Power Required Calculator, Results

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HP quoted should be considered as SHP (shaft horsepower) i.e. the power available at the shaft for the highest RPM's attainable. Most outboard manufacturers quote HP as SHP. For inboards consult your manufacturer, as most are quoted in BHP (break horsepower), SHP is typically 96% of BHP, depending on the number of bearings, gearboxes and ancillaries. The results and the chart plot are based on an SL Ratio of 1.34. this “magical” 1.34 is just a guide as to the usual theoretical hull speed, beyond which the increasing amount of power required to push a hull past a certain speed becomes impractical. 1.34 is typical but lighter displacement boats may be allowed a higher number. Note that this number only applies to displacement hulls.

Hull Speed Chart

The chart displays speed at different percentages of theoretical hull speed and the SHP required to reach these speeds. Typically you will not try to push a displacement the vessel past 100% of hull speed but I have shown the it at 125% to demonstrate the increase of power required that makes going past hull speed impractical.

Formula

  • Horsepower = Displacement / ((150)2 / (Hull Speed)2)
  • See SL Ratio for further information.

Also see:

Boat Hull