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Old 20 April 2003, 17:15   #1
Country: Ireland
Town: Dublin.
Boat name: Tender 2.
Make: Valiant DR 600
Length: 6m +
Engine: 150 Optimax
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Join Date: Oct 2002
Posts: 443
Cavitation ???

Out today in a force 5-6 with a very large swell. Several times the prop lost its grip on the water and the reves went up by 1500. After the revs were reduced it took a while longer to build up speed again and to get onto the plane. All comment welcome.

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Old 20 April 2003, 17:54   #2
Country: Ireland
Join Date: Jan 2003
Posts: 22

Are you experiencing Cavitation or Ventilation.Cavitation can be caused by the water boiling on the surface of the prop blades from a reduction of water pressure on its surface, if the prop is incorrectly matched to the application it is being used for you will get this,if it is incorrectly pitched or worse if the surface or tips of the blades are damaged, you will get Cavitation, some cavitation is normal but not on the scale you suggest,or is it Ventilating, look at the motor height on the transom, are you sucking down air from the surface or catching exhaust gases from the outlet due to motor trim angle, is it happening all the time or in sharp turns, look at trim angle,mounting height or the prop surface, in the mean time find out the correct pitch prop for your Mercury.Is the prop stainless or ally ,I think Paddy in Western Marine could tell you the correct prop pitch for your engine . jordan

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Old 21 April 2003, 17:13   #3
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Scotty, at the risk of teaching you to suck eggs... Were you working the throttle? or just leaving it at the same setting until almost airbourne (which I guess you would be quite often in f5-6) and then reducing the revs after they'd risen too high?
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Old 21 April 2003, 18:23   #4
Country: Greece
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Make: MAR.CO
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Scotty Cav or Vent who knows?? May the engine was trimmed up?? (possibly???)
What is the size of the engine?? what is the prop pich (is it SS or Aly??)?? how heavy was your boat with pax o/b??
All this as well as the hight of the engine mounting and trim play a major role in what you describing.
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Old 21 April 2003, 23:06   #5
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
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Join Date: Oct 2002
Posts: 5,596
It's a common problem. If everything is OK after checking as Jordan suggested, it's simply a crap prop. The blades are stalling and won't grip properly until you are nearly stopped. Then it starts over again. Less pitch will help, but it's not a real solution.

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