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Ranman
08-01-2000, 03:25 PM
Here's another one for you experts. When planing my C-18 from idle, I get what seems to be a strange missing sort of sound from the thru-hull exhausts. This sound is not real obvious, but does not sound super smooth either. The boat does not miss at idle and it sings pretty well at top end. If I put my head below the windshield while running and listen to the engine hum it sounds fine. Even if I cruise at low speeds (just on plane) it just sounds strange. Is this just the way thru-hulls are or should I look into something? BTW The engine is a stock Merc 350 MAG and I'm running a Mirage 25p and so the boat is not a "hole-shotter".

PaulO
08-01-2000, 03:42 PM
Can't say for sure but, my guess is that since it idles fine and runs fine at full tilt, it is probably some strange acoustic thing going on. Too many factors to compare to another boat though. By the way, that is a very stout prop to be running with that HP. What are you getting for max RPM? Is this a 1.5:1 ratio drive?
PaulO

Ranman
08-01-2000, 04:19 PM
PaulO, I'm not sure what the ratio is, I will look tonight. The boat pulls 4900 RPM at 67.1 mph on the GPS and takes a long time to wind out.

RickSE
08-01-2000, 05:21 PM
If you have a Thunderbolt V ignition, which you should on that boat, it could be that the ignition controller feels it needs to change the timing for some reason. The noise you hear may be the reason for the timing change or the timing change itself. Thunderbolt V ignitions will change timing for knock control and power optimization.

RickR
08-01-2000, 07:59 PM
madpoodle is right it sounds like flappers to me.
If you can reach them, try attaching a couple of clothspins to hold the flaps together and see if that makes a difference in the sound.

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RICKR
mailto:riggerb@aol.comriggerb@aol.com</A>

Spitfire Dave
08-02-2000, 08:26 AM
It may be hard to believe but it is probably the flappers in the exhuast tips. I know two guys that had the same problem. The cure was removing the flappers and installing Eddie Marine silencers. I run a 3" silencer in a 4" tip with good results. Spitfire Dave