OP-ED: Marine Propulsive Efficiency in Speed Restricted Waterways

Written by Harry Valentine

Ongoing concerns about exhaust emissions from ship engines and collisions between marine propellers and some marine animals, invites discussion on various possible solutions to such concerns. Small recreational watercraft incorporate inboard enclosed propellers that develop a propulsive water-jet, with intake water passing through a screen filter. However, the propulsive efficiency of such water-jets is comparatively low. A propeller or water-jet reaches a theoretical 100% propulsive efficiency when the speed of the water-jet or propulsive water stream matches the speed of the watercraft, except that there would be zero thrust.

Propellers on large commercial ships rotate at 75 to 80RPM while pumping water at some 90% efficiency, producing a propulsive water stream that measured against ship speed, can reach 86%. The result is a propulsive efficiency of 86% x 90% = 77% on the open ocean. At 50% engine thermal efficiency, the ship would sail at an overall efficiency of almost 39% measured from the fuel tank. Like other forms of turbo-machinery, hydraulic propellers operate at peak efficiency over a very narrow RPM-range and pushing a narrow range of water volume flow rate.

While ships may sail the open ocean at 20-knots, they may sail inland waterways at 12-knots. The water drag of the ship varies with the square of the sailing speed, meaning that the ship propulsion system will need to generate some 2.75 times the thrust at 20-knots than at 12-knots. Thrust is a function of the amount of water that the propeller pushes, measured in cubic units per second. Slowing the propeller to reduce thrust can drop its pumping efficiency from over 80% to about 40% as it generates the propulsive water stream. The propulsive efficiency between that water stream and ship speed may remain high.

Ship transport companies can live with a situation where their vessels will achieve optimal propulsive efficiency sailing over several thousand nautical miles of ocean, with reduced propulsive efficiency occurring over a few hundred nautical miles of waterway canal.  During earlier times of very cheap fuel and non-existent exhaust emission regulations, it was quite feasible to operate an inland bulk ore carrier at 40% propeller pumping efficiency, 80% propulsive efficiency between water stream and ship speed, with a marine engine that operate at 40% engine thermal efficiency. The ship operated at an overall 12.8% efficiency measured from the fuel tank.

Water Jet and Duct:

The British company Pursuit Dynamics developed a converging-diverging (venturi) duct that can convert a small, high-speed jet of turbulent water to a slower moving, large mass of mainly laminar water flow. Such ducts combine a large-diameter intake and large-diameter outlet with a small diameter throat at the mid-point. A high-speed stream of water or saturated steam flows through a jet pipe and into the throat of the converging-diverging venturi duct.

The water jet induces a flow of water from the surroundings to flow into the venturi throat, where the high-speed and marginal-speed streams mix. A large volume mass of water will then flow smoothly from the outlet of the diverging section of duct, at a fraction of the speed of the water-jet or steam-jet and at a conversion efficiency of about 99%. Optimal propulsive efficiency occurs when the speed of the outlet stream is marginally higher than the speed of the watercraft that uses this propulsion system.

Pump technologies that include piston pumps, positive-displacement rotary pumps and hydraulic turbines enclosed in a duct, can reach pumping efficiency of over 80%. Multiple water pumps with matching outlets can push different volume flow rates of water at high velocity into the venturi throat. If the throat were rectangular, there would be scope to vary its cross-sectional area and optimize the conversion efficiency to a lower-speed propulsive stream. The intake to the converging-diverging duct may include additional water scoops and water outlets that may serve to optimize propulsive efficiency.

A combination of 3-different sizes of water pumps could deliver water in a 1:2:4 water volume flow rate ratio, or 7-different water volume flow rates at peak pumping efficiency of about 80%. The installation could theoretically provide optimal propulsive efficiency at 7-different sailing speeds, allowing the vessel to sail efficiently along inland waterways as well as on the open ocean.

Paddle Wheels:

Waterwheels that had a history of several thousand years before a propulsion version of a waterwheel was adapted for use on steam powered ships. While paddlewheel technology can deliver high propulsive efficiency at low sailing speeds in shallow waterways, ship directional control is problematic on both stern paddlewheel vessels and also side paddlewheel watercraft. In the modern era, electrically driven bow and stern thrusters can enhance directional control on a paddlewheel driven vessels.

A precedent from the road construction industry combines a central steering pivot, hydraulic steering control and massive rollers for wheels. There is scope to adapt the central-pivot hydraulic steering system to a stern paddlewheel vessel, where a paddlewheel can be steered like the propeller on an azipod. A system of twin side-by-side stern paddlewheels can allow a mechanical drive system to connect the paddlewheels to a transversely mounted engine. Such a vessel would sail shallow inland waterways at restricted speeds while delivering relatively high propulsive efficiency along with a measure of directional control.

At the present day, some tugboats use a vertical-axis “paddlewheel” with adjustable paddles. This technology can offer good propulsive efficiency along deep draft, speed-restricted waterways. However, most inland waterways are of relatively shallow draft. Extreme shallow draft waterways offer an advantage to transverse-horizontal-axis paddlewheel propulsion over the vertical-axis variety.

Wind Propulsive Assistance:

It is possible to use sails and kites to assist in watercraft propulsion on several inland waterways, depending on prevailing winds. On the open ocean, several ships already use sails and kites to reduce fuel consumption and the trend is likely to continue. Overhead power lines and bridges pose the greatest obstacle to using sails and kites to provide propulsive assistance to watercraft that sail inland waterways at restricted speed.

Conclusions:

Higher fuel prices, the need for greater safety along certain waterways and more stringent standards for engine exhaust emissions will require higher propulsive efficiency for vessels that sail along inland, speed restricted waterways. Several technical developments can raise the low-speed propulsive efficiency of vessels while reducing hazard to aquatic life and other users of the waterway. There may be scope to undertake further research into raising the overall propulsive efficiency of commercial watercraft that sail along inland waterways, as well as on watercraft that sail on both the inland waterways as well as the open ocean.

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The author, Harry Valentine, can be reached at [email protected].