Stern paddle-wheel and side paddle-wheel propulsion operated partially submerged, and evolved from water wheels that converted river flow to mechanical work. Axial-flow boat propellers evolved from windmills and delivered greater propulsive thrust when fully submerged, remaining unaffected by wave-induced vessel rolling motions. Precedents involving gear pump technology provide a basis by which to improve the propulsive efficiency of paddle-wheels.
Introduction
Following the development of high-pressure steam engines at the University of Glasgow, Scotland during the early 19th century, large steam cylinders directly drove side-wheels and stern paddle-wheels in steam-powered ships and river vessels. Paddle-wheels evolved from the classical water wheel that operated along river banks, driving saws, flour mills, and machinery used in early manufacturing. Despite low power output, some designs of the undershoot water wheel that were partially enclosed in a channel, could convert river flow energy to mechanical work at over 90% efficiency.
Water wheels that operated in short man-made channels rotated with minimal clearance on either side and even below the water wheel, in turn reducing water leakage from each of the paddles. By comparison, the submerged section of propulsive paddle-wheels pushed water rearward without a duct or channel partially enclosing the working paddles. As each submerged paddle moved rearward, some water would move sideways as well as downward and even upward and over the paddle, reducing overall forward propulsive efficiency of paddle-wheels.
Modern Paddle-wheels
Paddle-wheel propulsion still exists in the present day, most commonly at waterfront parks where visitors can rent pedal-powered catamaran two-person boats. Two sets of pedals directly drive a centrally located paddle-wheel. An examination of these modern paddle wheels reveals the problems of the classical paddle-wheel involving water being pushed sideways and downward, with only a small percentage of the energy being devoted to actual propulsion. There are many builders of these pedal-powered paddle-wheel boats, and perhaps a few of them might be interested in increasing propulsive efficiency to increase vessel speed or operational distance.
Combining the catamaran twin-hull configuration with a centrally-placed paddle wheel has resolved the problem of all or part of a paddle wheel lifting above water during vessel rolling. The combination of vessel configuration and paddle wheel placement provides the basis by which to explore alternate methods by which to resolve deficiencies in both traditional commercial and modern recreational paddle wheel design. Improving modern recreational paddle wheel propulsive efficiency could provide the basis by which to develop a modern commercial paddle wheel vessel that sails along inland waterways, as a replica of vessels of a bygone classical era.
Reducing Water Leakage
Riding aboard a pedal-powered paddle-wheel boat reveals the low efficiency of the paddle-wheel, with riders quickly experiencing fatigue. Raising propulsive efficiency of the paddle-wheel requires a reduction of energy moving sideways and downwards from each paddle during rotation, in turn requiring partial enclosure inside a duct or channel. A pair of discs secured on either side of the paddles would prevent sideways movement of water while a drum connecting to the inner radius of each paddle would prevent upward movement of water over the paddle. A curved guide below the paddle would prevent downward movement of water.
Reducing water leakage would in turn increase the mass flow rate of water being pushed rearward and indirectly raise propulsive efficiency. Gear pump layout encloses mini-size waterwheels inside a casing with minimal clearance between gear and casing to minimize leakage and increase outlet dynamic pressure. In large scale, an adjustable rectangular outlet would adjust outlet cross-sectional area and water outlet speed, allowing people who ride pedal-powered paddle-wheel boats to pedal the vessel over greater distances with less effort at moderate speed, or travel over shorter distances at higher speed than possible with present technology.
Hull Revision
Pedal-powered paddle-wheel boats are often stored on land with the bottom of the paddle-wheel barely above ground. When in water, only a small amount of paddle would be partially submerged. A revision in hull design would lower the boat into the water and increase the depth to which each paddle would partially submerge. Some pedal-powered paddle-wheel boats borrow from wheelbarrow precedent with paddles on either side of a central wheel that rides on a ground surface when the boat is being moved on land.
Placing paddles between a pair of large-diameter discs would assure rolling on a solid ground surface when moving the boat on land, like a wheelbarrow. Extending wheel discs beyond paddle radius would allow for installation of partial enclosure below the paddles, to minimize downward leakage and enhance propulsive efficiency when on water. The revision would partially submerge a greater portion of each paddle that would push a higher volume of water rearward. A rectangular outlet channel of adjustable cross-sectional area installed downstream of the paddle-wheel would adjust water outlet speed and vessel sailing speed.
Low-speed Hydrofoils
The development of low-speed hydrofoils applied to surfboards has influenced the development of human-powered vessels. Hydrofoils have been applied to oar-powered kayaks and pedal-powered boats that use geared propellers for propulsion. In both cases, vessel hulls lifted above water to reduce water drag and allow for higher speeds. Pedal-driven tail-fin and tail-fluke propulsion has also been tested on small boats, to raise propulsive efficiency at low sailing speeds. It is theoretically possible for a pedal-powered, direct-drive paddle-wheel that is partially enclosed inside a duct to operate at high enough propulsive efficiency to allow the vessel to sail on hydrofoils.
Conclusions
The pedal-powered twin-hull catamaran boat with centrally placed paddle-wheel resolves the shortcomings of earlier ocean-going paddle-wheel vessels that alternately lifted a paddle-wheel above water during roll motions. There is scope to increase paddle-wheel propulsive efficiency by partially enclosing a re-designed wheel inside a duct to minimize leakage and unproductive flow of water from each paddle, as well as partially submerging more of the paddle-wheel. Transferring the rearward flowing propulsion stream into a rectangular outlet duct of variable cross-section offers the combination of higher propulsive efficiency and an ability to change sailing speed, while paddle-wheel rotational speed remain constant.
