Upgrade Yamaha 40hp

I am considering upgrading my 40hp two stroke engine.
My boat is 16ft aluminum 42 inches wide.
Currently, my motor pushes the boat at about 31mph at full throttle.

My boat is rated to handled up to a 75hp motor.

Anyone have any idea as to how much speed would be gained by going to a 70hp (two stroke) from my 40hp on this type of boat.

I would estimate that I currently get about 7.5 - 8mpg on my 40hp now.

Any idea, what gas mileage would be on a 70hp (two stroke).

Thanks for any insights.

P.S. I just had the carburetors cleaned by a Boat Dealer, so I will going on the water this weekend to give it a good test run. Labor charge was $126 parts $26 - (Turn around time was two (2) days).

[boats.net]

You can go to Yamaha's web site and look at Performance bulletins to see if your boat or boat like yours is listed. www.yamaha-motor.com

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Well this may help, My 18 foot gregor has a 140 horse Suzuki. Four seater wave runners have a hard time hanging with me. 10 mile round trip with idlling repositioning and carrying three men and a bait tank with 400lbs
water around 5 gals gas.and this is not full speed running as it is ocean fishing. Boat is a real E ticket ride but very unstable in turns. Consequently I have bought a yamaha 40 to replace it. Aluminum boats don't need speed, Just reliability and safety. Stick with what you have it is a good combo. Use the money you save for extra fishing trips. BMC

[bigfish1229]

years ago i owned a 16 foot flat bottom aluminum boat. it had a 1987 25 mercury on it with just a tiller handle and gas tank and battery. nothing else in boat. ran somewhere around 30 mph all daylight to dark on 6 gallons of gas. some time later i replaced that 25 with a 40 merc 2001 model with the same setup tiller handle same battery and same 6 gallon gas tank. with this engine it ran over 40 mph however it was too much power. keep in mind this boat was rated for a 50hp motor. it also burned more double the fuel even if you ran it easy to stay in control. i think you may be better off with a 50 2 stroke, the 60/70 models are a whole different beast, much like my expierence.

Thanks for the insight!!!!

The motor runs fine. Good Advice given.
Thanks.