[Gsoloway]

I have a 15-4 Gheenoe with a Nissan 9.8 2 stroke short shaft. My cavitation plate is 2-1/2 inches below the bottom of the hull.  I want to make a riser plate to bring the motor up. The question is, how high? If I build a riser plate (with no setback) can I raise the motor 3-1/2 inches so the cavitation plate is an inch above the bottom?

On another similar question, if I build a jack plate, is there a rule of thumb that I can use that says for every X inches of setback I can raise the motor another X inches?

 

[Brett]

The best answer I can come up with is the result of testing several outboards
on a flat bottom plywood skiff over the past three years. I think I just like test rides.
Without adjustable trim tabs on your transom, this will produce the best results...

Make an inclinometer, a simple tool for measuring angle from vertical.
Simple to assemble from easily obtained components (sinker, fishing line, protractor)

Using a carpenters level, set your hull so the keel is horizontal.
Then install the inclinometer on the hull so it reads 90 degrees. (vertical)
After installing the inclinometer, raise the bow until the inclinometer reads 85 degrees (5 degrees from horizontal)
With the hull inclined bow up to 5 degrees from horizontal, adjust the trim setting
of the outboard so that the anti-ventilation plate is horizontal using the carpenters level.
The result looks like this...

Now adjust the outboard, using the clamps and shims, to raise the outboard
until the anti-ventilation plate is level with the bottom of the transom.
This height setting will work for most factory propellers and looks like this

With a cupped propeller it's possible to raise the outboard higher
until the top of the water inlet is just below the level of the transom
looking like this...

Without power trim tabs to reshape the flow of water, that is about as high as you can get.
doesn't matter if the engine is transom mounted or on an 8 inch setback.
Water flow out from under the hull at planing speeds stays almost level for about 2 feet
and the water inlet has to be below the surface of the water.

 

[Gsoloway]

Brett- That is an awesome answer. Way more than I expected! That all makes sense.

So, if I can do the same thing with a riser as opposed to a jack plate, the jack plate offers no advantage other than the ability to adjust. (that could be important) Being such a small boat, I am thinking I don't need the extra weight of a JP.

Do you think I will be safe setting the cavitation plate one inch above the bottom and leaving it there?

 

[Brett]

For a stock factory prop, this is your best setup.
Hull at 5 degrees from horizontal, a-v plate level with underside of transom.
Should allow greatest efficiency without ventilating or losing water pressure.
Easy to achieve with a bolt on transom riser

Homebuilt riser courtesy of forum member un-shore

Note the level of the a-v plate relative to the bottom of the transom

Here's the original thread posted by un-shore

http://www.microskiff.com/cgi-bin/yabb2/YaBB.pl?num=1246382329

and in case you wonder why I chose the 5 degree tilt to set the hull
it's typical of a 'noe running at planing speeds...

 

[Gsoloway]

Thanks Brett, very insightful. Everything I read said 4 degrees so I am with you on 5.  I bought a protractor today. My first since school. I am weeks way from working on this because of my work/travel schedule but I will report back. I am an engineer/geek so I appreciate the data points to start from.

You are a gentleman AND a scholar! There aren't many of us left!

 

[chuckm310]

DUH.....HUH :-/