Innovative boat buoyed by bubbles
Designer's hunch about aerated water may have big payoff in global competition
PORTSMOUTH, R.I. — SAILBOAT racers like to brag about their high-tech boats, designed on supercomputers and made from exotic materials usually found only on spacecraft.
Now solo around-the-world racer Steve Pettengill may have a breakthrough in sailboat speed that is more the result of a guess than millions of dollars of scientific effort.
Pettengill, sailing Hunter's Child, cut a ``Venturi slot'' on his 60-foot sailboat - a vessel he will pilot single-handed in the BOC Challenge around-the-world race beginning Sept. 17 from Charleston, S.C.
The slot is a gap of about 1/4 of an inch that goes completely across the bottom of the boat. A hole leads from the slot to the cockpit drain tubes on deck. As the boat moves forward, air is forced through the tubes and out the slot.
Pettengill explains the theory that the sailboat is dragging about an inch of ``dead water'' under the last third of the hull. This dead water is weight that must be moved with the boat. Since air is 1,840 times lighter than water, the idea is that the back part of the boat will move through a less dense medium - aerated water. The bubbles replace the dead water.
Lars Bergstrom, the boat's designer, based in Sarasota, Fla. tested his design this summer on a much smaller sailboat to try to get some measurements. A much earlier version of the concept seemed to work at high wind speeds when the boat would get up on plane.
``If it works, it would be good to use on more low-speed motor boats and a lot of different things,'' Bergstrom says.
The Venturi slot is not the only innovation on Hunter's Child, which is sponsored by Hunter Marine, an Alachua, Fla., sailboatmaker.
The bottom of the rudder is on a track and can slide from side to side across the stern while the top of the rudder is fixed at the center of the boat. In essence, it allows the rudder to tilt relative to the boat.
The idea is that as the boat heels over in the wind, the rudder can always be kept perpendicular to the surface of the water. This means that all the rudder's energy can be used to steer the boat, rather than trying to lift or bury the bow as well. Without this wasted effort, the rudder can be can be smaller, producing less drag and saving weight.
The boat was designed with a razor-sharp bow, which gives the boat a fine entry into waves. Boats with a more blunt bow tend to burrow into the back of the wave in front as they surf down the 50-foot seas common in the Southern Ocean.
In addition, Bergstrom moved the boat's keel much further aft than usual. This gives the boat a very long entry into the water.
With the keel so far aft, the designer had to come up with some innovations with the mast, which usually sits above the keel. With the mast far aft, it would be hard to achieve good sail shape unless the boom, which holds the mainsail, extended past the stern of the boat. Normally, this can't be done, since there is a backstay helping to hold the mast upright.
Bergstrom, however, designed a mast that does not need a backstay. Instead, he swept the spreaders, which hold the rigging out from the sides of the mast, back 30 degrees. The spreaders act as a backstay, so the boat can carry a much longer boom. With the longer boom, the boat can also carry 18 to 20 percent more sail area, producing more power.
After 4,500 miles of sailing over the last month, Pettengill says he thinks the systems will work.
At one point, the boat moved at 15 knots in 18 knots of breeze for more than four hours, he says. This is far above the boat's theoretical maximum hull speed of 10.3 knots.
Pettengill will find out for certain how well the systems work when the race starts.