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Designing a Hydraulic Ferry

Harry Valentine explores how to hydraulically-propel ferry vessels

By Harry Valentine

The world price of oil has long affected the economics of airline, road and marine transportation sectors. Ferry vessel technology is unique in the wide variety of possible alternative forms of propulsion. There are propeller-less ferry vessels known as kinetic ferries that are propelled from land or by river currents and oceanic tidal currents. In other ferry operations, an electrically driven overhead cable that is driven from shore pulls a ferry vessel back and forth across a narrow water channel.

Rising oil prices encourage examination of alternate forms of propulsive energy for a variety of vehicular applications. The physical size and weight restrictions that apply road, railway and air transport vehicles restricts the choice of alternative energy storage and propulsive technologies. The sheer physical size and fully laden weight of marine transport vehicles combined with an extremely low power-to-weight ratio (EG: 1-horsepower moving 10-tons of payload) broadens the range of possible alternative energy storage and propulsive technologies.

Hydraulic Energy Storage:

Hydraulic accumulator technology has long been used in the construction and automotive industries. In construction equipment, the compressed-air-over-oil mobile hydraulic battery has been used to keep hydraulic equipment operational in the event of a sudden engine shut down. Hydraulic accumulators have been used to operate the brake systems of road and railway vehicles. Mining trucks built by Caterpillar and cars built by Citroen applied hydraulic accumulator technology to vehicle suspension systems.

Several city transport buses in Germany and in Sweden were modified to operate with hybrid air-over-oil hydraulic propulsion, during acceleration. A diesel engine of half the power output of conventional city transport buses maintained vehicle speed between bus stops. Size and weight restrictions impose limits on the physical size of air-over-oil hydraulic propulsion for a road vehicle. The sheer physical size and weight capability of some short-distance marine transport vessels allows for possible adaptation to compressed-air-over-water propulsive energy storage.

Hydraulic Accumulator:

Accumulator energy storage involves an air-over-liquid accumulator system operating at pressures of between 3000-psi and 5000-psi (20mPa to 35mPa). It is possible to adapt the concept to propel ferry vessels used in short-distance services across deep channels, where building a bridge would be impractical. The water current in the channel would be negligible and using a tension cable across the channel to propel the vessel would otherwise be impractical.

The energy storage system would involve an array of multiple, spherically shaped pressure vessels made of a maraging stainless steel. The process of maraging involves pumping super-cooled liquid nitrogen into the pressure vessel at extreme pressure, to raise the tensile strength of the steel. In a ship, each pressure vessel would include a membrane or separator to prevent contact between the liquid (water) and the gas (air). The gas would be atmospheric air kept at a minimum pressure of 3000-psi (20mPa) in each tank that would connect to an extensive coil of stainless steel pipe that in turn would connect to a hydraulic pump.

Electrically driven hydraulic pumps would pump water through the coiled pipes and pressurize each accumulator to 5000-psia (35mPa) during layovers at port, at each end of the journey. The electric power would come from the power grid, or local electrical generation installation. At some offshore locations, a submarine would carry electrical power from the mainland, for use on an island. The use of stainless steel would allow the accumulator system to operate in either a seawater or inland water environment.

The choice of compressed-air-over-water involves economics. Hydraulic pumps generate practically no heat in the liquid being pumped during the pumping process. They require less energy that air pumps to pump to the same pressure and are better suited for rapid recharge applications. In transportation service, the efficiency of pumped hydraulic systems has been high enough to offer cost benefits that result from the reduction in diesel fuel consumption.

During operation, the compressed air in the hydraulic accumulators would push the highly pressurized water through any of several propulsive technologies to provide propulsion. Given the inertia and momentum of a large marine vessel that is moving at near constant speed, the propulsive system may operate in a series of short bursts of power as a means of conserving energy and extending operating range. The propulsive technologies may involve hydraulic motors and propellers, or it may involve a venturi water jet system.


An accumulator ferry vessel propulsion system may combine the highly pressurized water driving a propeller via a hydraulic motor and a reduction gearbox. There is also the alternative of installing accumulator-driven water jets may on the outer edges of a large-diameter, slow turning propeller to convert the high-velocity low-mass flow rate of water to a low-velocity large-mass flow rate of water. The layout would raise propulsive efficiency and dispense with the hydraulic motors and gearboxes.

A propeller-less water jet propulsion system would combine small high-speed jets of highly pressurized water with a system of concentric venturi-type ducts. The objective of the layout would be to use a small high-speed jet of water to drive a much larger volume of water at lower velocity, to raise propulsive efficiency. Accumulator pressure could propel a water jet at 20°ree;C up to 1400m/s or 4600-ft/sec. The concentric duct system would include an adjustable, large-area water intake, with accumulator-driven water jets being installed upstream of the smallest section of duct.

During operation, the high-speed water jets may only account for 1% to 5% of the total water volume flow rate leaving the outlet to provide propulsion. Propeller-less ferry vessels may operate in environmentally sensitive waters where marine life or people may otherwise come into contact with propellers. The absence of rotating machinery also offers the prospects of reduced maintenance requirements. Bow and stern thrusters would use water-jet driven propellers to provide navigation control while the ferry is at port.


Most ferry services are presently powered by diesel fuel or by bunker oil. A future oil prices rise, ferry vessels powered by a variety of alternative technologies are likely to enter service around the world. Cost-competitive energy storage technologies that involve low complexity and greatly extended service life would likely see service in future designs of ferry vessels. A compressed-air-over-water hydraulic accumulator propulsion system could form the basis of propulsion for some future ferry services, especially at geographic locations where oil prices greatly exceed the local price of electrical power.

Times Article Viewed: 5310
Published: 18-Apr-2011


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