I found someone with the same boat as mine. I was looking to see what kind of speeds to expect out of the same boat with 575HP . This is what the other guy has


1988 Scarab 28' Excel 4600 lbs dry, Holds 80 gal. of fuel
MerCruiser 454 Mag 365hp
Bravo One outdrive 1:50 gears with 23p Mirage Stainless Prop Max Speed 62 mph @ 4700 RPM


Thanks
Paul

The formula for approximate speed is as follows:

( pitch X r.p.m. ) divided by drive ratio divided by 1056

Try bams site go - fast .com

73 to 75mph in my guestimation

62mph in a 28 ft boat with 365 hp seems a little fast to me. I'd bet that is not GPS or radar.

Hot Boat
Use Checkmate statement but subtract 12% slippage

rpm x pittch
__________ = speed with no slip

gear x 1056


108100
_______ =68.24 - slip ??13% = 59.37 mph

1584

15 hp to go 1 mph????

62 mph would be a 4.4% slip I DON'T THINK SO.

365 was at the crank
if you are 575 at the crank and not the prop it will be apples to apples.

575 - 365 = 210 / 15 = 14 more mph + 59.4 = 73.4

How did I do???????????

Here is a link to our on-line speed calculator.

Speed Calc

Thanks for all the help here. I am going to go with what MIKEHTMSR24 said. It seems well thought out and close to what the base speed of the boat should be. I didn't check out the other sights, Mike seems close enough to me. What I need to find is some honest GPS speeds. Its hard to find someone that will tell the troth on how fast there boat goes unless it's something to brag about. Thanks

Paul

rule of thumb that also works well

Sqrt(new HP)
___________ x old max speed = new speed

Sqrt(old HP)

so:

Sqrt(575)
________ x 62mph =77.8mph

Sqrt(365)

If you start with 59mph which as MIKEHTMSR24 pointed out seems more reasonable then the new speed will be 74mph

And y'all told your 6th grade math teacher when will I ever use square roots in real life