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Author Topic:   Calling all the wrenchheads! 40hp johnson w/performance parts
EasyE posted 03-30-2002 05:48 PM ET (US)   Profile for EasyE   Send Email to EasyE  
I have been using a 16' whaler with a 40hp johnson and im thinking about buying it depending on you all's predictions.

Right now with a foam dry (it has a few holes all over the hull) I get 27.5mph at WOT spinning a 12x15 aluminum, trim hull(engine), me my anchor and 6 gallons of gas. My goal is to get the speed up to 32mph using the same engine. I know with a 50hp which comes in the same engine block, it would be easy to make that top end or faster. Im not try to turn the 40hp into a 50hp because im not planing on changing any exhaust manifolds or porting anything but the closer the better! Do u'all have any suggestions?

What i have thinking is to rebuild carbs using parts from a 50hp, raise the engine 1 inch off the transom or whatever u'all suggest, change to either stock 50hp reeds or 50hp boysen performance reeds, Stainless steel cupped prop...I have choise of 11x15 or 12x15...what should i do?
Also, I will remove all the patches from under the hull and do it over properly and drain and seal all holes.

Any other suggestions would be great. Its a while since I have owned a Boston Whaler! Thanks-EasyE

Salmon Tub posted 04-03-2002 10:20 AM ET (US)     Profile for Salmon Tub  Send Email to Salmon Tub     
With a Tohatsu 40 (1996) that I previously had on my Montauk, I was able to get (with myself, 15 gal. gas,...) 20 mph cruise (4000rpm) and 29-30 at WOT (5400 rpm) - no wind, tiny wind ripple) into a headwind, lost 2-3 mph, with a tail wind, gained 2+ mph. Swung a 10.5" x 13.5" prop, and trimed in and out until best speed reached. Carefull messing with the carbs. If they have different jets, you may lean it out to the point where you will burn the block. Too much air is a bad thing. What RPM's are you at? What is the Gear ratio? Believe it or not, I found out that a little ripple (wind not wine) is better for speed. Flat (milk pond) water tends to make the hull stick, and will slow you down.
EasyE posted 04-03-2002 09:00 PM ET (US)     Profile for EasyE  Send Email to EasyE     
Thats interesting, my 16' should be quite a bit lighter than your Montauk and a 40hp Johnson has more displacment than a 40hp tohatsu which has an extra cylinder. Then again you can probably see your reflection lookin at the bottom of your montauk while bottom is very rough with patches and dents.
A tohatsu has a 1.85:1 gear ratio while a Johnson has a 2.42:1 gear ratio. Im swinging a 12x15 prop with no cupping and have no tach. It looks as if I have to play with props to get the right speed. Even though you have a 13.5 pitch you are also swingin that prop almost twice as fast as me judging from the ratios. I think 12" diameter is too wide and is better for the power, perhaps the 11x15 cupped prop from Turbo is a better choise?

When I was checkin out the carb rebuilt kit for this engine i notice that for the 40/48/50/55 horse power boats was all under one part number! All using one set of jets. Then I checked the reeds and it was the same thing! So im wondering, what is the diffrence? All in the ports? Still need more help. Thanks-EasyE

PS- thats interesting a boat would run a little faster in rippled water. It does make sense, since in dead calm there is hardly any air under the boat. A stepped hull would help. Thats a new project! A stepped hull 13',I'd would love to see that.

EasyE posted 04-03-2002 09:05 PM ET (US)     Profile for EasyE  Send Email to EasyE     
Typo...<while my bottom is very rough>...3rd line from top
JohnAz posted 04-03-2002 09:19 PM ET (US)     Profile for JohnAz  Send Email to JohnAz     
exhaust tuning

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