DUDLEY DIX YACHT DESIGN

Boat Design Coefficients


Below are explanations of the various terms and coefficients used for comparison of boat designs. Designers use them when comparing designs from their own drawing boards and those of other designers. The same info can be used by the layman to assist in selecting a design to build.

Block Coefficient

Ratio of the underwater volume of the hull to a rectangular block of the same overall dimensions as the underbody, ie the portion left if you were to carve the underbody out of a solid block.

Formula: Cb = Underwater volume / (LWL x BWL x canoebody draft)

Displacement

Weight of the boat, including crew and stores. This is a variable figure, depending on what loads are on the boat at the time. Quoted displacements are normally to a datum or design waterline and are an estimate of total weight in a particular load condition.

Displacement / Length Ratio

Non-dimensional value for comparison of displacements of hulls of various types and lengths. Low D/L values apply to lightweight racing hulls (40-50) and high D/L values apply to heavy displacement cruisers (350-400).

Formula: D/L = Displ in tons / [(.01 x LWL)^3]

Fineness Coefficient

See Waterplane Coefficient

Midship Coefficient

Ratio of the largest underwater section of the hull to a rectangle of the same overall width and depth as the underwater section of the hull. This defines the fullness of the underbody. A low Cm indicates a cut-away mid-section and a high Cm indicates a boxy section shape.

Formula: Cm = Max underwater section area / (width x depth of largest underwater section)

Prismatic Coefficient

Ratio of the underwater volume of the hull to a rectangular block of the same overall length as the underbody and with cross-sectional area equal to the largest underwater section of the hull. This is used to evaluate the distribution of the volume of the underbody. A low Cp indicates a full mid-section and fine ends, a high Cp indicates a boat with fuller ends. The range for sailboats is about 0.48 to 0.65.

Formula: Cp = Underwater volume / (LWL x max section area)

Sail Area / Displacement Ratio

Non-dimensional figure for comparing sail power to displacement of boats of various sizes. Low figures (15) apply to cruisers and high figures to racers (25-30).

Comparisons involving sail area can be inaccurate because of differing methods of reporting sail area. The traditional method includes the triangle areas of mainsail and 100% foretriangle in the calculation and is mostly used for cruising designs. High performance designs are more likely to be reported with total sail area, including mainsail roach and largest headsail area.

Formula: SA/D = Sail area in sq.ft / [(Underwater volume in cu.ft)^(2/3)]

Sail Area / Wetted Surface Ratio

Figure for comparing sail power to wetted area, indicates performance in light wind conditions. See comment under Sail Area/Displacement Ratio re differing methods of reporting sail area.

Formula: SA/WS = Sail Area / Wetted Surface Area

Waterplane Coefficient

Ratio of the waterplane of the hull (area of water surface supporting the hull) to a rectangle of the same overall length and width of the hull at waterline. This is related to Cp and is used to evaluate the fineness of the hull at waterline. A low Cw figure indicates fine ends, a high Cw figure indicates fuller ends.

Formula: Cw = Waterplane area / (LWL x BWL)


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This page was updated 27 October 2016

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