SA/D Calculator

Below is a quick SA/D calculator (Sail Area to Displacement ratio) – metric and imperial calculator.

This is an important design ratio (Sail Area-Displacement Ratio (SA/D)), an indication of the boat’s sail power given its weight. Let’s say you have 2 boats with the same displacement. Boat A has an SA/D ratio of 25. Boat B has an SA/D ratio of 20. Boat A therefore flies more sail area than Boat B and is more “highly canvassed”.

Compare Like for Like (esp. Foretriangle Area)

It is important to try and compare “apples with apples” with this ratio. 

The sail area is normally calculated as the sum of the foretriangle area and the mainsail area.

A good approximation is to use the main plus jib or solent (upwind sail area) but bear in mind that a genoa will overstate the SA/D and a self-tacking solent will likely understate it.

One trick is to apply a manual adjustment to the foresails as follows:

Self tacking jib 90% of foretriangle. So add 11% to the solent area for the calculation.
Jib 110-120%. Use 91% of the overlapping jib area (100/110)
Genoa: 130-150%. Use 77% of the genoa area (100/130).

In the end, you might have to “eyeball it” to get to a reasonable estimate.

Equations

Imperial:
SA⁄D = SA (ft²) ÷ ((D (lb) ÷ 64))^(²⁄₃)

Metric
SA⁄D = SA (m²) ÷ (D (tonnes))^(²⁄₃)

Sail Plans

A catamaran with an SA/D under 18 would be considered under-canvased; values above 20 would be average, anything above 25 suggests high performance, and values over 30 translate to very high performance. 

The important thing when comparing the SA/Ds is to try and make sure that the sail area and displacement values are measured in the same way for each boat. 

Manufacturers

Bear in mind that manufacturers may overstate sail areas and understate displacements to maximize the SA/D ratio for marketing purposes. So they may use the area of an overlapping genoa for example. The important thing, is to try and maintain a consistent methodology so that you can compare boats.

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