The great white sharks hanging out around the Farallon Islands 25 miles west of the Golden Gate have been greeted by an unusual visitor in recent days: the Derek M. Baylis, a 65-foot vessel designed not just for research but to demonstrate how quieter and more fuel-efficient boats can be built.

The Baylis is a hybrid that can operate under sail, diesel or a combination of the two, using a fraction of the fuel required for a comparable-size boat of conventional design. The craft is the creation of Tom Wylie, a local yacht designer, and Dave Wahle, his partner in sailboat manufacturing, who began work four years ago assuming that marine researchers would welcome such an environmentally friendly vessel.

That was certainly the case with Barbara A. Block, a Stanford professor of marine sciences. She chartered the Baylis this month for a research project that was placing remote telemetry tags on the great whites. The devices record navigational information, body temperature, depth and ambient temperature data.

''We appreciate the design and efficiency of the Baylis and the capacity to get out to the Farallones by sail,'' Dr. Block said. ''Our project is designed to impact the refuge and the sanctuary waters around the Farallones as little as possible, and there is no better way than to do this by sail.''

Like the Toyota Prius, which derives its fuel savings in part from its hybrid gas and electric power system and from an aerodynamic shape and narrow, high-pressure tires, the Baylis gains its efficiency from a smooth hull shape as well as the addition of sail.

''This is a very straightforward invention where my friends and I have repositioned off-the-shelf items so that even without the sail, we get a 20-to-1 improvement in fuel economy,'' Mr. Wylie said. ''I'm really trying to use finesse, not force, working with Mother Nature, not against her, and the outcome is actually free boat speed.''

In yacht-racing circles, Mr. Wylie is known for designing slippery hull shapes and innovative rigs, both of which are featured on the Baylis. Speed in a sailboat is a product of lift versus drag, and naval architects seek to increase the former while minimizing the latter, within the often conflicting requirements of the boat's intended use.

The Baylis is a modern variation of the classic catboats -- the simple single-sailed boats used a century or more ago for fishing and transportation in the coastal waters around New England, New York and New Jersey.

Technically, the Baylis is a cat-ketch, as it has two masts, with the larger main mast forward of the smaller mizzen. There is a single sail on each mast with wishbone booms -- much like the systems used in windsurfing -- which makes the rig virtually self-tending. The carbon fiber masts are also self-supporting, with no steel shrouds or stays, to reduce drag and weight aloft.

Sailboats are propelled by the lift created when wind passes over the curved surface of their sails, but they would simply scuttle sideways were it not for the counteracting lift generated by their two underwater foils, the keel and the rudder. Like a modern racing yacht, the Baylis has foil shapes as efficient as that of any airplane wing.

Optimizing the hull shape for minimum drag is where things become complicated, and where Mr. Wylie resorts to technical terms like prismatic coefficient and deadrise. Basically, he tries to reduce volume in the bow and stern, concentrating mass in the middle of the boat.

He also designs hulls with an uncommonly deeper and rounder shape below the waterline. Curved shapes are inherently stiffer than straight ones, so the hulls can be built lighter for a given strength. A curved shape also has less wet surface than a flat one, and boats with a curved underbody tend to have more gentle motion than those with a steeper, flatter shape.

''I try to put a slight curve even in the flattest line, and I get a lot of benefits from that,'' Mr. Wylie said. ''It's very difficult, because when you start doing three-dimensional curves, improving one parameter inevitably pushes another out of shape.''

When not chartered by marine biologists, the Baylis makes excursions twice daily for paying customers under the Monterey Bay Aquarium Science Under Sail program. A glance behind the boat when it is under sail shows the benefit of a smooth, easily-driven hull, which simply uses less power to glide through the waves: sailing at 10 knots in 10 knots of breeze, the Baylis barely leaves a wake.

Even under diesel power, the Baylis uses about one-twentieth as much fuel as the aquarium's other research vessel, the Zephyr, or two gallons an hour at a cruising speed of 10 knots, Mr. Wylie said. The Baylis's svelte shape means it can be driven by a single 100-horsepower diesel that consumes far less fuel than the Zephyr's two 1,000-horsepower motors.

Seeing the Baylis berthed at Fisherman's Wharf gave Hedley Prince, the harbormaster, an idea. He had seen firsthand the decline of the local fishing fleet due to restricted catches, foreign competition and surging fuel prices. Mr. Prince could do little about Russian trawlers or the salmon population, but the Baylis showed a potential solution to the high cost of fuel: a hybrid fishing boat.