Quote:
Originally Posted by Dave R
Just divide the MPH by the GPH to get MPG. 30/5=6 so 6 MPG.
6MPG in a 24 foot bowrider seems pretty optimistic though.
Assuming you have a 300 HP 5.3, I'd expect the top speed to be close to 50 MPH. Since drag is a function of the square of speed we can calculate the HP needed to go 30MPH if the boat tops out at 50MPH
50x50=2500 2500/300=8.33
30x30=900 900/8.33 = 108 HP
It takes roughly .08GPH to make 1 HP so 108 HP would require 8.64 GPH so I think you are getting around 3.5 MPG, 30/8.64= 3.5
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I think most of your assumptions are spot on - with my current prop setup, my top speed is just a little over 50 mph.
I may be too simplistic in my approach here, but I basically take the gallons used in filling up the tank divided by the engine hours between fill-ups. I've been doing this since I bought the boat, and I have about 320 engine hours. With this math, I usually come out between 5 and 6 when I haven't been towing skiers, tubers, etc. When not towing, my cruising speed in decent conditions is usually around 30 mph. Not too scientific, and probably not very accurate.