Good top speed! 66 Boaton Whaler with a 75 Johnson 70HP!

Jet Wrench

Petty Officer 3rd Class
Joined
May 26, 2010
Messages
97
OK, here is the set up. Bare Hull - 850 lbs, console - 60 lbs, battery (installed fwd in under console)- 20 lbs, 10 gallons of fuel (two, 6 gallon tanks placed aft, port and starboard) - 65 lbs. No RPS seat and the railings and rub rail have not been installed yet. Engine, 1975 Johnson 70HP with a standard aluminum prop. Engine mounted two inches higher than a standard configuration on a CMC trim plate that puts the engine aft of the transom by approximately 6 inches. Only myself piloting the boat.
Weather: No wind, water is glass, temperature about 78 degrees. Get out past the no wake zone and slowly bring her up to plane. A nice, even plane is about 3900-4000 RPM. I take her up to max power (5600 RPM). I am accompanied by a friend who runs along side of me with a ski boat. He has a GPS and mechanical speedo to determine my speed. I don't have either, just the tach.

At 5600 RPM, I am pleased to learn from my friend that my Whaler is tooling along at approximately 38-40 MPH! The speed reported justified my final engine trim adjustments! I had no idea she would run this good! The engine is thirsty! It will burn about 7.5 GPH at this RPM, but what a way to have fun! At 4000 RPM, the fuel burn is closer to 5 GPH. When the RPS seat is finally installed, I will get the Moeller 26 gallon tank and install it under the seat!

This was/is such a great hull! Excellent handling, turns on a dime and very confident at higher speeds! I can't wait to finish her out with the seat, rub rail, aft and bow rails and my own custom made t-top!

Jet Wrench
 

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H20Rat

Vice Admiral
Joined
Mar 8, 2009
Messages
5,204
Re: Good top speed! 66 Boaton Whaler with a 75 Johnson 70HP!

The engine is thirsty! It will burn about 7.5 GPH at this RPM, but what a way to have fun!

Pretty much right on the money for the rule of thumb estimate! (1/10th your horsepower is the WOT GPH consumption rate)
 

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: Good top speed! 66 Boaton Whaler with a 75 Johnson 70HP!

My 1966 Sakonnet would reach 38mph with my '79 Johnny 70.
 

scoutabout

Lieutenant Commander
Joined
Oct 14, 2006
Messages
1,568
Re: Good top speed! 66 Boaton Whaler with a 75 Johnson 70HP!

Nice boat. Try another run when the water is just slightly rippled by a little wind - you might find you get even better numbers as there will be even less hull in the water. Glass smooth surfaces can suck your hull down and create more drag. Every little bit helps! :D
 
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