Sail Area-to-Displacement Ratio

The Sail Area-to-Displacement ratio is based on the relationship of the power developed by a boat's sails to her total weight (displacement should include the total weight of the boat, her supplies, and crew). The rating is derived by dividing the sail area by the boat's displacement (expressed in terms of volume) to the 2/3 power. It produces values in the following ranges:

Displacement-to-Length Ratio

The Displacement-to-Length ratio is a means for categorizing hulls based on their "density". It is figured by dividing displacement (in tons) by 1/100th of the LWL in feet cubed. The generally-accepted ranges are:

Theoretical Hull Speed

Theoretical Hull Speed is calculated based on the length of a displacement-type hull at the waterline. At her theoretical maximum hull speed a boat rides in the trough between two wave crests. Waves travel at a rate based on the distance between their crests. The formula is: Speed = 1.34 * the square root of LWL (LWL being equal to the distance between wave crests). Planing hulls (as in multi-hulls and centerboard dinghies) attain faster speeds based on their ability to "climb over" their wave.

Capsize Screening Ratio

The Capsize Screening formula was developed by the U.S. Sailing Association to judge the safety of offshore sailboats. The beam is divided by the cube root of the displacement (expressed in cubic feet). Values lower than 2.0 are considered safe for offshore sailing. The rating improves as the displacement increases and the beam decreases.

Waterplane Area

Waterplane area is the area of a horizontal slice through the hull at water level.

Ballast-to-Displacement Ratio

The Ballast-to-Displacement ratio simply expresses ballast as a percentage of a vessel's total displacement.

Comfort Ratio

This is a ratio formulated by Ted Brewer as a measure of motion comfort. It has been widely accepted and, provides a reasonable comparison between yachts of similar type. It is based on the fact that the faster the motion the more upsetting it is to the average person. Given a wave of X height, the speed of the upward motion depends on the displacement of the yacht and the amount of waterline area that is acted upon. Greater displacement, or lesser WL area, gives a slower motion and more comfort for any given sea state.

Ratios will vary from 5.0 for a light daysailer to the high 60s for a super heavy vessel, such as a Colin Archer ketch. Moderate and successful ocean cruisers, such as the Valiant 40 and Whitby 42, will fall into the low-middle 30s range.