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Sailing breaks free of the math that has held it back for centuries

2020 May 1

remember my distress when I was introduced to Old Devil Hull Speed.

I was just getting into offshore racing, working at being a semi-competent bowman, and I was gung-ho for speed. I read everything I could find about sailing fast, eventually including a rather technical article from which I learned there was a mathematical rule that limited how fast a sailboat could move.

I wrote down the formula—hull speed in knots equals 1.34 times the square root of the waterline length in feet—and looked up the specs for the boat I was crewing on. The waterline length (LWL) was exactly 28 feet. I calculated the square root, multiplied it by 1.34—and was so appalled by the answer that I did the math again, expecting to find a mistake. Same answer: 7.09 knots.

Seriously? All that work, setting, trimming, taking down, resetting all of those sails, getting beat up by waves and doused by spray, to move at a lousy 7 knots!

I tried to defeat hull speed in heavy air races by urging the owner to let me pile on sail area, but the boat would have none of it. In one memorable downwind blow, there were five sails set on the 41-foot yawl—spinnaker, staysail, mainsail, mizzen spinnaker and mizzen—but rather than move faster through the water, which was impossible thanks to the law of hull speed, the overstressed hull moved in an arc, rolling rhythmically at great speed, gunwale to gunwale, shipping water on each side.

The essence of hull speed is that displacement hulls make waves as they go through the water. As speed increases the wave at the bow and stern are merged into one with a deep trough between the crests traveling at a speed determined by the length of the boat.

When pushed to the limit, that full-keeled 41-footer built a wave that was a veritable tsunami at the aft end of the boat. Once, when zooming along in a photo boat beside the majestic competitors in a J-Class regatta, I got a closeup look at the wave generated by one of those heavy-displacement, 130-foot dinosaurs, and I swear the trough was so deep that the dangling feet of the hiked-out crew were 15 feet above the water. At hull speed, the mighty J had dug an enormous hole in the sea.

Boats pushing hull speed make such a fuss going through the water that they drag along a series of waves. Some of the boats I owned after my apprenticeship on the yawl were big wave makers, which in one race that lives infamy delighted our archrival in the racing fleet. His boat was smaller and lighter and, according to Old Devil Hull Speed, slower, which was reflected in his generous handicap rating. As we passed him on a breezy reach, his boat climbed up one of the waves radiating from our transom and just stayed there, like a surfer locked into the perfect wave, for most of the 70-mile race. Great merriment was evident among the crew of the drafting yacht. In the blustery conditions they were just out of hearing range, but we were pretty sure they were mocking us.

The mathematical shackles on boatspeed were tighter in those days. Today I am delighted to report what many sailors have already seen for themselves—Old Devil Hull Speed is a shadow of its former self, its influence petering out like a dying breeze.

Thanks to smart design and boatbuilding technology, today’s offshore sailboats are lighter and have less hull in the water, and many are designed to limit wave resistance with increased buoyancy aft and  near vertical sterns. You know the look—plumb (or plumbish) bow and straight transom, resulting in waterline lengths almost as long as the overall length. Many current generation 41-foot boats have LWLs at least 10 feet longer than the 28 feet of the boat that was a slave to hull speed. For good measure, they will also do something that was nearly impossible for boats of yore—break free of the waves and surf, even in moderate conditions, rendering hull speed irrelevant.

While such boats have weakened the fetters of hull speed, a new class of monohull sailboats has, once and for all, driven a stake through the heart of the Old Devil. They are the foilers that sail on the surface of the water rather than through it. Though most of their centuries of history, boats had to obey a speed limit, but there is no speed limit for a boat the rides on hydrofoils. The most exotic of them, the new America’s Cup boats, have sailed faster than 50 knots. That computes to roughly 60 mph. Call it the new math of sailing.