not the slap of water against the hull and it is not something slapping nearby (like the traveler, as one suggested). The sound was occurring in a quick, rhythmic fashion that remained fairly constant once a certain speed was attained. Although the phenomenon may have been present at lower hull speed, the sound may not have been audible until higher hull speeds are obtained.

My best guess is that the sound relates to the spinning prop shaft because I never saw the prop turn whilst sailing. However, because I never heard the sound, I don't know that I had an occasion to look at the prop shaft while sailing.

If the transmission does have a built-in shaft break and if that brake is now broken, then that would, most likely, be the cause of the sound. Of course, the cutlass bearing could be worn causing the shaft to vibrate in a rhythmic way resulting in the repetitious tapping sound we heard.

The one indisputable comment you received was that these sounds can sometimes be extremely difficult to trace. Good luck and keep me posted.

Best,

Glenn

-------------- Original message --------------
From: "Dixon Hemphill" <dixonh1925@verizon.net>

Hi Glenn,

If you go to the CDSOA website and click on Discussions you will read my request for info and the many possible reasons from other readers.

I have printed the replies and will take them to the boat this week to "test" them . Do any stand out to you as probable reasons for the noise?

Dixon


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[Ed Haley]

After reading the description of when the thump is made, at a certain sailing speed, can it be that the mast is pumping? If the rigging is set up correctly, the mast shouldn't pump but if it isn't, then you may get pronounced thumping especially if the mast is really badly tuned.

[Dean Abramson]

Dixon,

Try this: while sailing (engine not running), and you hear the noise, have someone else steer. Make sure that the transmission is in reverse. Go into the cabin, open the engine compartment, and with a flashlight, look behind the engine and see if the shaft is turning. If it is not, then you know that your tranny is locking the prop (which would be the case on most of our boats).

If the shaft IS turning, now try putting the trans in forward, and see if that makes a difference. Maybe some trannys lock the prop better in forward. If the shaft still turns, put the tranny in neutral, and see if the shaft turns faster (and the noise repeats faster.)

In any event, if you cannot lock the prop/shaft, I would guess that that is the sound of the prop freewheeling, maybe combined with a bad cutless bearing. In that case, the problem is still there while motoring, but you just don't hear it.

I am also wondering if there is a problem with the attachment of the rudder, or issues with the rudder post. Try tying the tiller tightly, then diving under and seeing if you can shake the rudder.

But I am guessing this will be more like Carter's mystery: just some loose item, which only makes a racket when the boat starts boogying, but can't be heard above the engine. I had a hand-crank which mounted on the inside of the engine cover on my 25D. One day in rough seas, it decided to un-mount itself, and fell down by the engine's drip tray, where it made a hell of a noise.

Another thing I had on that boat was that the Perry Nut zinc came loose; it backed down the shaft aft of the prop, and proceeded to knock on the rudder. I figured out later that it had been trapped from falling all the way off by the rudder; the rudder was pretty beat up at the point of contact. I noticed that day, that the racket disappeared for good once I made a sharp turn; that enabled the zinc to fall all the way off. I deduced all this when I later found the Perry Nut gone, and the rudder gelcoat messed up right behind the prop.

Please let us know what you find. Good luck.

Dean