SAD ratio and sailboat classifications

[sailingunity]

The interwebs seems to be in conflict about what the SAD ratio means about the performance of a boat. In some places I have found that 13-14 means it's a motor sailor (though I've certainly seen some regular sailboats in this range). On one site 18-22 is a "medium cruiser", with 22+ being racing boats, while other sources say 17-19 is a racing yacht and 20+ is a ultralight racer.

There seems to be no standard for either the value ranges or what the class is even called.

Perhaps cruising world can explain why the differences seem to be so stark here: https://www.cruisingworld.com/how/ho...boats-measure/

"Boats measured in the 1970s and the 1980s for racing under the International Offshore Rule for the most part had SA/Ds between 16 and 17, based on the sum of the mainsail triangle (M = PE/2) and 100-percent foretriangle area (100%FT = IJ/2). The measurement system favored small mainsails and large headsails, and since designers of cruising boats stuck close to the IOR sail plan, the IOR value for SA/D became the yardstick. An SA/D above 17 said “fast boat,” and anything below 16 said “slow boat." "

Given that measure - my 1974 boat is on the cusp of being a fast boat, but my knot meter would beg to differ. Obviously there is a that goes into what makes a boat go fast or slow beside the SAD ratio, things like keel shape and wetted surface area, but generally speaking having ranges is probably a "close enough" approach to give folks a general representation of a boat's performance. Piecing together what I have found around here and there, these are the buckets that feel like they make the most sense:

13-15: Heavy cruiser (underpowered)
15-17: Cruiser (average performance)
17-22: Performance cruiser (above average)
22+: Racer

What kind of classification system would you come up with?

[psk125]

The ranges you describe seem to make some sense. As you say, there are other factors involved. The SA/D ratio gives a good indication of the horsepower available to move the boat. The more power available, the faster you can go (to a point). On the other hand, it is essentially a statement of how much sail area there is relative to the displacement of the boat. If you want to move in light air, you will want a high SA/D ratio, regardless of whether you have a racing-type boat or not. If you sail in a heavy-air venue, a lower SA/D ratio would mean not having to reef so much. Different strokes for different folks.

[Bowdrie]

[QUOTE=sailingunity;3767784 The measurement system favored small mainsails and large headsails

13-15: Heavy cruiser (underpowered)
15-17: Cruiser (average performance)
17-22: Performance cruiser (above average)
22+: Racer
[/QUOTE]

^That chart is pretty much in the ballpark in what we tend to use for evaluations.
As regarding "small mainsails", yes, mainsails became more heavily "taxed" than they were under the CCA rules.
What this led to was not only to make the mains smaller, but to increase the aspect ratio, (taller/skinnier mains,) and mast heights started going up.
The taller masts also gave larger headsails, the end results were a sailplan that because of its greater luff lengths for a measured area, gave better speed upwind, especially in lighter air where the wind up high is usually greater than lower down near the surface.
Out on the open ocean on a passage, (assuming good Trade Wind sailing,) the "sail area/wetted surface" ratio is a better measure as wetted surface has a direct influence on drag.
On a racecourse the SA/D becomes more important as it has a big effect upon how quickly a boat can accelerate around the marks and when tacking.
When doing those maneuvers the boat speed is low and wetted surface drag is not as big a deal, (drag going up exponentially with speed,) but it takes lots of horsepower to accelerate the weight.
There is so much more involved with hull shapes/waterplane areas and a host of other factors than my simplistic answer can furnish.
In the end they are just numbers that can give us a "rough idea" of performance parameters.
To give an example, if you go to one of the "data" sites for boats, that give all kinds of numerical values you would find that the numbers for an early '70s S&S PJ/Swan 43, (with the original single-spreader rig,) are not all that much different from an Alajuela 38, (a heavy double-ender).
They're very different boats, yet "by the numbers" they are closer than might be expected.