is there any rough way to calculate the amount of hp it takes to get 10mph more ? does this change when speeds will be over 70mph ? i know there are different variables that come into play . am thinking of pro charging my boat with 502 mpi,(415hp) am running 70mph gps right now and hope to see 80 mph to 80+ mph ,is it possible?

The only way to get to those numbers are either build your motor more H/P or Super Charge. You will need at least 600 or better horse. To get that hull over the 80 mph mark,

physics says that power is a function of Velocity cubed (i.e. to the third power) for viscous fluids (air and water)

so the theoretical answer to your question is.

if 415 hp to go 70mph, then to get 80 (14.3% more speed)
will require (1.143 cubed) or 1.49 x power, or 619.38 HP.

This assumes same prop slip/efficiency. so being realistic, you will probably need a little more that the 619. but 619 is the minimum requirement.

you can use this function in reverse too. if you build an engine up from 415 to 500Hp for example you should expect a speed of approximately ((500/415) cube root) x 70 =

74.48mph

Where does torque fit into this equation? Supercharging has dramatic effects on boats because they add a lot of torque, which is needed to push through the water.

You will definately gain more than 4.5 mph!

Where's Tomcat? he's the man to ask.

Screw the math. Old rule of thumb, not considering hull variables, is 15 ponies per mph gained....Anyone ever hear anything different?.....

Rick

Here are two formulas that are suprisingly accurate.

New Speed = Old Speed X [the squareroot of (New Horsepower / Old Horsepower) ]

or

New Horsepower = Old Horsepower X [ (New Speed / Old Speed) squared]

It takes alot more horsepower for each mph.

Note - These formulas are simular to the ones that Rambunctious stated. However, he uses cubes and I use squares.

Good Luck!

Clay,
I'll have to agree with your method above. I've also used that formula before and like you said is surprisingly accurate. I think the hyperbolic(square) function is more accurate:

Horsepower is what we're all accustomed to using as a determinant in our speeds, however, in essence what you're really interested in is how many pounds of thrust you can translate that horsepower into.

Maximum speed will be achieved at the precise point when the thrust produced by your engine equals drag(hydrodynamic=hull, outdrive & aerodynamic=hull).

And drag(hydro, aero) is a function of velocity squared.

There you go. That's the first explanation I've heard that sort-of takes into account hull variations...I'm jottin' that down dude...

Rick

Just stick a blower on it and see what happens. Remember you will need a bigger prop too. Go to 650 HP so you get over your 80 MPH. mark.