proper outboard height

[rcfn43]

I have a long shaft 28hp on a short transom. I am trying to raise the motor to the proper height for the best performance. I have been told that cavitation plate should be approx. Even with or a little below the lowest point of the keel. I have done this and still having issues.
The issues are that the boat will not plane out and wants to over rev. I have adjusted the trim and moved the motor up and down to no avail.

Help

 

[Davem3]

Re: proper outboard height

i would love to help, but need some more information, please.

they have transom jack plates, that some have said they love having.

the boat you have, company, year, transom height, and is it glass or aluminum?

motor make, year length of shaft , please............

pics do work the best though .............

Welcome to I Boats

 

[roscoe]

Re: proper outboard height

Long shaft on a short transom will always give you some trouble, the bigger the motor, the worse it gets, IMO.


How exactly did you raise the motor 5" ?
Jack plate? Does it position the motor back away from the transom?

Photos please.

How heavy is your boat?

 

[cr2k]

Re: proper outboard height

What is your trim angle?

 

[Mr. Demeanor]

Re: proper outboard height

The diagram above is agood starting point but you will likely need to go up a little higher than that. If the engine is mounted directly to the transom and there is no notch (running surface of hull runs all the way back to the transom) then you will need to mount he engine with the anti-ventilation plate (sometimes called a cavitation plate) a little above the running surface. The "plate" should be skimming the water surface when normally trimmed and running at a typical cruise speed. having someone take a pic over the transom like the one below will help determine your setup. Propeller type can greatly effect how high you can go along with water pressure and boat handling.
Generally speaking, running the engine as high as possible will yield the best performance and economy. Higher engine heights also reduce some boat tendency to porpoise. Every boat I have owned has been set up from the factory with the engine too low, even my Fountain Center Console and you would think they would have it figured out for performance. I gained 4mph just raising the motor.
Every boat/motor/owner combo is different so dont be afraid to play around with the setup.

This boat has a notch and will have the engine mounted lower in relation to the running surface do the amount if setback (distance from motor to end of running surface). It is like having a jackplate with a lot of setback.

 

[rcfn43]

Re: proper outboard height

Thanks for all of the info, can't get back on the water until the weekend to take pics, here is more info on my boat and motor
The boat is a fiberglass, 1976 15 ' Tom Boy bass boat . Weight is about 600 lbs. I raised motor with a Jack plate that does not extend off the transom.
The distance from the back of the keel to the front of the motor is about 9".
I have a feeling that part of my problem is not enough weight in the bow. I have been by myself while trying to get this adjusted.

 

[Philster]

Re: proper outboard height

Your motor is too low.

That's the educated guess and most likely answer. You need a jackplate or other accessory to get the motor higher.

TH Marine sells the Atlas jack on iboats. If you are way off, even a jackplate might not help.

 

[MH Hawker]

Re: proper outboard height

While your out this weekend try and get the WOT rpms , it may also be a prop issue but sounds more like its not hooking up, but it is good to have that info.

 

[rcfn43]

Re: proper outboard height

The cav plate is 3" above the keel now that I raised it.