Outboard Hight

[quesnel]

I have a 89 Alumacraft V170 bass boat powered with a 90 hp. Johnson. All runs well but I have a cavitaion problem in turns at high speed or large waves. There is no problem if the motor is in the way down. Would lowering the motor help?

[L.DOSSO]

What level is the cavitation plate of your outboard engine located at? (as compared to the lowest point (keel) of the transom ?

[quesnel]

the cavitation plate is 1/2" above the lowest point on the keel.

[L.DOSSO]

You should try to lower the engine down 1/2 '' and thus have the cavitation plate at the same level with the lowest point on the transom.

[woodboat]

A stainless prop with a good cup wil help allowing the engine to stay high for decreased drag.

[marshmat]

Terminology correction: You guys are talking about ventilation, not cavitation. If the engine is too high, and air gets sucked into the prop arc when you turn and the prop ventilates. On almost all hulls operating at normal planing speeds you want the antiventilation plate (that plate has absolutely nothing to do with cavitation) parallel with the bottom, or very slightly below. Higher mountings are only for high-speed applications where the engine is set back and drag is a major problem.
For reference, cavitation refers to a condition where pressure on the trailing side of a propeller plade drops so low that the water against the blade turns to steam. Usually this is the result of spinning a prop too fast for the speed of the boat; you can get this if you ram the throttle to max while sitting still.

[woodboat]

Don't say "you guys" just because I didn't think it was important enough to correct the terminology. That's why I stay out of the sailboat section because it is all just a rope to me
Anyway the boat is an aluminum bass boat. Very light, high speed craft. It is VERY common to raise the engine to increase speed. The boat must be fitted with a proper propeller to do that though. I would never mount the engine with the plate BELOW the bottom of the boat on this particular application.

[quesnel]

I am curently running a three blade aluminum cuped prop. My max rpm is 5500, the max rated rpm for this engine. Should I get the same configuration in stainless steel?

[marshmat]

I'd start by dropping your motor 1/2" to correct the ventilation problem. Once you do that you may find the boat performs according to expectations.

WB- As I said before, you will only see engines with the plate above the keel in specialized high-speed applications. 90hp on a V170 is probably not such an application. Manufacturers' guidelines for most standard outboards do state that the plate should be at or slightly below the keel.

[woodboat]

I had a 15ft McKee craft with a 70HP evinrude. I used two props. The 13X19 SS ran best with the plate raised slightly. The prop had enough bite that speed increased 2 MPH by raising the engine on the transom all while not ventilating. Manufacturers recommendations are usually "safe" so yes it will help to lower it slightly. I just wanted to point out that the other option was a prop that could hold on a bit better.

[quesnel]

Dose anyone out there know, when I get this boat tuned correctly. What should I get for top end speed. This boats weight is 1100 #. Powered by a 90hp Johnson.