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,

'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.

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 value | Description | Examples |
---|---|---|

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,

SA/D distribution

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.

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 for the average boats

References