Cavitation

[Wassachu]

I have a 22 foot boat with a 70HP Johnson on it. I bought the whole package new in 1990. I installed a Dolphin on the outboard because of stern slide in cornering. The Dolphin corrected that problem. I've haven't had anymore problems until now. I moved to Maine and my boating is in the Piscataqua River, seperates Maine and New Hampshire. This river is tidal and has a strong current on incoming and outgoing tides. When we have extra high or low tides and I try to run against the tide my outboard cavitates. I've played with my trim and that doesn't make it better it sometimes makes it worse. If the outboard cavitates a lot my outboard overheats. When the alarm goes off I have to shut down and use my kicker to move to the edge out of the current. The river is narrow and has rocky shorelines or few places to anchor. This has happened twice and is scary and dangerous in strong current. If anyone has any ideas, I'd appreciate it. Thanks.

 

[steelespike]

Re: Cavitation

First that is ventilation, cavitation is a different animal.
My first thought is the motor is too high.I hesitate to lower the motor until we try other things.Is the prop in excellent condition?What size prop what is your present max rpm and speed?Dolphins can cause any number of problems.I'd like to see how it acts with the dolphin removed.Smart tabs may help with slide the smart tab guys may be able to help there.You must do some serious turning if you are sliding.Is there no external keel?A stainless prop may solve the venting problem.If you intend to buy a prop a dealer may let you try some to see if it will solve the problem.
Anti vent plate (just above prop)should be about even with the bottom of the boat. But again would like to try other solutions as the more motor in the water the more drag.

 

[Wassachu]

Re: Cavitation

I changed my prop last year. I went from a 19 pitch to a 15 pitch to give me more RPM at WOT. With the 19 pitch prop I was only getting about 5200 to 5400 RPM at WOT and you Johnson is designed for 6000 RPM. The new prop is a little above 6000 RPM. I think I've had the ventalation problem with both props. I still have the old prop. Both are aluminum and in good condition. My boat is a dory with a semiflat bottom and not much of a keel so it would break loose in a turn and slide. I have trim tabs on the boat and power trim on the motor. I've played with both. The current is strong and wavy so sometimes the prop almost comes out of the water. My speed is just enough to make headway and keep out of the way of larger boats. Until I get this figured out, I'll be avoiding those days with extra high or low tides. I'm trying to talk my wife into a new E-Tec 90HP. That extra 20 HP might help. My 70 sucks gas too.
The motor is as far down on the transome as it can get.

 

[steelespike]

Re: Cavitation

Other than removing the Dolphin I think a stainless prop is your best bet.
They are very effective.Both at moving the boat efficiently and combating ventilation.You may lose a little rpm.Hopefully you can try a couple of props somewhere.

 

[ddaigle]

Re: Cavitation

Since your motor over=heats ,your motor is coming out of the water bucking on the waves. This is also causing your ventilation problem. You need to get the motor as low as possible and get rid of the dolphin and trim tabs. These are raising the back of your boat even higher. Also when you are running in those conditions, move most of your weight to the stern.You might also think about a new waterpump. If you are still having cooling problems, you might want to look into a Bob's machine transon water pickup. I have one on my boat, customized with a screen and it works great.

 

[Wassachu]

Re: Cavitation

I've read and have been told that a four bladed prop might help my performance and cavitation problems. I know this 3 blade 4 blade issue has gone round and round but I've never heard anything about cavitation or some say my problem is ventilation. Another question is does anyone have any experience with a composite prop? I've read that they are better then aluminum and not as good as stainless steel.

 

[chad tha duck comander]

Re: Cavitation

could i bother someone about a cavitation problem

 

[walleyehed]

Re: Cavitation

Chad, I started you a new thread in the top part of this forum...Title is "Cause of cavitation"......and you need to tell us, in that thread, what boat/engine/prop you run now.