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Old 11-04-2019, 04:45 PM   #1
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City: San Francisco
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Vessel Name: Sarah McLean
Vessel Model: Mainship 30 Pilot
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Cavitation in mainship 20

Spent nice day yesterday crabbing outside the Golden Gate in 70 degree weather and 5kts of breeze. The only annoying thing is the cavitation noise coming from the rear of my 2002 Mainship 30.
It depends on the water and the engine speed. It was very loud in the turbulent flood eddy mix under the bridge at 2800 rpm but got a lot quieter when moved into smoother water near Sausalito
and gets less at full WOT. Very distinct sound from engine noise. More of a rumble

The question is: could moving from a 4 bladed prop to a 5 bladed help significantly? Or maybe just re pitch the prop.. My WOT runs to 3250 at 20 kts which is just where a Yanmar 4LHA-STP should be at WOT.
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Old 11-04-2019, 05:06 PM   #2
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First of all 3,250 is not the wot rpm of that engine. 3,300 rpm is. And it is recommended to run about 100 rpm more as a contingency for bottom fouling, increased weight, etc. Also check the rpms with an accurate photo tach. Dash tachs can be quite a bit off.

So back to cavitation. Virtually all Pilot 30s and 34s suffer from this problem. My former Pilot 34 would run smooth with a clean bottom but would progressively get louder as fouling built up.

Various things have been tried to solve it. I faired the deadwood in front of the prop to no effect. Some have suggested changing to a 5 blade prop, but no one so far as I know has reported doing it. Switching to 5 blades should theoretically help, but will also be less efficient.

Maybe you could be the first .


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Old 11-05-2019, 08:59 AM   #3
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I remember this discussion several years ago in the Yahoo site.
Going to a 5 blade solved the "cavitation" as I remember it.
More than one reported this as a "fix".
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