Hey Guys,

I recently came across a formula for calculating top speed/ hp and it’s been pretty accurate from running the numbers on some of my friends boats. I don’t think it’s perfect but so far is seems to get close.

Here it is:

Horsepower/Weight of Boat = X

Square Root of X = Y

Y x 250 = Hypothetical MPH.

Hypothetical MPH x 90% = True MPH (accounts for prop slip).

I’m curious to see if this holds up, especially considering the many variations in Hull Design. Post your numbers from the formula vs what your boat will actually do if you’d like.

Works for my straight V bottom within a mile per hour.

Very good, though prop slip is still relevant. This table gives us a Constant to use based on Hull Type as well as provides more accurate prop slip numbers.

So instead of generic Yx250, we can get more specific with this table and choose the proper constant for out equation as well as adjust the prop slip portion of the formula.

doesn’t work for me.

Interested in my next config, testing the formula with a previous known config:

9600 lb boat (not factoring in fuel or passengers)
1200 hp
top speed actual was 68.9 mph

using the formula...

Horsepower/Weight of Boat = 0.125

Square Root of 0.125 = 0.35355

0.35355 x 250 = Hypothetical MPH of 88.38

= 22% error

Thats how I I initially ran the formula as well.
Take a look at the little paragraph on the Black Table above, it states that you need to use full loaded weight in the calculation. Also, you forgot to multiply by .9 to account for prop slip. I wonder if that would get you closer.

If I take your info, add 1k lbs for passenger and fuel, subtract 10% from your hp to account for drivetrain losses and then remove the 9% for prop slip, we end up at 73mph.

Still not perfect, but much closer to reality.