Calculated by multiplying the LWL area by 5.333 for sea water or 5.2 for fresh water.īN – Bruce Number: The Bruce Number is a power-to-weight ratio for relative speed potential for comparing two or more boats. The weight required to sink the yacht one inch. The maximum speed of a displacement hull (referring to a hull that travels through the water rather than on top of it, e.g. For boats of the same length, generally the higher the S#, the lower the PHRF. The equation incorporates SA/Disp (100% fore triangle) and Disp/length ratios to create a guide to probable boat performance vs. S# first appeared (that we know of) in TellTales, April 1988, “On a Scale of One to Ten” by A.P. The displacement in cubic feet can be found by dividing the displacement in pounds by 64. The lower the better.ĬSF = Beam/Disp (cubic ft)^.333. The boat is better suited for ocean passages (vs coastal cruising) if the result of the calculation is 2.0 or less. The CSF compares beam with displacement since excess beam contributes to capsize and heavy displacement reduces capsize vulnerability. 3 LOA) x Beam^1.33), where displacement is expressed in pounds, and length is expressed in feet.ĭesigned to determine if a boat has blue water capability. Over 50 indicates an extremely heavy bluewater boat.Ĭomfort ratio = D ÷ (.65 x (.7 LWL +. Numbers below 20 indicate a lightweight racing boat ģ0 to 40 indicates a moderate bluewater cruising boat Ĥ0 to 50 indicates a heavy bluewater boat

Consider, though, that the typical summertime coastal cruiser will rarely encounter the wind and seas that an ocean going yacht will meet. It is based on the fact that the faster the motion the more upsetting it is to the average person. It provides a reasonable comparison between yachts of similar size and type.

This is a ratio created by Ted Brewer as a measure of motion comfort. The lower a boat’s Displacement/Length (LWL) ratio, the less power it takes to drive the boat to its nominal hull speed. A sail area/displacement ratio below 16 would be considered under powered ġ6 to 20 would indicate reasonably good performance Ībove 20 suggests relatively high performance.Ī Ballast/Displacement ratio of 40 or more translates into a stiffer, more powerful boat that will be better able to stand up to the wind.īal./Disp = ballast (lbs)/ displacement (lbs)*100

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