Cavitation or Ventillation

[sirrufus]

I have a 9.9 Evinrude on a 15' Sea Nymph with a modifed-V hull. I am getting severe ventilaton or cavitation at all speed ranges - even just starting out. I have reviewed this web site extensively and know the difference between the two. I just can't figure out my problem. I checked the prop and it is OK. I've had the motor looked and it checks out. The anti-vent. plate is about 1 1/2" below the bottom of the hull. I can't lower it any more I've tried moving the motor up about 2" and have tried running on all of the different trim holes. Nothing has helped so far. Is it possible that it is just underpowered causing cavitation or just that the motor and boat don't work together? Any ideas would be greatly appreciated. Would a doel fin stabilizer help this situation? Thanks.

 

[archcycle]

Re: Cavitation or Ventillation

If possible try a prop with a little less pitch. Cavitation is when you create an area of drastically lower than normal pressure so the and the low pressure causes the water to boil at ambient temperature. Lower prop pitch means smaller difference in front/back side pressure so the prop slips through the water more easily and begins acting on a different chunk of liquid leaving the somewhat lower pressure area behind. You really shouldn't be ventilating with the prop several inches below the water so it's probably safe to rule ventilation out.

 

[sirrufus]

Re: Cavitation or Ventillation

Thanks. I'll see if I can try another prop. Is this unusual for a boat/motor combo of this size. I've never heard of having this problem with a smal aluminum boat and 9.9 motor.

 

[orbanp]

Re: Cavitation or Ventillation

Is that the original prop? How big is it?
There should be about 1/2" clearance between the bottom side of the cavitation plate and the tip of the propeller.

Peter

 

[Chris1956]

Re: Cavitation or Ventillation

Are you sure it is not a bad bearing, bent prop or propshaft? Ventilation or cavitation would be unusual on a rig like that, especially at all speeds, with the prop set below the transom.