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Boat Speed Predictions

Please note - by using this calculator you agree to the following license provisions:
This calculator is for non-commercial 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.

Boat Speed Calculator

Weight Total Shaft Horsepower Constant Speed

This information can be used to pick a starting point when propping out a new boat or engine combination. You can  plug these results into the formulas on the Prop Slip page or use our Prop Slip Calculator  and come up with a propeller pitch.  Just remember there is no substitute for accurate testing!  

If you don't have Internet access on your boat, Mercury has a neat slide rule that makes all this easy.