Technical Report Prepared 12/1/98
Early results of the Surface Skimmer Hydrofoil Technology are based on the performance of a prototype which weighs approximately 650 pounds including the weight of the fuel and driver, has a Johnson twenty-five horsepower outboard motor (with no performance modifications other than a long shaft), and attains a top speed between forty and fifty miles-per-hour. This can be compared to a safety chase boat (which is involved in the testing) weighing approximately the same, 650 pounds, which includes the weight of the fuel and driver, has a twenty-five horsepower outboard motor, and attains a top speed of approximately 15 miles-per-hour.
During normal lake conditions with swells of approximately one to two feet, the ride of the Surface Skimmer prototype hydrofoil boat is extremely solid, level, and predictable with respect to handling characteristics, this is owing to the wave-piercing capabilities of the hydrofoils---the prototype did not have a vertically adjustable dampening system. Instead of the hydrofoils riding up and over an approaching swell, they pierce the swell crest. As the Surface Skimmer hydrofoils pierce approaching swells, the curved upper surface and flat lower surface generate the required lift to return the hydrofoils to the surface of the body of water.
One advantage of Surface Skimmer wave-piercing hydrofoils at high speeds compared to a traditional mono-hull watercraft is safety. When a traditional high speed mono-hull watercraft encounters a cresting swell of approximately two to six (or more) feet in height, this traditional mono-hull watercraft may be lifted off the surface of the water (somewhat like a ramping effect water skiers experience when using a ramp for jumping) and become airborne with sometimes an unpredictable result which may then lead to a complete airborne rotation owing to the relatively large hull surface acting (temporarily) as an airfoil (at speeds approximately 80 miles-per-hour or more). In contrast, the wave-piercing characteristics of the Surface Skimmer design cuts a relatively level path through the water wave, it might also be thought of as cutting off the top of the wave. Once the hydrofoil enters the water wave, the lift properties of the Surface Skimmer wave-piercing hydrofoil design take effect, and if the design incorporates a vertically adjustable strut assembly (which mounts to the forward and rearward ends of the center beam), then the Surface Skimmer wave-piercing hydrofoil design rises to the surface of the body of water. The Surface Skimmer design incorporates a mechanical dampening effect and thus, the Surface Skimmer wave-piercing hydrofoil design avoids the ramping and subsequently unpredictable airborne effect which might otherwise be experienced by a traditional mono-hull watercraft moving in a forward direction at high speed.
Previous hydrofoil watercraft designs are fully wetted, meaning the hydrofoil operates under the surface of the water and requires a sophisticated active control system to maintain the desired distance between the bottom of the hull and the water surface, and to keep the watercraft in an upright vertical position with respect to the earth while traveling in a forward direction. This is because forces acting on the hydrofoil wing (or wings) are relatively the same above and below the surfaces of the hydrofoils, opposed to the Surface Skimmer wave-piercing design which operates in the boundary layer between the water (which may be thought of as a dense liquid) and the air (which may be thought of as a less dense liquid). The dense liquid or water exerts the greater forces on the hydrofoil resulting in the hydrofoil wing operating at the surface of the water (at forward speeds), when the hydrofoil wing is inclined with relation to the surface of the water (and in consideration of the forward motion of the marine vehicle). To summarize, the Surface Skimmer wave-piercing hydrofoil design results in a simplification of the necessary controls which combine to keep a traditionally submerged hydrofoil level (with the surface of the water), and increases overall marine vehicle stability, inherent safety at high speeds, and fuel efficiency---fuel efficiency owing to the low drag-to-mass ratio characteristics of the Surface Skimmer hydrofoils.
The prototype was tested at Lake Cachuma, Santa Barbara County, California. On about the fourth outing the research team successfully urged the Surface Skimmer wave-piercing hydrofoil to the surface of the lake by using a quick burst of power from the twenty-five horsepower outboard motor, and reached speeds between forty and fifty miles-per-hour in three to six seconds. Three additional test outings were successfully completed, one at Lake Cachuma, California, one at Lake Cacitas, California, and the last at the University of California at Santa Barbara enclosed salt water pond.