Sunday, December 17, 2017

OP-ED: Gearless Mechanical Power Transfer for Ferries and Tugs

December 5, 2012 by  
Filed under Port Services & Renewables

Written by Harry Valentine

There are mechanical power transfer drive systems that date from the heyday of steam powered ships that may still have application in modern day maritime propulsion. While modern heavy-duty power transfer systems use expensive electrical methods or gear trains, earlier generation steam-powered technology distributed power using gearless mechanical technology. Steam railway locomotives used pairs of side rods spaced at 90-degrees apart bolted to multiple driving wheels to distribute power.

Railway side rods carried both compressive and tensile loads. A Swiss inventor modified the concept to carry only tension loads using lighter side rods, to carry equal or greater power. The Swiss mechanism involved 3-tension rods being spaced 120-degrees apart that allowed power to transfer smoothly between 2 or more parallel drive shafts (figure 1). Extended length metal tensile members will carry greater load in tension than in compression, for equal cross-sectional area and shape profile.

The Swiss mechanism bypasses the problem of buckling that could occur in extended-length compressive members. The lighter weight of the Swiss mechanism allows it to carry greater loadings while operating at faster crankshaft rotational speeds, while involving lower capital cost and higher efficiency than electrical transmissions. It can also transfer greater torque at lower RPM over a longer service life than costlier gear transmissions.

Power Distribution:

A large low-speed diesel engine could drive into a Swiss mechanism and distribute power between 3-output shafts that drive propellers. Such a layout would allow multiple smaller-diameter propellers to propel wide-beam, shallow-draft heavy vessels through mainly shallower waterways.

Mechanical Azipod:

The Swiss mechanism could operate with an azipod. Power would flow from the engine through a double-jointed drive shaft, where the mid-point of 2 x closely spaced universal joints would be coaxial to the azipod steering axis. The propeller could steer up to 45-degrees from the straight-ahead while additional rudders would redirect the propeller water stream to flow at 90-degrees from the vessel centerline, allowing the vessel stern to move sideways toward or away from a pier.

90-Degree Power Transfer:

A gearless 90-degree power transfer system that includes roller crankshafts, tension rods and sliding crossheads on rollers (figure 2), could transfer power through an angle of 90-degres at higher efficiency than a gear-based system and operate with a Swiss mechanism. The gearless 90-degree mechanism would incur far less friction than a gear-based system while carrying high power levels at low engine rotational speeds.

A marine diesel engine may drive into a Swiss mechanism that would distribute the power amongst 3 or 4-parallel output drive shafts that would then carry the power into a crosshead 90-degree transfer drive. Parallel input cranks at one level would pull on primary tension rods that pull on sliding crossheads that then pull on secondary tension rods that pull the throws of a crankshaft that drives the output shaft.

Each output shaft may drive either a vertical-axis Voith-Schneider propeller or a conventional axial-propeller. Power would flow from the engine through both the Swiss mechanism and 90-degree crosshead mechanism, from where it would leave at an angle of 90-degrees to the engine rotational axis. The 90-degree mechanism may drive a single output shaft or 2-output shafts placed on either side of the input shafts.

Mega-Ferry Application:

A ferry ship powered by a lengthwise-mounted low-speed engine (75 to 90-RPM) may use the Swiss mechanism and 90-degree crosshead mechanism to transfer power to a single or double pair of vertical-axis Voith-Schneider propellers. The 2-output crankshafts of the 90-degree drive may be arranged to rotate in the same direction, or to counter-rotate. A twin counter-rotating vertical-axis propeller system could afford a ferry ship easy maneuvering at space-constrained terminals.

Some maritime engines can deliver maximum power at either end of the engine, allowing for the installation of a Swiss mechanism and a 90-degree power transfer mechanism at each end. The output shafts would drive 4 x vertical-axis Voith-Schneider propellers that provide propulsion and navigation and easy maneuvering for a mega-ferry vessel. The same power system may also propel a mega-tug that maneuvers massive ships at port.

Twin-Hull Oceanic Tug:

A twin-hull catamaran tugboat may carry a transversely mounted engine capable of delivering full power at either end of the engine. It may drive into Swiss mechanisms and 90-degree mechanisms installed at both ends of the engine, at the twin hulls centerlines. These gearless mechanisms would each drive an identical propeller installed below each hull. Large marine engines can generate up to 3,500,000-lb-ft of torque at 75-RPM, power levels that would exceed the capability of cost-competitive gear-based power transmission systems.

A lengthwise-mounted engine may drive into Swiss mechanisms installed at both ends of the engine, to transfer power to the centerlines of the twin hulls where secondary vertical Swiss mechanisms would transfer power to lower elevation and 2-output shafts that each drive a propeller. This layout would include transversely mounted hollow box structures capable of withstanding extreme axial compressive loads. By comparison, the transverse engine option would use much shorter structures to withstand such loadings.

A twin-hull mega-tugboat may carry 2-propellers spaced at up to 90-metres or 300-feet apart. The mechanical transmission would operate at higher efficiency (99%) than a more costly diesel-electric power transfer system (80% to 90%). The wide propeller spacing would allow the twin-hull mega-tugboat to pull oceanic trains while the propeller backwash bypasses the bow of the lead vessel and flow near the sides of the towed vessels to reduce hydraulic drag.

Water Jet:

Adapting Wartsila’s water jet technology to a twin-hull mega-tugboat may assist in balancing the thrust between the widely spaced propellers and assist steering and navigation. The mega-tugboat would use a triangular cable system to pull an oceanic train of multiple barges. There may be scope to design the tugboat to couple the tug to the stern of a lead vessel that it would push and steer while simultaneously towing a second equivalent vessel.

Twin-hull tugs may pull oceanic trains on such voyages as Hong Kong/Taipei – Long Beach/Los Angeles, or Rio de Janeiro/Sao Paulo – Mumbai/Cochin, Rio de Janeiro/Sao Paulo – Rotterdam/Dover, Seoul/Shanghai – Long Beach/Los Angeles plus several other routes than bypass the Panama Canal. The technology could offer reductions in fuel consumption while increasing crew productivity aboard ships that use computer-assisted navigation for oceanic trains.

Conclusion:

Efficient and cost-competitive gearless mechanical power transfer technology from another era may have application in modern maritime propulsion. It may find application on short-haul mega-ferry ships, large tugs at ports and specialized twin-hull oceanic mega-tugs that may pull oceanic trains over extended voyages of 2 to 3-weeks duration each.

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