Future Hydrofoil Development and the Aeronautical Fan-Wing

Several years ago, an aeronautical designer installed an aeronautical version of a paddlewheel into model airplane wings, to achieve the phenomena of “lift” at much lower speeds. Maritime hydrofoils are based on aeronautical wing cross-sectional profiles, potentially allowing an aeronautical wing innovation to be adapted for hydrofoil use, to reduce the speed at which a vessel hull would rise up in water.

Introduction

In the area of fluid dynamics research involving transportation vehicles, it is not unusual for a researcher to submerge a small-scale aeronautical component into a fast flowing stream of water, for testing purposes. There is great similarity in the lengthwise cross-sectional profiles of aeronautical wings and hydrofoils, even similarity in the cross-sectional profiles between aeronautical and some maritime rudders. Paddlewheels evolved from waterwheels that extracted power from river flow and were adapted for vessel propulsion. Aeronautical designers have adapted paddlewheels to aeronautical propulsion and installed them into the leading ends of aeronautical wings to generate “lift” at lower speeds.

While transverse-axis paddlewheels have largely disappeared from maritime propulsive application, the old technology that can operate at high efficiency at low sailing speed is still used on some passenger cruise vessels that operate along inland waterways. Paddlewheels have been adapted to operate using a vertical-axis with moveable paddles, the basis for tug-boat propulsion. In aeronautical application, the transverse-axis paddlewheels produce a fast moving air stream that flows over the top surface of each of a pair of wings. Scale models of the concept have been built and tested, proofs-of-concept dubbed “fan-wing” planes. 

Hydrofoil Possibility

The aeronautical “fan-wing” precedent provides a basis for further research into maritime hydrofoil technology, specifically to install a transverse-axis paddlewheel into the forward region of a hydrofoil, with the purpose of testing and refining the concept for possible future hydrofoil vessel application. There is the possibility of developing the paddlewheel-hydrofoil concept to produce upward lift at lower sailing speed than traditional hydrofoil vessels, potentially reducing energy consumption required to achieve lift. One possible application of the paddlewheel-hydrofoil concept would be on ferry vessels or small passenger cruise vessels, to achieve smoother sailing in severely choppy water.

Hydrofoil boat technology achieves peak weight at around 400-tons and based on the combination of factors that include vessel speed, its power requirement and hydrofoil surface area. Aeronautical precedent involves the paddlewheels providing the combination of forward thrust to accelerate the plane forward along with a fast air flow over the wing upper surface while maintain boundary-layer surface contact that produces upward lift. That precedent suggests than a maritime paddlewheel installed into a hydrofoil’s forward region would achieve the same result, possibly assisted by additional stern area propulsion to achieve slightly higher sailing speed and much higher propulsive efficiency.

Scale-Model Research

Hydrofoil surfboards are among the smallest sailing technology that utilizes hydrofoil technology resembling scale-model airplane wings. A small hydrofoil built with mini-paddlewheels installed at the leading edge would provide both a proof-of-concept and also a basis for further evaluation and design modification. Some hydrofoil surfboards are self-propelled using batteries and electrically driven mini-propellers. A self-propelled hydrofoil surfboard that uses paddlewheel-hydrofoils would likely lift above water at very low speed. It would likely provide a very smooth ride through choppy water at the low speed while potentially using less battery power than a more traditional propeller driven competitor.

The people-powered recreational boat sector includes bicycle-based and other pedal-powered hydrofoil vehicles. There should be scope to develop a pedal-powered paddlewheel-hydrofoil craft as a basis for further research and refinement. It would likely lift on to the hydrofoils at comparatively low speed and provide a smooth and comfortable ride through choppy water. Pedal-powered paddlewheel-hydrofoil craft would be appropriate scale-model research vehicles and could serve as the basis to develop large-scale variants that could operate as commercial ferry vessels or short-distance sightseeing cruise vessels. There may be scope to develop tandem paddlewheels on extended length (long - chord) wings.

Tandem Paddlewheels

The first research objective would be to produce a paddlewheel-hydrofoil proof-of-concept. Aeronautical precedent suggests that such an object would be achievable and serve as a basis for further and future research. An extended length (chord) of wing would potentially offer the possibility of being to carry additional weight and exceed the 400-ton restriction. The threat of stalling on the upper surface of a hydrofoil built with an extended chord length invites consideration of “fore-and-aft” tandem paddlewheels as a possible means by which to maintain sustained boundary layer contact between water stream and the extended chord length of hydrofoil upper surface.

An alternative arrangement would combine a forward paddlewheel followed by a spinning Magnus/Flettner rotor built into the hydrofoil. The objective of the spinning rotor would be to redirect the fast moving water stream over and on to the trailing surface area of the upper side of the hydrofoil, while the paddlewheel would both direct water flow over the forward surface area and provide all or a significant proportion of propulsion. While the tandem arrangement involves additional system complexity, it offers the possibility of increasing vessel weight capacity and also allowing the vessels to sail on hydrofoils at greatly reduced speed.

Energy Usage

Reducing sailing speed by 50 percent can reduce power by a factor of 8. Assume that a conventional hydrofoil vessel requires 8000 units of energy to travel at cruising speed. If the propeller operates at 80 percent propulsive efficiency comparing propeller water stream speed to vessel speed, the engine driving the propeller would need to deliver 8,000/80 percent = 10,000 units of energy.

When sailing at half the speed, 1,000 units of power would be required to overcome hull water drag. If paddlewheels accelerating water streams over the hydrofoils operate at 50 percent efficiency, engine power requirement would be 1,000/50 percent = 2,000 units of energy.

A paddlewheel-hydrofoil vessel could theoretically travel on its hydrofoils at half the speed of a conventional hydrofoil vessel and require 20-25 percent of the energy requirement. The market niches for such a paddlewheel-hydrofoil vessel would include ferry service across severely choppy water and transportation through such water to connect remote coastal communities not connected by direct roads, as well as water taxi services. There may also be a market niche for short-distance sightseeing cruise vessels that sail through such water, and possibly small cruise vessels that undertake multi-day voyages sailing across rough water conditions.

Conclusions

The precedent of installing paddlewheels into aeronautical wings provides a basis to undertake research into and develop maritime versions of the concept involving hydrofoils. Such development offers the prospect of reducing energy consumption of traditional hydrofoil vessels while simultaneously offering the prospect of smoother riding characteristics when sailing through severely choppy water. Paddlewheel-hydrofoil ferry vessels could find market application across bays, lakes, to and from nearby offshore islands and even some forms of coastal transportation service where vessels would be expected to sail through extremely choppy water conditions.

Harry Valentine is a regular contributor to The Maritime Executive.