Preview: What About a Sailboat’s Displacement? Doug Hylan Discusses Light and Heavy

April 5, 2013

Doug Hylan

Among the dockside pundits, the discussion of light vs. heavy displacement usually revolves around the ability of a cruising sailboat to carry the necessary provisions and gear for extended cruising. I would like to consider the question from another angle: appearance and cost.

Light displacement boats have some real advantages. Up to a certain point, lighter displacement saves money, both in initial cost and continuing expenses. Cy Hamlin pioneered this idea with his Controversy yachts produced at Mt. Desert Yacht Yard in the 1960s. Many people are surprised to learn that boats, like meat, tend to cost by the pound, not the foot. Compared to a heavy displacement 40-foot sailboat, a lighter boat of the same length will require smaller sails, lighter rigging, smaller ground tackle, a smaller engine, and less ballast.

Under many conditions, lighter boats can also be faster. For one thing, they can cheat the devil of hull speed, even plane if they are very light and have the proper hull shape. This was one reason that MARY ANN, the first Barnegat Bay A Cat, was able to sweep aside the older heavier competition on the bay, even though they sported vastly bigger sail plans.

Nat Herreshoff, the Wizard of Bristol, took advantage of both of these factors. One of the many facets of his wizardliness was his intuitive feel for light, stiff and strong construction. His boats generally had lighter structure than the competition, making them both cheaper to build and faster under sail. Even when a rating rule demanded that boats be of a certain displacement, lighter construction meant that more of that weight could be put into ballast. Ballast is not only one of the cheapest components per pound in a boat, but a greater proportion of ballast increases stability, meaning that the boat can stand up to more wind before reefing.

Aesthetics, however, is one area where lighter boats have trouble competing. While they may not be able to enumerate the reasons, most people will admit that older boats tend to look more graceful and appealing than their modern sisters. Although several factors are involved, a major one is freeboard, or the amount of hull that shows above the waterline. Older boats generally have less, and just as with cars, lower and sleeker usually looks better.

In the world of boat design numbers (called hydrostatics), the relative “lightness” or “heaviness” of a boat is defined by its displacement/length ratio, usually abbreviated as D/L. I'll spare you the formula, but keep in mind that this is a dimensionless number – in other words, it makes no difference if the boat is a dinghy or an ocean liner. If her D/L is 400 she is considered to be in the heavy displacement realm. If the D/L is 150, she is pretty light.

Why do lighter boats tend to be higher sided? Archimedes who purportedly jumped out of his tub and ran naked through the streets of ancient Syracuse shouting “Eureka!” is credited with the answer. The heavier a boat is, the more there is of it under water. The more there is under water, the less of it there needs to be above water to get the same amount of headroom.

Consider the two boats shown below, one an old style heavy cruiser (D/L 400), the other a modern light displacement sister (D/L 150). Both boats have an overall length of 33', a waterline length of 28 feet and a draft of 5 1/2 feet. They both have the same 6-foot headroom, but the cabin sole of the lighter boat must be much higher, so either the topsides or the cabin (usually both) has to be moved up to get the same headroom.

Sailboat Displacement - Light Displacement vs. Heavy Displacement

So, tall people who are unwilling to bump their heads (or bend over to avoid it) can be the ruination of good looks in small, light displacement boats. Daysailers, where headroom is not considered essential, are generally immune to this problem, and once a yacht gets to 60 or so feet in length, there is plenty of height in either type for homo erects.

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