water rise? (adding setback & flotation)

[jerry1865]

hello all, need just a bit of help. ive lost my little scrap note that tells me how much water will rise behind the boat.
im thinking it was about 2" per foot.

im adding setback and flotation in the form of a swim platform to a 22' cutty with a v8 johnny.
thanks in advance, jerry cockrell

[Jay Smith]

Jerry,
If your are refering to the amount of elevation the gear case can be jacked up in proportion to the extention of jack plate length , I've always took the dimension of for every 1" the jack plate move the power head is moved toward the rear you can ( in theroy ) come up 1/8 th inch and still have an equivalant water feed to the prop and water intake.. In other words if you applied a 4" jack plate your should be able to elevate the engine 4/8 ths OR 1/2" and still have the same water inventory applied to the prop and water intake. This has worked fairly well for me in the past where engines with low water pick up begin at a starting point of split shaft with the motor trimmed at exact neutral trim and bolted up flat to the transom. Then as a jack plate is added for the enhancement of Center of Gravity balance the motor/gear case can be jacked up with the 1/8" UP per inch of jack plate adds length BACK .. Most hulls have a sweet spot and this theroy can be used as a starting point . Again after reading your question this may or may not be applied to your question.

Good luck,
Jay

[baja200merk]

on a big heavy slow boat it raises much faster..

test then test again

[jerry1865]

thanks to both of you. jay, yes that was the formula i was looking for.
1/8 per inch by the foot would be 1.5". then factor in the heavy, sounds like ill start this process at 2" up and see how close that will get me.
im figuring a foot of set-back to start with, then if i need more it will be easy to add if i build this platform correctly.
years ago i went back 34" with a v6 johnny, handled great but nearly drowned the motor if i wasnt real careful coming off the throttle.
this one will get flotation right away,

baja, you are right on the money. big =yes, heavy=3100 lbs, slow....by your standard, yes= 55mph so far. fully expect to see 62-65mph after this mod.
for a hull that was designed to plow water and run at appr. 32 mph that aint too bad.
most of you would not imagine the attn, this rig get every time i drop her in the river.

[baja200merk]

at 5mph it will take about 3inches to rise 2 feet, just start low and work your way up. Watch water pressure/temp and Good luck :cheers:

[sschefer]

I' agree with ya'all my PL65 has a 5-1/2 setback and the manual said go up 1" I can actually go up three with my deep V but the water pressure needle starts to bounce at about 35 so I keep it at about 2 and it's great.