What is the sail area-displacement ratio (SA/D) of a sailboat?

When exploring the purchase of a bluewater sailboat, or any sailboat for that matter, you’ve probably encountered various ratios that offer insights into the sailboat’s handling or traits, tailored to its intended application. Among these is the Sail Area to Displacement ratio (SA/D), a valuable metric that can shed light on the performance potential across all types of vessels, from daysailers and racers to bluewater sailboats. In this article, we aim to provide you with an understanding of what this ratio signifies, the method for calculating it, and the important considerations to bear in mind when applying it to assess sailboat performance.

What is the sail area-displacement ratio (SA/D) of a sailboat?

The fundamental insight provided by the SA/D ratio is an estimation of a sailboat’s sailing power in relation to its weight. This metric offers a glimpse into the vessel’s power efficiency based on its sail area, for instance, indicating potential speed in light wind conditions. It is derived from the sail size required to propel a specific volume of water, corresponding to the boat’s displacement.

This ratio serves as a straightforward method to gauge performance and speed. However, it’s important to remember that numerous other factors also play significant roles in affecting a sailboat’s capabilities, and these should be considered for a comprehensive understanding of its performance.

How do you calculate the sail area-displacement ratio (SA/D) of a sailboat?

What do you need to figure out the SA/D ratio? Just the sail area in square feet and the boat’s weight in pounds. In the equation, you divide the pounds by 64 to change it into cubic feet (since a cubic foot of salt water weighs 64 pounds). Then, the bottom number of the formula is raised to the power of 2/3.

You might ask, why make the formula this tricky? Why not just divide the sail area by the weight? Well, the inventor wanted the SA/D ratio to not depend on units, meaning they preferred a plain number like ’20’ instead of ’20ft²/lb’.

To get the specific details for your boat, sailboatdata.com should have everything you need. Usually, they’ve even done the math for you, and you can find the ratio already calculated on their website.

So lets try to calculate this according to the data of our Fellowship 28:

The specifications:

  • Smallest sail area according to the original brochure (Jib + Main): 283 ft² (largest according to the brochure would be 363 ft²)
  • Displacement according to the original brochure 7,937 lb

The Fellowship 28 features an SA/D ratio of 11.38, a figure that positions it closer to a motorsailer than a high-performance sailing yacht. This highlights a critical issue with the ratio: its reliability is contingent upon accurate data. If we use the maximum sail area listed in the brochure, the ratio increases to 14.60, shifting its classification towards that of a more leisurely ocean cruiser. This topic will be explored in greater detail further on in our article.

Categories of the SA/D

The SA/D serves as a metric to classify sailboats by their respective values. In the following section, we explore how renowned yacht designer Ted Brewer, who also came up with the Comfort Ratio, organizes these vessels in his book “Understanding Boat Design.”

Boat TypeSA/D
Motorsailers13 – 14
Slow auxiliary sailboats14 – 15
Average offshore cruisers15 – 16
Coastal cruisers16 – 17
Racing yachts17 – 19
Ultra light racers20+

Important things to consider when interpreting the CR value

Always compare apples with apples

The SA/D ratio by itself doesn’t offer much clarity—yes, it allows you to categorize a boat as we’ve discussed, but it only provides a ballpark figure of performance. As seen in our previous calculation, the presence of a larger Genoa significantly influences this metric. Thus, exercise caution when comparing different boats. Ensure that the sail configurations and types are alike. Particularly when purchasing a new boat, pay attention to how the manufacturer calculated this ratio. Some might inflate the ratio by opting for larger sails, such as overlapping genoas, or by basing the weight on the boat’s bare-bones delivery state, devoid of any additional load.

Taking other factors into account

While the SA/D ratio serves as a handy benchmark for comparing similar boats, it’s not the ultimate determinant of a boat’s behavior in specific sea or wind conditions. Remember to consider other aspects like the shape and form of the keel, the design of the bow and stern, weight distribution, and other such traits to determine the best fit for your needs. Reflect on your intended use for the boat; the requirements for a cruising vessel differ markedly from those of a racing yacht, for example.

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