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
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
'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'
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
|16 - 18||Heavy offshore cruisers|
|18 - 22||Medium cruisers|
|22 - 26||Inshore 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.
As an example, the figure below shows the distribution of the SA/D for 9 meter (30 ft) sailboat designs.
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.
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
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).