Speed of Boats

Transport on water is a comparatively slow way to go
The speed of vessels on salt water is measured in knots, a term that comes from the days when a ship’s speed was measured by the number of knots that ran out in a certain time in a line cast into the water astern. The knot is defined as 1 nautical mile per hour; it is therefore not necessary to refer to boat speeds in knots per hour. Vessels operating on the Great Lakes and other freshwater bodies, however, measure their speed in miles per hour—and those are statute, or land, miles.Compared with many other forms of transportation, speeds on water are slow, particularly those of boats under sail. The fastest run by a clipper ship in 24 hours was 465 miles, achieved by the Champion of the Seas in December 1854, west of Cape Horn. That’s an average of a little more than 19 knots.Only in recent years have we learned how to substantially improve on speed. In March 2001, the 110-foot catamaran Club Med broke the world record for a circumnavigation under sail. The winner of the Race of the Millennium, Club Med took 62 days to cover 26,500 miles at an average speed of 18.2 knots. But one day she sailed 650 nautical miles—a 24-hour average of more than 27 knots.She was a highly specialized boat, of course. Most small sailing yachts average between 5 and 8 knots, making a modest 120 to 190 miles in 24 hours, a distance a family automobile could cover in 3 or 4 hours. For planning and provisioning purposes, the rule of thumb for small sailboats is to allow for progress of 100 miles a day.

Before the age of electronic knotmeters and GPS, speed was measured with a chip log. The pie-shaped wood, when deployed over the stern, would strip line off a rotating drum as the vessel moved forward. Knots were worked into the line at measured intervals, and the number of knots running out in a given time gave the boat’s speed.

The top speeds of displacement powerboats are similarly limited by waterline length, although trawler yachts are increasingly likely to be capable of speeds into the semidisplacement range as diesel engines get lighter and more powerful. Whereas a full-displacement trawler yacht of 40 feet or so might top out at about 8 knots, its semidisplacement counterpart—marked by a fuller bow, wider waterline beam, and tightly radiused or hard-chine bilge to give a flatter bottom for hydrodynamic bearing and lift—offers speeds up to 15 knots or more, the trade-off being less efficient operation at low speeds.A similar trend toward higher speeds is noteworthy among production powerboats capable of operating at planing and high-end semidisplacement (or semiplaning) speeds. The 40-plus-foot Bertrams and Vikings that topped out at 16 to 20 knots in the 1970s might be capable of 32 knots or more today, speeds at which pounding becomes much more pronounced. To counteract it, the new designs carry more deadrise aft than their forebears, yet they have to remain flat enough in their stern sections to provide lift at high speeds. These boats are not fuel-efficient.See also Bilge; Chines; Hull Shapes; Multihulls; Speed Limits.

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