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post #1 of 39 Old 10-15-2007 Thread Starter
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Question Funny noise

When motoring in a heavy chop in my Bristol 29.9, I hear a "funny" mechanical noise -- i.e., a noise that isn't present when motoring in flat water. It's not the engine noise, which doesn't change, and seems to be coming from the back of the engine compartment. At first I thought it might be cavitation, but cavitation would be intermittent (I assume), and this is constant. Should I suspect that a bearing is going bad? If it's a bearing problem, why don't I hear the noise during a long motoring run in smooth water? (For instance, the funny noise never cropped up while I motored up the Elk River and through the C&D canal last week.)
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post #2 of 39 Old 10-15-2007
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Could be a worn strut bearing. Shaft alignment might change as the hull flexes with the chop, causing mis alignment vibration/chatter.
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post #3 of 39 Old 10-15-2007
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What kind of noise?
thumpata thumpata thumpata
whomp whomp whomp
ding ding ding
crunch crunch crunch

Usually I hear this kind of thing from my wife, not when she gets home from work but usually when I am in bed half asleep - "Oh, I forgot to tell you, the car's making a funny noise! Goodnight"

Gas of diesel engine? Is the sound coming from the waterlift muffler, the rudder, transmission or maybe the exhaust system? Could be the cutlass, very rhythmic dull knocking sound.

Last edited by Gene T; 10-15-2007 at 05:29 PM.
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post #4 of 39 Old 10-15-2007 Thread Starter
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"Chatter" is the best word I can think of to describe it. FWIW, the motor is diesel. I'm sure it's not the motor, because, like I said, the "funny" noise is separate from the engine noise and comes from farther astern. The motor noise is the same as usual when this occurs.

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What kind of noise?
thumpata thumpata thumpata
whomp whomp whomp
ding ding ding
crunch crunch crunch

Usually I hear this kind of thing from my wife, not when she gets home from work but usually when I am in bed half asleep - "Oh, I forgot to tell you, the car's making a funny noise! Goodnight"

Gas of diesel engine? Is the sound coming from the waterlift muffler, the rudder, transmission or maybe the exhaust system? Could be the cutlass, very rhythmic dull knocking sound.
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post #5 of 39 Old 10-15-2007 Thread Starter
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Could be a worn strut bearing. Shaft alignment might change as the hull flexes with the chop, causing mis alignment vibration/chatter.
Vibration/chatter is what it sounds like to me, but there is no strut on my boat. The prop goes through a tube to a propeller aperture in a skeg. If the alignment changes in a chop, does that mean the hull is too weak?
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post #6 of 39 Old 10-15-2007
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How long is the prop shaft? All boats have some flex. If it’s a short shaft it’s not affected as much as a long shaft. Is it possible to put a wooden dowel from your ear to different areas where the sound seems to be coming from? This will pin point the location of the noise. Then you can find out what is wrong.
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post #7 of 39 Old 10-15-2007
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does it sound like girl groups from the 60's?
Motown?
bubble gum rock?
Pray it's not heavy metal!
I prefer smooth jazz in my running gear

Denise, Bristol PA, Oday 30. On Tidal Delaware River, Anchor Yacht Club.
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post #8 of 39 Old 10-15-2007
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You still have a cutlass bearing, but a bad one probably wouldn't sound like a chatter. Your rudder might be making the noise, being buffeted by the prop.

Heavy powering in waves would cause the stern to squat, or sink lower than normal. This could change several things, one is the exhaust. If the exhaust is now submerged that could cause interesting noises. Does the sound change if you change the engine RPM?

I doubt we can diagnose it for you, I would try to repeat it with someone else driving the boat so I could crawl around below deck and listen. Sound travels so you might need to use a listening stick.
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post #9 of 39 Old 10-15-2007
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My exhaust glub a bubs when pushing hard. Boat actually goes slower on max rpm. Hard to say on tidal flows but hull speed seems to be just under 7 knots.

A friend's catalina 30 after he installed a new cutless bearing, shaft, and prop just recently lost the flex plate between the hurth trans and the universal engine. major job! . cheap part you don't want to know the sounds it was making

Denise, Bristol PA, Oday 30. On Tidal Delaware River, Anchor Yacht Club.
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post #10 of 39 Old 10-15-2007
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I will put my $20 on the rudder pivot points.
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