ventilating problems

[FAC]

i am experiencing what i think is ventilation. when i go full throttle from a stop the boat will accelerate until it is almost on plane then the rpms will rise drastically but the speed stays the same if i let of the throttle a little bit the prop seems to "bite" into the water and i can get on a full plane once the propeller bites i can again go full throttle and it works fine. The same thing seems to happen when i am barely on plane at about 3/4 throttle and make even a small turn, but i can turn fine at full speed
my boat is a 15' open bow tri hull with only a 25hp motor it is rated for 75hp
the a/v plate is about 1/3 of an inch above the keel the a/v plate also has six holes in it from a hydrofoil it also seems like it is underwater when on plane
Do you think my motor is to high or could the ventilation be caused only by the holes in the a/v plate?

 

[trendsetter240]

Re: ventilating problems

i am experiencing what i think is ventilation. when i go full throttle from a stop the boat will accelerate until it is almost on plane then the rpms will rise drastically but the speed stays the same if i let of the throttle a little bit the prop seems to "bite" into the water and i can get on a full plane once the propeller bites i can again go full throttle and it works fine. The same thing seems to happen when i am barely on plane at about 3/4 throttle and make even a small turn, but i can turn fine at full speed
my boat is a 15' open bow tri hull with only a 25hp motor it is rated for 75hp
the a/v plate is about 1/3 of an inch above the keel the a/v plate also has six holes in it from a hydrofoil it also seems like it is underwater when on plane
Do you think my motor is to high or could the ventilation be caused only by the holes in the a/v plate?

Yep, the motor could be too high or it could be trimmed too far out. If you have power trim and tilt try tucking the motor down until you are well onto a plane.

If you have manual tilt try adjusting the tilt lock bar down one hole and see if it goes away.

 

[JaseBosto]

Re: ventilating problems

too high. My baja boat with an outboard used to do the same thing. Your prop is cavatating.

 

[FAC]

Re: ventilating problems

i did mess with the trim the innermost position is the best performing position anything farther out it would get so bad that i could not plane out, i will try lowering it a bit.

 

[JaseBosto]

Re: ventilating problems

I had my outboard all the way down on the transom, probably was too far. Before that ,I remember I would drop the hammer to plane off. The bow would come straight up to what seemed like if you were standing, you would fall off the stern. At about that point, the screw would cavitate and the rpms would fly up. When the bow dropped the screw would catch and off I would go. If I cut the wheel(and in this boat, you could do it at WOT and have no worries), the screw would cavitate again. Wound up changing the prop because it was only 15" pitch and the baja wouldn't beat 35mph. Changing the prop alone got rid of most of my cavitation issue.

 

[steelespike]

Re: ventilating problems

What make and year motor?
A carefully selected prop may stop your problem and get you a little more speed as well.Tell us your prop size(usually on the barrel or under the prop nut.With a tinytach and a gps you could get us your max rpm and speed.
With the right prop you may be able to raise the motor even more for a further slight improvement in speed.

 

[FAC]

Re: ventilating problems

my motor is a '82 johnson the prop is a 9 1/4 X 12 i have not found very many props that will fit. that said i am not really concerned with getting the absolute most speed out of it. all i want is an easy to pilot and reliable fishing boat

 

[steelespike]

Re: ventilating problems

OK it appears you have the old style gearcase which does limit how high you can go and prop selection.I would guess your 12" prop is too much. And an 11" or even a 10" may be a better match and may vent less.and perform better. A 12" isn't available here; tells me may not be a common size.the 11 is available in 2 types. and a 10 and 9 1 type each.
It is important for the health of the motor to prop it within its rpm range.
Again the tiny tach will help this.If your motor is lugging ,which i would suspect with a 12" on that boat could lead to premature engine problems and poor performance.You can solve the venting by lowering the motor but won't solve your prop mismatch.