For boats with adjustable keels (centerboards, daggerboards, lifting and swing keels), Draft (max) is with the board down. A boat’s actual draft is usually somewhat more than the original designed or advertised draft. Like the LWL, it will vary with the weights of fuel, water, stores and equipment. Beam WL: Greatest width of the hull at the waterline.ĭRAFT: This is the depth of the hull from the LWL to the bottom of the keel or fin. The LWL will increase as the yacht sinks into the water with the added weight of stores and equipment.īEAM: This is the greatest width of the hull and is often expressed as Beam (Max). LENGTH WATERLINE (LWL): LWL is the length of the hull at the level where it sits in the water (the waterline) as measured from the bow ending at the waterline to the stern ending. Length Over All (LOA) may be the LOD but often includes rail overhangs, anchor sprits, bowsprits and even boomkins. Length On Deck (LOD) is the true length, omitting rail overhangs. LENGTH: Different designers and builders have different ways of expressing Length. A cat with a number 0.6 is likely to sail 6kts in 10kts wind, a cat with a number of 0.7 is likely to sail at 7kts in 10kts wind. The higher the number the faster speed prediction for the boat. It takes into consideration “reported” sail area, displacement and length at waterline.
Kelsall Sailing Performance (KSP): Another measure of relative speed potential of a boat. A boat with a BN of 1.6 or greater is a boat that will be reefed often in offshore cruising.ĭerek Harvey, “Multihulls for Cruising and Racing”, International Marine, Camden, Maine, 1991, states that a BN of 1 is generally accepted as the dividing line between so-called slow and fast multihulls. 100% fore-triangle only, no overlapping sails.Ĭhris White, “The Cruising Multihull”, (International Marine, Camden, Maine, 1997), states that a boat with a BN of less than 1.3 will be slow in light winds. It takes into consideration the displacement and sail area of main and jib. 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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