Crouch’s Planing Speed formula
This formula applied to planing boats only and only takes into account the displacement of the hull, ignoring length. This may seem odd but as opposed to a displacement hull, the most important factor in determining speed is displacement.
If you think of skipping stones as an analogy, first off a good skipping stone should be flat so that it can skim (plan) over the water at high speed, secondly it’s got to be small/light enough so that you can throw it fast enough to allow it to skim. If that stone isn’t the right shape or don’t throw it right I’ll break the surface and slow down rapidly. These two characteristics apply to the planing hull.
The shape of the hull’s run needs to be flat to be conducive to planing (skimming) and the power to weight ratio needs to be sufficient to get it up on plan.
Hull Speed Calculator, Results
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, as calculated here, is important as it is a building block to attaining figures like, what power is required and ultimately calculating what propeller may be optimum for a particular boat.Hull Speed ChartThe chart displays speed at different percentages of theoretical hull speed.
Typically you will not try to push a displacement the vessel past 100% of hull speed but I have shown the it at 125% so that in the next step it can be used to illustrate how much power is required to push the hull past this speed.Formula1.34 * LWL^½See SL Ratio for further information.
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.