Small Watercraft Innovation and Future Propulsion

A recent innovation applied to the most basic form of watercraft offers potential for alternative future propulsion. Innovative lateral thinking resulted in the installation of a hydrofoil based on the equivalent of the wing layout of a scale model aircraft under a surfboard. It has revolutionized recreational surfing and has been applied to small watercraft.  

Introduction

The history of maritime transportation includes a multitude of innovations that over a period of centuries, has steadily improved water-based transportation. Some of the innovations seemed almost insignificant, such as the lengthwise installation of a vertical wood plank under the central axis of a flat-bottomed boat, to improve directional control. The additional of a movable sail allowed ancient mariners to use coastal crosswinds as the source of propulsion, even winds blowing toward the boat at a 45-degree angle. Such innovation made commercial sailing more competitive with overland transportation of goods using animals. 

Applying additional innovation led to the development of multiple-masted schooners and eventually yachts. Perhaps as a result of unintended mishaps, early recreational ski enthusiasts discovered that when traveling fast, their skis could actually carry them across narrow bodies of liquid water. Widened skis became surfboards propelled by coastal sea waves, possibly the result of people observing seals using waves to travel to shore. Efforts aimed at increasing recreational vessel speed led to experiments using modified skis to minimize contact between a solid surface and water, with ongoing experimentation resulting in the development of hydrofoils.

Hydrofoils

The development of hydrofoils offered increased vessel speed with greatly reduced vessel pitching and rolling motions. However, early hydrofoil technology also required higher engine power for a vessel to make the transition from mono-hull to rising upward to sailing on the small hydrofoils. Designers and builders of competitive yachts added small hydrofoils to the bottom of the central keel, to partially lift the vessel so as to reduce hydrodynamic or water drag by reducing the vessel/water wet surface area as vessel speed increased. Builders of recreational surfboards experimented with keels and hydrofoils and with surprising results.

A surfboard built with a small scale-model airplane wing arrangement at the bottom of the keel would actually rise above the water surface when propelled by a wave, carried only by the pair of forward and rear water wings. Surfers soon discovered that they could induce pitching motions to the raised surfboard, by rocking themselves forward and backward on the board, or “pumping” the hydrofoil board to generate propulsion in a comparable manner to marine mammals that “wag” their tail flukes to achieve propulsion. Installing hydrofoils has changed recreational surfing to include “pump-surfing”  on calm water.

Resonant Frequency and Propulsion

A recreational surfer pumping a hydrofoil surfboard may be compared to the activity of a little child jumping and bouncing on a spring mattress or trampoline. The relationship between the stiffness of the springs, the child’s weight and its movement coordination determines the nature of the bouncing. Proper synchronization of all relevant factors allows the child to bounce almost effortlessly for a prolonged duration. The duplicate leg muscles of the Australian kangaroo functions like mattress springs, allowing the kangaroo to bounce forward over greatly extended distances while expending minimal effort. Both the child and kangaroo maintain resonant frequency. 

The design of a hydrofoil carrying a moving surfboard above water allows the rider to partially duplicate the resonant frequency of a child bounce-jumping on a mattress or kangaroo forward-hopping. Hydrofoil wing profiles, angles and springiness or flexibility along with board rider’s expertise at pump-propulsion has allowed some surfers to ride up to a dozen consecutive waves approaching a beach, often achieving a measure of resonant frequency when pump-riding between waves. Some hydrofoil surfboards are specifically designed for resonant-frequency-like pump-propulsion on calm water at small inland lake or river locations, with rider using paddle oar to start moving. 

Ocean Wave Propulsion

Several boat designers and builders in several different countries have built operational wave-powered vessels, usually two-section vessels involving mono-hulls, with a short forward section and extended length rear section. The short section is more vulnerable to wave action and drives the propulsion technology. One enthusiast from Japan chose to sail a wave-powered boat from Hawaii to Japan except that due to unusually calm seas, the voyage took three months. The precedent of the pump-propelled hydrofoil surfboard offers the possibility of a pitch-prone short forward section using hydrofoils as the basis of wave-driven propulsion.

One design from France uses the forward short mono-hull section to pump a duplicate of the equivalent of the tail fluke of a whale, while pulling a much longer mono-hull barge at speeds of up to six knots. There may be scope to combine a short forward mono-hull to provide pumping-propulsion with a much longer small-water-area-twin-hull catamaran-type barge trailing section that would involve less water drag. A recent small ferry boat design uses a surfboard style of large forward with small trailing hydrofoils and rises above water surface at nine knots, perhaps a future design objective of wave-powered coastal vessels.    

Alternate Resonant Frequency Propulsion

A surfboard rider pump-propelling a hydrofoil surfboard and ocean wave propulsion represent variations of resonant frequency based propulsion. In both cases, the propulsion is based on a variation of the mathematical sine wave, as is alternating current (AC) electrical power. It is technically possible to design and build mechanisms that can generate alternating up and down mechanical forces. One such mechanism involves weighted counter-rotating balance shafts found inside some designs of large-displacement four cylinder automotive engines to minimize transmission of engine vibration into the passenger area. Such mechanisms could duplicate the forces of a rider pump-propelling a hydrofoil surfboard.

A pair of geared, electrically driven counter-rotating shafts with weights could generate alternating up-and-down mechanical forces in the vertical direction and pump the mechanical equivalent of a large whale-like tail fluke up and down to generate propulsion. At low speeds along inland waterways, traditional propellers operate at low efficiency. By comparison, a whale-like mechanical tail-fluke could move a much greater volume of water at much lower relative speed than a propeller, thereby offering greater low-speed propulsive efficiency. A future vessel could combine small water area twin hulls placed above hydrofoils and use resonant frequency to activate mechanical fluke propulsion.  

Compressed Air-over-Water Acceleration

Bottle rockets use compressed-air-over-water energy storage to achieve both vertical take-off and propulsion. On a larger scale such energy storage can be applied to boats to provide fast short-distance acceleration and/or short-distance propulsion, including using retractable venturi pumps to avoid cavitation. It could theoretically accelerate a small-water-area-twin-hull (SWATH) catamaran vessel to sufficiently high speed for it to transition to riding on low-speed hydrofoils, when ocean wave energy conversion using additional hydrofoils or flukes would then blend in to maintain hydrofoil sailing speed. When the vessel is moored, ocean wave action could replenish its compressed air energy storage system. 

Conclusions

Innovation involving the smallest of personal watercraft has not only made ocean wave surfing more attractive, it has been adapted to operation on calm water found at inland locations along rivers and small lakes. The low-speed hydrofoil combination of a wide forward wing and narrow tail includes a box-shaped forward wing reminiscent of a bi-plane. Small boats including a battery-powered water-taxi utilize large-scale versions of surfboard-style hydrofoils to achieve smooth low-speed sailing and at higher elevation with minimal pitching and roll motions, potentially the future of small ferry vessels required to sail across choppy water.