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balaobill

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I have a 23.5'walk around

I have a 23.5'walk around trophy. weight of boat is 4400lbs. deep vee. it has a 190 hp 4.3 mercruiser stern drive alpha. Wide open throttle boat runs around 26 -27 knots. Want to put 315 hp 5.7 vortex in her. engine is 100 lbs. heavier.
would like to know estimated boat speed with the 315 hp engine.

thanks for your help.
bill
 
"A good rule of thumb I read f

"A good rule of thumb I read for increase in boat speed is that it takes hp equal to the square of the speed increase to do the job. So if you want to increase your speed by 20%, it will take 44% more power. (1.20 x 1.20 = 1.44).
So for your case you can expect the sq. root of 315/190 which is 1.288 times 27 knots = 35 knots. Better. Meanwhile you can expect to burn 315/190 = 1.66 times as much gas doing it.
Nasty, ain't it?
I am replacing my 180 hp 4.3 in a 20' Mirage (2500 lbs) with a Merc bobtail 383 stroker MPI motor rated at 350 hp. My top speed at present is 47 mph; so I expect sq. root 350/180 = 1.39 times 47 = 65 mph max. We will see what happens.
How often will I want to go that fast? Maybe twice a season.
Good luck,
Rod"
 
"Mark makes a good point.
I a

"Mark makes a good point.
I am changing from 1.81 upper gear ratio to 1.47 as used in most V-8 applications.
You will need to do the same, or go from a 19" pitch prop up to 23" or so.
Rod"
 
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