 
Boat Speed Predictions
Please note  by using this calculator you
agree to the following license provisions:
This calculator is for
noncommercial use by the general public only. It is proved for
entertainment only and not intended to be used for any other purpose.
Commercial use for any reason is prohibited.
A license for commercial use may be available upon application to BAM
Marine on a per use or annual basis.
There are so many factors that affect boat speed that
it is hard to make accurate estimates of what the real top end will be. Mercury Marine
uses the following formula to estimate potential top end,
this calculator uses this formula.
The Square Root of (Total Shaft Horsepower / Weight )
x Constant = Speed
The constants have been developed from experience, and are
as follows:

Accurate data is important
when using this calculator. Weights should be actual running weight as
determined by a scale and are to include driver, fuel, engines, supplies
etc. We have found manufactures published weight to be unsuitable for these
calculations. Shaft horsepower is to be the actual uncorrected engine
horsepower less drive train losses. Adjustments of 5 to 10% of corrected
dyno figures are typical. 

Optimum Slip Percent (For max
top speed) 
Heavy V bottom 
Low drive height, cabin, side by side engines 
225 
10  13 % 
Fast V bottom 
High Drives, boxes, notched transom, good
bottom 
250 
8  9 % 
Catamaran 
Pleasure type cat 
275 
6  8 % 
Fast Catamaran 
Late Skater
full race, etc 
300 + 
4  6% 
Some of the newer cats require an even higher constant than
those above.
The calculator below uses the above formula. Plug in your
info, and click "Calculate" on the item you want to compute. By using
known values for the weight, power and speed you can come up with a more accurate constant
for your boat. You can then see what effect changes to weight or power will have on top
speed. Using the calculator in this fashion will produce surprisingly accurate results.
