how fast

[Jon Fuller]

Originally posted by FrenchPhil
Isnt there a speed calc out there with boat weight ? It definitly has an impact on speed.

There's a formula for comparing boats of a similar design, with differing weight and Power, that I know as the 'Levi' formula. but it basically says, to double your speed, you must quadruple your power.

[BluFin]

Originally posted by Jonny
......., to double your speed, you must quadruple your power.

How strange that works almost exactly for take home salary for ex wives to double your income you must quad your salary

[Jon Fuller]

Ouch!

[BluFin]

Attached zip of Speed calc on Excel S/Sheet
Type known info in yellow boxes, you must know your gear ratio

Slip can vary greatly

[dbe1]

I ski raced a Yam Prov 200 set on a power lift on a Phantom 21, hit 75mph ,6800 revs. ran a 23 Prov stainless prop race prepared by Pole Propulsion, rev limiters removed, blue printed, 250 yam heads, Boysen reeds, terrific acceleration out of the hole, never missed a beat in over 4 years of hard racing.

[Matt]

Also the knowledge base on www.go-fast.com is pretty good for calculations & predictions.

[FrenchPhil]

http://www.go-fast.com/boat_speed_predictions.htm

Is what I had in mind weight wise.

[Jon Fuller]

Phil, gotta be a fluke, but that calculator said of my boat, based on 6600lbs, 650hp and 225 'constant = 71mph, which is precisely what it does.

There has to be hint of luck in there tho', coz you could have the exact same input figures, with another hull design, weight distro etc, and get a very different outcome.

[FrenchPhil]

Well John, I tried a few scenarios & it's pretty close. Anybody try with 16-20' Phantoms
with 90-280 hp ?

[BluFin]

Dont rely on it for multihulls I have just run the thing from my old raceboat data from 84 it calculated out at 116 MPH
(I my what would have been very wet dreams)
Never not even close.

It does state cats are a variable though

What does the specter weigh TD have you run it through that link?

[Jon Fuller]

yeah, but did you put your all up weight in as 'pounds'?

[jw.]

Originally posted by Jonny
Phil, gotta be a fluke, but that calculator said of my boat, based on 6600lbs, 650hp and 225 'constant = 71mph, which is precisely what it does.

There has to be hint of luck in there tho', coz you could have the exact same input figures, with another hull design, weight distro etc, and get a very different outcome.

It must be that constant at 225 is dead right for your boat at the weight you put in.

[Jon Fuller]

Actually, when I read the instructions properly, and put in the correct HP (after transmission losses) and the exact weight, rather than approx, the numbers changed, now it seems my boat has a constant of 235

[BluFin]

Yep in pounds, could be in the transmission loss, being great than anybody realised .
great to play with eh !!