Is there a rule of thumb for how much mph you get when you add x amount of hp? Assume twin engine , 35' deep v boat and current top of 80 mph, if I add 100 total hp (50 per engine) what would mph be, what about 200 total hp?

On a single engine V it takes between 12 and 20 hp to gain 1 mph. This is assuming you are not near the limits of the hull.

Ron

the typical, approximate behavior of a v bottom follows a squared law

we need to know your current horsepower at the 80 mph

but, if i take a guess and say 2x500= 1000
then adding 100 hp total might give you:

80^2 / 1000 = x^2/1100

x = new speed = 83.9

for 200 hp increase:

80^2/1000 = x^2/1200

x = 87.6