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99 johnson to 15 more top end

T

timw

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" I have a 9.9 Johnson on a 12

" I have a 9.9 Johnson on a 12ft aluminumn boat and want to get more top end. I called a dealer and he said going to a 15hp motor would get me no additional top end, since they are the same rpm. I am wondering what a 20hp would give me. Is there a way to figure this out before going out to buy a new motor? "
 
" 4 years ago I had the same p

" 4 years ago I had the same problem.
I needed more power for my Zodiac boat and my 9.9 Johnson.
I achieved it by building serial 15hp-parts in the motor, which are:
The bigger carburetor and the more breathing diaphragm valve of the 15hp.
The performance, acceleration and top speed were greatly improved.
Your dealer is wrong in his opinion.
Higher power at same revolution means your boat will reach a higher top speed at higher revolution then before. "
 
" Most V hull 12' aluminum

" Most V hull 12' aluminum boats are rated 10HP Max.
If you are trying to go faster, you might want to try a propellor with a higher pitch. You will loose a little speed getting out of the hole, though. "
 
"Tim,

The 9.9 Johnson would

"Tim,

The 9.9 Johnson would need a different exhaust tube in order to convert to a 15HP , in addition to the carb work. The exhaust tube on the 15 has a circular cross-section and gets wider at the bottom, as opposed to the narrow, rectangular shape on the 9.9.

"
 
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