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.
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.