# Comparing Design Ratios

When buying a boat, consider how its ratios compare with those of similar boats in its category.
• Author:
• Updated:
Original:

Given the infinite number of variables involved, making any kind of objective comparison between two different sailboat designs might, at first, seem impossible. However, with the help of design ratios, you can not only compare and contrast different designs, but get a pretty good idea, sight unseen, as to how a boat is going to perform under sail.

The first of these, the Displacement-Length Ratio (D/L) is a nondimensional expression of how heavy a boat is relative to its waterline length. A D/L ratio is calculated by dividing a boat’s displacement in long tons (2,240 pounds) by one one-hundredth of the waterline length (in feet) cubed. Because the displacement in long tons can be expressed as a volume (cubic feet of seawater), and since length-cubed is also volumetric, the result of the D/L formula is nondimensional, i.e., it is no longer tied to a particular physical size.

This means the D/L can be used to compare the relative "heaviness" of boats no matter their size. For example, a boat with a 25-foot waterline and one with a 50-foot waterline are equally heavy relative to their respective waterline lengths if they have the same D/L ratio. The relative heaviness of boats is often categorized as follows:

These gradations are applicable to current designs, which tend more toward reduced overhangs and longer waterlines than earlier boats. Most sailboat designs of the 1930s through the 1960s had D/Ls (as they are often referred to) above 300, but with their long overhangs their sailing length was increased as they heeled, resulting in “effective” D/Ls at speeds that were well below their calculated values.

The significance of the displacement-length ratio is that the lighter a boat is relative to its waterline length, the higher its speed potential, especially when in displacement mode. If there is less water to push aside, wavemaking drag is reduced. Some ultralight-displacement boats, or ULDBs, are light enough to plane just like a powerboat, in which case a low D/L gives some indication how quickly and easily a design will transition out of displacement mode. (Although hull shape is also important.)

While at first it might seem desirable to always have as low a D/L as possible, as in most aspects of yacht design there are trade-offs. In general, the lower the D/L, the less comfortable the boat’s motion will be in a seaway and the more sensitive the boat is to overloading.

Another important design parameter is the Sail Area-Displacement Ratio (SA/D), another nondimensional calculation that expresses the relationship between a boat’s sail area and its displacement. Traditionally, it is calculated by dividing the nominal sail area in square feet by the boat’s displacement in cubic feet to the two-thirds power.

In this case, displacement is converted from pounds to cubic feet by dividing it by 64, the weight of a cubic foot of seawater. The standard displacement used by most designers for comparative purposes is the half-load displacement, calculated with the boat equipped for sailing with the crew and half the consumables (provisions, water, fuel, supplies) on board.

The nominal sail area is calculated as the sum of the foretriangle area (one-half the base, or J dimension, multiplied by the height, or I dimension) and the mainsail area (one-half the foot, or E dimension, multiplied by the hoist, or P dimension). It's important to note, however, that naval architects (and marine journalists) are increasingly using actual sail areas for mainsails, in particular, because the nominal area fails to account for the larger roaches often flown on modern rigs.

The SA/D is also nondimensional because sail area in square feet is divided by a displacement term that has been converted from cubic to square feet. This mathematical manipulation results in a parameter that again is independent of boat size. Therefore, a 30-footer with a SA/D of 20 will have just as powerful a rig as a 60-footer with the same SA/D.

In general terms, a boat having a sail area-displacement ratio under 15 would be considered under-canvased; values above 15 would indicate reasonably good performance; and anything above 18 to 20 suggests relatively high performance, provided the boat has sufficient stability and a low enough displacement-length ratio to take advantage of its abundant sail area.

The SA/D is analogous to an automobile’s horsepower-to-weight ratio, with sail area being a measure of power, and displacement being indicative of wavemaking drag. The principal components of drag are friction and wavemaking; since wavemaking increases disproportionally with speed, the SA/D is indicative of performance in moderate- to heavy-air conditions.

It is important when comparing the SA/Ds for different boats to be certain that the sail area and displacement values are measured in the same way for each boat. In SAIL's New Sailboat Review, we typically calculate ratios using the mainsail and 100% foretriangle area.

On occasion a boatbuilder may publish sail areas and displacements that are larger and lower than nominal so as to maximize a boat’s SA/D ratio in the interest of making appear to be higher performance. This is done by quoting the area of an overlapping genoa, or in the case of displacement, publishing the light-ship weight of the boat as shipped from the builder rather than the half-load displacement.

Finally, there is the Ballast Ratio, or the amount of ballast a design is carrying. Unlike the first two ratio, this one requires no fancy conversions of units: simply divide the weight of the ballast by the half-load displacement, and then multiply by 100. The higher the number, the greater the righting moment, i.e. the greater to the ability to resist heeling.

Typically, a ballast ratio of 40 or more translates into a stiffer, more powerful boat that will be better able to stand up to the wind. However, it's important to be aware that ballast placement also plays an important role in righting moment: for example, the same weight of internal ballast will provide far less righting moment than at the very tip of a deep fin keel. Therefore, ballast ratios are somewhat less determinant than the first two.

While righting moment if obviously important to good sailing performance, this is another area in which is it possible to have too much of a good thing. Specifically, a boat with an aggressive righting moment will have a faster, more aggressive motion in a seaway, which can be hard on crews. If you're only spending an afternoon racing around the buoys, then comfort may not be especially important. However, "sea-kindliness" as it's called, can make all the difference in the world on a long passage.

## The Extraordinary Life and Mysterious Disappearance of Thomas Thor Tangvald

The first boat Thomas Tangvald ever owned was just 22 feet long. She was an odd craft, a narrow plywood scow with a flat bottom, leeboards on either side, and square ends—little more than a daysailer with a rotting deck and tiny cabinhouse tacked on. Thomas paid just \$200 for ...read more

## USVI Charter Yacht Show Showcases a Flourishing Industry

As the U.S. Virgin Islands continues to attract sailors seeking to charter and explore the pristine territory on their own, the immense growth and expanded options for a crewed yacht or term charters have exploded here over the past five years. Last week, the USVI Charter ...read more

## Personal Locator Beacon Wins Top Design Award

The Ocean Signal RescueME PLB3 AIS Personal Locator took top honors at the 2022 DAME Design Awards, while Aceleron Essential, a cobalt-free lithium-iron phosphate battery with replaceable and upgradeable parts, won the first DAME Environmental Design Award. Announced each year ...read more

## EPIRB in the Golden Globe Race

Tapio Lehtinen’s boat sank early this morning southeast of South Africa while racing the Golden Globe Race, a faithfully low-tech reproduction of the 1968 Sunday Times Golden Globe. The boat went down quickly and stern-first according to the skipper’s emergency transmissions. ...read more

## Victory, Tragedy in the Route du Rhum

The 2022 Route du Rhum was a highly anticipated event in the ocean racing calendar, but few could have predicted exactly how challenging, dramatic, and tragic it would ultimately prove. French yachtsman Charles Caudrelier took home gold aboard the Ultim maxi trimaran Maxi Edmond ...read more

## Boat Review: Lyman-Morse LM46

Lyman-Morse has been building fine yachts in Thomaston, Maine, ever since Cabot Lyman first joined forces with Roger Morse back in 1978. With experience creating and modifying boats built of various materials, backed by its own in-house fabrication facility, the firm has ...read more

## Know-how: All-new Battery Tech

Until very recently, the batteries in sailboats used some form of lead-acid chemistry to store energy. Different manufacturers used different techniques and materials, but in the end, the chemistry and the process by which the batteries charge and discharge electricity remained ...read more

## At the Helm: When Things Go Sideways

I don’t like sea stories. My number one goal on every passage is to get the crew back in one piece. My number two goal is to get the boat back in one piece as well. If I can’t do both, I’ll take the former. Do this long enough, though, and things are going to happen, no matter ...read more