math formula for speed????
 

I know I saw something somewhere on how to compute probable top end speed in a boat....any math experts here? I want to know how much power it will take to move a certain hull to a certain top end. Can it be done via a formula? This may help in determining what engines to decide upon in any particular boat..... any takers?

$ + $ = MPH

ROFLMAO!!!!!:D :p :D

Here's a starting point: rpm X pitch X ratio X .00094697 divided by slip. Now comes the hard part. How do you know how to calculate slip? There is a different factor for every kind of hull; V, displacement, cat, etc. Its a very black mystery as is propellor technology. If you get 100 answers here, you'll get 100 different solutions. Remember, there is never too much horsepower or too many cubic inches or too many dollars...

ooooooooooooooh, that's how you do it...

http://go-fast.com/prop_slip.htm
this is pretty good

Actually, try this for speed predictions, the formula is there if you want to do it long hand, but I put a calculator online.

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

The formula we use has proven very accurate;
100 hull efficiency = RPM/gear ratio x pitch/12/5280 x 60

The tricky part is to have the % of efficiency value of your boat (less than the 100 value that your hull is) You would then at the end of this equation multiply the result by this percentage to receive the speed.

Hope this helps,

Larry Carpenter
E-Ticket Performance Boats

interesting, thanks for the help....only question now is how do I know what the hull efficiency is??????

Don't confuse prop slip calculations with predicting speed.

Once you already know how fast a boat goes, the prop calcs are fine for fine tuning or predicting RPM changes for different props and ratios.

Speed predictions need to take into account hull efficiency, weight and power. Nothing to do with prop and ratio. The prop / ratio stuff is after you figure out how fast it might go.

The speed prediction part is very difficult, as the hull efficiency can only be guessed at (can be a very educated guess) until the boat is actually tested. Picture 2 of the same hull, optimize drive height / setback on one and it might be 5-10 MPH faster than it's more conservative twin. Different efficiency.

makes sense Mark, I thought we might be getting off the path with the prop calculators....I used prop calculators before to determine speed after we had baseline

There is a coincedence here, someone mentioned Jerry Gilbreath a few posts ago. Jerry is one of the rare people who can look at a boat, learn the horsepower, look at the set up, and judge the speed very accurately. Thats another of his attriblutes and is better than all the calculations that will be shown here...

If you know your current horsepower and speed, then use this formula...

New Speed = Old Speed times [ the square root of ( New Horsepower / Old Horsepower ) ]

also

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

Good luck! :cool:

This is the way I do it as a broker:

### in MPH quoted by seller
Subtract 10MPH for every six months the liar has had it on the market.
then
Add 2 MPH if the boat has clear title
Add 2 MPH if his wife is within earshot
Add 2 MPH if he honestly does not want to sell the boat

On the other hand
Subtract 2 MPH if the owner owes more than the asking price
Subtract 2 MPH if the Wife is not around
Subtract 5 MPH if the owner has been drinking
Subtract 5 MPH if a hot 22 year old blonde is within earshot
Subtract 10 MPH if Stinsen quoted the speed!

IDRPSTF,

iI bought a boat using that formula once and it was pretty accurate, now if only I can remember where I put it

The boat or the formula?