ventilation propeller problem

hello

i have an inflatable boat 7,oo meters with a mariner long shaft outboard 200hp.(1996 model)
i recently put a static cmc aluminum braket (9 inch) and i have a problem with my 21 aluminum propeller...when i rich the 5400 rpm its taking air and goes at 6000-6500 rpm....(trim all down)
i have put the engine 7.5 cm highter than before (the bracket) and i CANNOT lower the engine....to fix my problem..
so i either change the motor from a long shaft to an extra long (and iearn 12.5cm )but its an expencive way or i am looking for something else like the permatrim solution (M9?)
i forgot to tell you that i have no trim anymore (when i touch the trim button to rise the motor even 1 cm ,the proptakes air ...or even i turn the boat a litle the prop takes air...)
i thing i need 4 cm lowring the motor to have no problem but i CANT.....)
thank you in advance
dimitrios archolekas

 

You may want to try a "stabilizer plate". It is a flat plate that bolts to the ventilation plate on the outboard. They prevent ventilation. Be careful, they can really change the way the boat handles.

See: http://www.allinflatables.com/shopping/custom/stabil.html

 

This is probably far more expensive and complex than you're thinking of, but have you looked at a hydraulic jack plate? Might help get the motor down a bit....

 

i cant put an hydravlic jackplate cause (i fotgot to tell you that the excaust is really close to the water so i cant get down) its a long shaft motor to an extra long trancom.....

 

Why did you put the motor 7.5 cm higher to get a deeper propellor?-------Does the boat look level from outside of it with a normal load or does it ride front down?

 

before the bracket (9 inch back) it was balanced...after the bracket in a calm sea and not running seems to be overloaded back...the excaust hole is close to the sea (4 inches now,,) so i cant go one hole down to earn one inch....
iam afraid of water in the pistons....
i made it to earn the place infront the motor head where i put a refreferator for fishes.....

 

Get rid of the bracket, and build a platform over the engine for the fish box.

 

You can take it a wee bit slower, stay on your plane.
Most waste fuel by taking it to WOT or near it, the loss in mph is negligible anyway as long as you maintain a plane.
Put her on plane, back off till you lose the plane, then find the rpm that 'JUST' keeps you on the plane; save mucho fuel and will not eliminate the ventilation, not on turns, but will discourage it big-time.
Cavitation is much more serious, prop damage.
Captain Lee