Cavitation problem

[Stoehrdp]

I have bought a 2008 West Marine HPV-420 inflatable boat. I have put a just rebuilt 1980 25HP Johnson 2 Stroke short shaft, on the back of it. This is the first time the motor has been in the water in about 2 years. BTW, the boat is rated for a 25HP outboard.

My problem is when I give it about half throttle and the boat starts to plane, the motor revs up high and I start losing power. The water at the gear box looks like it is cavitating, and from what I have read probably is.

Estimated total weight in the boat is 450 lbs or so and the boat only weighs 130. So I was looking at 600 lbs max.

On the trailer the anti-ventilation plate is sitting about an inch or two below the hull.

I?m assuming the prop is the standard (9x9 I think) and does have a couple nicks, however I believe it worked well with my last boat a 14? sears gamefisher.

Was attempting to try it with a different boat load but lost a coil wire and did not get the chance.

My questions are

Is the weight of the boat the problem?

Would something like a Doel-Fin Hydrofoil fix it?
Do I need a higher pitch prop?

Or is it some other problem?

Thanks in advance
Dave

 

[steelespike]

Re: Cavitation problem

I wouldn't think weight is the problem unless very poorly distributed.Cavitation is different than ventilation.Ventilation is probably what you have.
It may be a spun hub but not likely.
mark across the hub with a water proof marker see if the mark splits.
See if it helps to trim the motor in all the way.You might check the specs on the boat for transom height/shaft length recommendations.
My impression is that some inflatables may have turbulance problems.
I would think inflation pressure is critical.
You might try to get a straight line measurement from the planing surface to the antivent plate.Get the actual demension of the plate position.
I took a quick look at west marine couldn't find a HPV420 all inflatables rated at 25hp were refered to by RIB---.
??

 

[Stoehrdp]

Re: Cavitation problem

Thanks Steelespike,

I did at one point, trim motor in and out with same results. I was told to use a short shaft and I did measure the transom (15")

I will check Hub this weekend. I'm doubtful on that also but it is an easy check.

I did notice the bottom of the boat may just be prone to that. It is a new model, It has a high pressure V floor/Hull and the way it connects to form the V there seems to be a channel that could send air right into the screw.

I will aslo check pressure though I'm sure it was were it needed to be

Here is a link to the boat or just search west marine with HP-V

http://www.westmarine.com/webapp/wc...true&storeNum=5005&subdeptNum=10&classNum=825

Thanks again
Dave

I wouldn't think weight is the problem unless very poorly distributed.Cavitation is different than ventilation.Ventilation is probably what you have.
It may be a spun hub but not likely.
mark across the hub with a water proof marker see if the mark splits.
See if it helps to trim the motor in all the way.You might check the specs on the boat for transom height/shaft length recommendations.
My impression is that some inflatables may have turbulance problems.
I would think inflation pressure is critical.
You might try to get a straight line measurement from the planing surface to the antivent plate.Get the actual demension of the plate position.
I took a quick look at west marine couldn't find a HPV420 all inflatables rated at 25hp were refered to by RIB---.
??

 

[steelespike]

Re: Cavitation problem

As I understand it the water can cool it enough to lower pressure.
This might sound a little screwy but if you can't find a solution you might try mounting the motor to the left or right a little.
You also might try breaking on to plane slowly maybe it burps a bubble of air and once it vents it won't bite again