Posts: 433
Location: Cedarburg, Wisconsin | Underpowering is the absolute biggest mistake you could possibly make from a safety standpoint, especially in rough water.
If you look at the reports done by the outboard manufacturers, on their web sites, you will see the fuel economy tables with different boats and motors. Every table looks basically the same. The motor gets the best economy at idle. Then the mpg readings crash until the boat gets on plane. After that the second best mpg readings usually appear somewhere around the middle of the rpm band and drop off again approaching WOT.
If you want to run, let's say 35mph, you need x amount of horsepower to do it. That will not change a noticeable amount from motor to motor. Only the load in the boat will change x, and the larger the boat the more weight you will need to change x as it will be a proportion of the boat's total weight. If x equals 110 hp for your boat, obviously the smaller hp motor you hang on the back, the greater the rpms you will need to turn to get to 110hp, because the hp curve of the motor is not flat. That puts you into the poor mpg range with smaller motors to hit the desired speed. To get the same mpgs with a smaller motor you will have to sacrifice speed and if you go too small, you won't even have the hp to get on plane anymore and the mpg will be horrid.
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