Sail Area Displacement Ratio (SA/D)

What is Sail Area Displacement Ratio (SA/D)?
The Sail Displacement Ratio is an attempt to estimate the sail power of a given sail boat. The SA/D indicates how fast a boat is in light wind.

How to calculate the Sail Displacement Ratio (SA/D)?
Sail Displacement Ratio formula
Sail Displacement Ratio formula

One could wonder why this formula is so complicated? Why not simply calculate SA/D as the area of the sail divided by the displacement?!?
That is because the inventor of the formula wanted the SA/D ratio to be unitless, i.e. the inventor preferred the value to be e.g. '18' in stead of '18 ft2/lb'.

'Problems' with the formula
Even though the formula makes sense, it has some areas of concern: 1) Two people may disagree on the SA/D for a given boat because they don't use the same sail area or displacement. 2) How to compare two different boats? We have described these issues and how to mitigate them here: Issues with 'Sail Area' and Issues with 'Displacement'.

SA/D range of values
The values below are the most commonly used to categorize the SA/D ratio.

SA/D range of values
SA/D valueDescriptionExamples
16 - 18Heavy offshore cruisers
18 - 22Medium cruisers
22 - 26Inshore crusers / racing boats
26+Extreme racing boats

The larger sail you use the higher SA/D value, i.e. a 135% genua will result in a larger SA/D value than a standard (100%) jib. In order to be able to provide a fair comparison between different boat designs, we use the standard jib and main sail as described here: Issues with 'Sail Area'. So,... that's why you will find lower values in our statistics than in the table above. Its not an error, its a consequence of fairness.

SA/D distribution
As an example, the figure below shows the distribution of the SA/D for 9 meter (30 ft) sailboat designs.
SA/D distribution

For 9 meter sailboats the mean for SA/D is 15.9. Looking at the figure, the value 15.9 is pretty much in the center of the histogram. The standard deviation σ is 2.15.

SA/D scattering
SA/D scattering
This figure shows the scattering of the SA/D ratio for sailboat designs. Each dot represents a specific design.

SA/D for the average boats
SA/D ratio per length
This figure shows the mean value of the SA/D ratio as function of the length of the hull. Each dot represents the mean value for a given length, and the vertical line represents the uncertainty of the mean value. The figure shows that designers tend to give small boats (less than 7 meter / 23 feet) and longer boats (more than 12 meter / 40 feet) more sail relative to the boats in the midrange (8 - 11 meter / 26 - 37 feet).