The letter was one that many of us could write. “We have dreamed about liveaboard cruising for many years and have chartered any number of boats for a week or a weekend. Now we’re ready to live aboard full time.” So wrote a California couple looking for advice on choosing a boat for running down to Panama and getting into the Western Caribbean. “In short, we’re looking for a comfortable, liveaboard coastal cruiser.” They had been considering four -respected stock 36- to 40-footers built around 1980. Which should they buy?
While I couldn’t name a favorite, I did suggest that “coastal cruiser” may be too confining a category for a boat with such ambitions. Coastal cruisers spend most of their time in protected waters, but the Pacific Coast and western Caribbean require offshore work, including those slogs to windward that gentlefolk like to say they’ll never endure. I advised that they look at publications that do objective boat reviews, and that they retain a surveyor. And because it’s easy for the dream of a lovely cruise to lead, willy-nilly, to a dangerous fantasy about the proper boat, I encouraged them to be ruthlessly critical of their needs and goals. As the yacht designer Norman L. Skene wrote long ago, “The design in its entirety should be a frank, vigorous declaration of the use to which the boat is to be put.” The first step is to define that use. Once the goal is defined, what next? Among the considerations are the accommodation plan, strength of construction, access to the engine and electronic systems, and (not least) aesthetics, for we all want a boat that “looks right.”
And then there’s performance: stability, seaworthiness, seakindliness (the boat’s motion while underway), and speed. A few numbers can help you compare performance among several candidates.
The most important performance number is the
range of positive stability (also called range of vanishing stability or latent stability). That’s the largest angle of heel at which a boat resists turning bottom-up. A degree more than that angle and she may flip over. An example is a sailing
dinghy, like the Laser, that depends entirely on the weight of the crew to keep it upright. Once a Laser heels 80 degrees and is over on its side, the crew exerts no leverage and the boat goes right over. If she turtles (turns upside-down), she like all boats with a low stability range will be a chore to bring back upright. This doesn’t mean the Laser is a bad boat, for racing
dinghies come this way, but in a keel monohull, an 80-degree range of positive stability would be criminal. The range there should be at least 115 degrees, meaning that even when the boat’s knocked down so far by a gust or a wave that the mast is in the water, she’ll pop back up.
Stability range can only be provided by the boat’s designer, builder, or the International Measurement System (IMS) or other rating rule. Another, less precise stability gauge can be calculated from a boat’s published dimensions. This is the capsize screening formula: divide the boat’s displacement (weight) in pounds by 64, find the cube root of the result, then divide that result into the boat’s beam in feet. If the end result is less than two, then the boat’s relatively safe from capsize and remaining turtled. If it’s greater than two, she’s probably vulnerable and may be best suited for protected water. The capsize screening formula is not an exact measure of stability but an indicator that can be helpful in the tire-kicking stage early in the process of selection.
Here are a few other performance numbers:
Displacement-length ratio (abbreviated D/L) indicates a boat's weight for her size. (A boat’s weight is called “displacement” because it’s equal to the weight of water that the hull displaces, or shoves aside, below the waterline.)
D/L = displacement in long tons divided by (.01 x LWL)3
A long ton is 2,240 pounds. Boats with a D/L ratio of more than about 325 are heavy cruisers. A number between 200 and 325 indicates a light- to moderate-displacement cruiser, and less than 200 is very light displacement. If the D/L ratio is low, you can tell without looking that she’s probably fast, bouncy, and tricky to sail well. Light displacement does not mean the boat is unsafe. Stability is the best test here, and many modern racers with their deep keels have excellent stability
—more than old-fashioned heavy cruisers with stubby keels and heavy masts. True, added displacement does provide inertia against waves, which is no small thing offshore. One of the absolutes that sailors must face is that water is heavy
—64 pounds a cubic foot for salt water, 62.2 pounds for fresh. A five-foot breaking wave may throw 8,000 pounds of water at 20 knots at you, the equivalent of 500 bowling balls, and you want to be able to stand up to it.
The sail area/displacement ratio (SA/D) allows you to rate horsepower among several boats:
SA/D = sail area divided by (displacement in cubic feet) 2/3
Sail area is the size of the mainsail plus the size of the foretriangle (the area bounded by the headstay, mast, and deck). To find displacement in cubic feet, divide it by 64, then square the number and, finally, find the cube root on a calculator. A SA/D ratio around 12 indicates an extreme underigged heavy cruiser. A ratio double that one is typical of racing boats, and most modern cruisers fall between the two extremes. Of course, you can carry all the sail you want for light air so long as you can reef quickly.
Hull speed is the best known performance number. That’s the theoretical maximum speed of a relatively heavy keel monohull:
Hull speed = 1.34 times the square root of LWL
A boat with a 25-foot waterline length (LWL) has a hull speed of 6.7 knots, while a 28-foot LWL gives 7.1 knots. You’ll go a little faster when surging down the face of a following wave. Getting to hull speed is easy on a reach in 15 knots of wind. The challenge is making that speed when conditions are less than optimum. It pays big dividends to have a cruising spinnaker and large jib for light air, and to have well-made, properly shaped sails, and to know how to trim them correctly.
An emphasis on speed may seem contrary to good seamanship, but while too much speed is risky, in the light to moderate conditions that usually prevail (even offshore) the ability to find and maintain high speed is a sublime tool. Consider that in eight hours of sailing, an extra knot of boat speed gives you eight more miles of distance
—or, to look at it another way, gets you to your destination an hour earlier. You don’t
have to sail one knot faster, of course, but being able to do so provides flexibility and the ability to make seamanlike decisions, like beating off a lee shore, making a fair tide or sailing through a foul one, reaching a protected harbor before a squall sweeps in, or saving
fuel by sailing instead of powering.
Many novice sailors make the mistake of buying clunkers because they seem like nautical equivalents of bank vaults—big, bulky, and apparently safe. Many of these boats do not have good stability and are pigs in light air or when sailing to windward. An old Swedish sailor said of one of these scows, “She’ll go a loooong vay an’ take a loooong time a-getting’ dere, too!” Perhaps “a loooong time” is your dream schedule, but wouldn’t you rather have some choice in the matter?
