Cavation...Trim...Jackplate.........RIDE PROBLEMS HELP!

[SC Largemouth]

OK, Here we go...this thread might answer mine and alot of others questions.

I have a 1988 Hydra-Sports VS 150 T (18'), 150hp Black Max Merc, jack plate, tilt/trim, 24p stainless prop 3-blade, hot foot throttle.  

Here are the problems.....

1)  Any speed under 40mph, the boat jumps jumps up and down

2)  The prop cavitates when cornering hard, and hard throttle at low speeds.

3)  The rooster tail is very wide and tall (loosing performance)

Here is what I've done to correct it...

1)  "Whale Tail" fin on foot

2)  Dropped motor 3/4" down on jack plate

This corrected the jumping problem, but I lost ALOT of top speed, and the cavitation is worse.

Any Suggestions?

[thetr20one]

That is a trim angle problem. Make sure your motor is trimming all the way down or in and nothing is in the way. A guy in my old club had the same boat and it was very sensitive to trim angles. That bouncing is commonly reffered to as porpoising. Your rooster tail shouln't be any higher than your cowling on plane. The whale tail can magnify problems, take it off, until you get it right at least. If you have a ported prop plug all the holes and start there. Even with a 150 this boat should fly.

[BassinDave]

Are you sure the hub is not slipping on your prop ?

[Glenn]

You're trimmed up way too much.  Way waaaay too much.

As mentioned, keep that rooster tail down at WOT, about the same height as the cowling.  At lower speeds, trim down until the porpoising stops.  

Same thing when cornering - trim down first.  Not only will it stop the cavitation, but it will improve handling while turning.

From a dead stop, trim ALL the way down first before taking off.  Then trim up slowly as speed increases.  If it starts porpoising, trim back down or speed up.  

Watch your RPM as you're wide open and trimming up.  If it suddenly jumps up, you've trimmed up too much.

Hope that helps!

[Bean Counter]

On a high speed turn you can back off the throttle until the stern drops and then accelerate through the turn and the cavitation will stop.

[Kevin Davis]

well i got a newbie question that im sure i know the answere to but dont know the term. What is cavitation?

[bassdocktor]

Kevin I'm sure someone will correct me if I am wrong but cavitation results from basically a big air bubble forming around the prop.

Scroll down and check out the sections on cavitation. Lets you know whats going on and why it is a problem.

http://www.boatfix.com/how/props.html

http://www.outboardrepairs.com/cavitation/

[Kevin Davis]

very informative bassdoctor thanks very much, alot of things i did not know in those links, i will save them to my favorites.