cavitation

[torcano]

During a recent sea trial the person who surveyed my boat thought the props were cavitating. Here is what I have:
Mercruiser Avenger SS props
Pitch 17, not sure on diameter
WOT RPM 4500
Twin 4.3 L MPI Mercruisers
2002 Searay Sundancer 28 feet (Overall 31.6) weight 8000lbs
He suggested larger diameter and smaller pitched props
I am willing to sacrfice top speed for more power getting out of the hole.
What are the maximum diameter and least pitch props I can put on this boat.

 

[Lyle29464]

Re: cavitation

I sort of agree with him. You really need as much diameter as you can fit in there and then the most pitch that will let you reach the recomended max rpm. Would need all of the numbers to be sure. was trim all the way in? Is bottom clean. was the speed a lot slower than normal. Your pulling a bunch of boat . Is it kept in the water? could you have water in the double bottom?

 

[torcano]

Re: cavitation

The boat was trimmed in. The bottom was power washed before the sea trial. There was no significant water in the boat.I can't say about speed being slower than normal because this boat is new to me.

 

[H20Rat]

Re: cavitation

cavitation on an I/O is rare, to the point of almost being impossible. (if the props are in good shape, if they are dinged up badly its another story...) Simply not enough horsepower to boil the water away from the props. (which is what cavitation is)

First question is, what is your GPS speed?

 

[tpenfield]

Re: cavitation

The problem may be that the Avenger is not really meant for that application (heavy boat, I/O). The Apollo would be more suited. 17" may be the right pitch, so try switching to a prop that is more suited to a heavy I/O. Mercury Mirage Plus .

 

[steelespike]

Re: cavitation

These guys probably know best what prop should work.
Present day terminology cavitation is not something you can feel. You may note the affects by looking at the prop.
Probably surveyer was referring to venting.When the props lose bite do to air fouling the prop usually
noted by rpm increasing while speed usually slows.
Sure would be easier to tell what is going on if we knew the wot rpm and speed by gps.Gear ratio usually stamped on the out drive would help as well.

 

[torcano]

Re: cavitation

I had Mercury Mirage Plus on my previous boat which was 800 lbs heavier with the same engines and drives. They seemed to work well. Unfortunately, I did not track GPS speed during the sea trial. He was checking to see what RPMs I was getting at WOT. I was going to start with aluminum props before I invested in a set of stainless steel. Tpenfield, I think the Avengers are a 17 pitch so that would not be a change. So even if it was not cavitation, what would be the optimum prop size to give me the most power coming plane at the expense of some top end speed. The most I ever normally run is 3500-3800 RPM.

 

[steelespike]

Re: cavitation

4 blades usually help hole shot and it is usually suggested to drop an inch of pitch to maintain rpm.
Usually not as many to choose from in even number props.