Cavitation problems now!

[SteuryLover]

Well I finally got the boat back from the shop and the Johnson 88 spl on it is running great. After noticing the motor being mounted a little low I also raised the motor 2 holes. I took her out (18' V hull CC 1200lbs dry no motor) in pretty rough seas and wind to test her. The boat had much more power then before and easily jumps over 4k. The hole shot was much improved from the previous setup (poorly rebuilt carbs and the outboard 2 holes lower). When I would gun it the RPM will jump to 5k and cavitate for a few second then grip and get down to 4500k. If I eased into it, no cavitation problems and none in turns. Here are my questions. Prop 13.25x17 QL minor cup.

1. It was only myself and 15gal of fuel. (usually 40gal fuel, 30 gal bait tank, gear, 5.5 kicker and 2-4 people) Will the hull ride deeper when on plane with a full load? Or when a boat planes is it always at the same height out of the water no matter load?

2. Do you think with a typical load as stated it will reduce the cavitation?

3. Is there a better prop style or size to fix this if the load does not.

By the way the boat is cooling fine.

Thanks for everyone's time on this.

 

[SteuryLover]

Re: Cavitation problems now!

One more bit of info, the motor has manual T&T so no trimming out while under way. The trim pin is currently set one hole from the bottom. Will moving this to the bottom help with the cavitation? Currently the boat planes pretty easy. Thanks again.

 

[steelespike]

Re: Cavitation problems now!

While I hate to give up the height I would lower it a little; see how it does.
After a test maybe trim out a notch.Rpm should be about 5,500 with a light load.Some folks tollerate a little venting to get the best speed which usually is best economy.I would hesitate to lower until you see how it runs in calmer water.When making changes do one at a time and test.

 

[Dhadley]

Re: Cavitation problems now!

In your other thread you talk about going to a ss prop. I wouldn't lower the motor until you decide on what prop you're going to use. If you stay with the aluminum prop you may indeed have to lower it. Going to either of the other props you mention may mean raising even further.

Then you have to consider the shape of the current aluminum prop. If it's worn and or beat up you're not getting a true baseline now.

Then you have an rpm issue. In the other thread I thought you mentioned you now get 5200 rpm. In this thread you say 4500. With that motor your target is at lest 5600, 5800 is better with an average load. If you're truly at 4500 now you might want to make sure there are not more running issues.

Things can get a bit confusing with 2 treads on the same subject.

 

[SteuryLover]

Re: Cavitation problems now!

Well took the boat out today in much better condition. I did not have the same cavitation problems in calm water as I did in rough water. So here are the real #s.

Top Speed 34-35 MPH (GPS)
Top RPM 4500k
Light Load
13.25x17 alum slightly cupped prop
Fresh gas pre-mixed

Does it seem here I need to go down a pitch? I am now thinking a 15 SS prop. Any help here is appreciated.

 

[john from md]

Re: Cavitation problems now!

The rpm looks a little low to me. I think you need to be in the 5000-5500 range with speed 35 to 40 when light. I don't know what you had done at the shop but has the engine ever been decarbed? If not, this is a place to start before you go changing props.

Changing from an aluminum prop to stainless will increase pitch without a pitch change. This will not help you as your rpm is already too low.

Another inexpensive addition you may consider is a set of smart tabs. They will get you up on a plane faster and better stabilize the boat all around.

Regards,

John

 

[Silvertip]

Re: Cavitation problems now!

You have some other issues as the numbers you provide do not work in the prop calculator. You either have an inaccurate tach, a mismarked prop, a very heavy boat (much heavier than you posted) or the engine simply isn't making power. As was mentioned, that engine should twist a 17P with no problem and speed should be in the 40+ range at 5500 - 5800 rpm. I suggest you start with the tach, then ensure the engine is in proper tune, has good compression and is actually performing as it should.

 

[SteuryLover]

Re: Cavitation problems now!

Thanks for the advice, I do have another post regarding my low RPM issue. It has been to 2 shops to solve the issue. Check out my other post marked " 88 SPL low RPM problem". So I agree there is a running issue that I need to solve before tackling this issue. Once I solve the low RPM issue I will return to this post and go from there. Please feel free to chime in on my low RPM thread.