Diesel Tug Propulsion Machinery

Direct Reversing Diesel Engines

The earliest applications of diesel propulsion to tugs used direct reversing diesel engines. These prime movers were usually low speed direct drive engines with no reduction gearbox. To reverse, the engine was brought to a complete stop, the valve and injection timing camshaft was shifted (usually by sliding it horizontally in the bearings) to a reverse profiled set of lobes, and the engine was restarted in the reverse direction. Direct reversing engines were normally fitted with air starting, using the admission of compressed air to the cylinders through a rotary air distributor. The advantages of direct reversing engines were that they were relatively simple, they did not require a reversing gear box, and operated similarly to the reciprocating steam engines that preceeded them. The disadvantage of direct reversing diesel engines was that control was very imprecise, reversing took several seconds and was by no means definite, and the time delay and lack of confidence demanded great attention and considerable anticipation in operation. Direct reversing engines declined in popularity following the development of electric drive systems and small and efficient marine reverse reduction gears.

Above: Winton Model W24 Direct Reversing Diesel Engine

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Diesel-Electric Propulsion Systems

The development of marine diesel-electric propulsion systems closely paralleled the application of this type of machinery to submarines, railroad motor cars, and locomotives, and used very similar components and control arrangements. From relatively simple early applications in the 1920s, the technology and system capabilities expanded rapidly until large multi-engine propulsion plants were common. Some of the largest diesel-electric tug propulsion applications were used in US Navy fleet tugs of World War Two, but much larger multi-engine plants were installed in Navy submarines, submarine tenders, destroyer escorts, and some commercial ships. Diesel-electric drive had the advantage of making propeller speed completely independent of engine speed, and offering fine increments of control that were not available in other systems. Its primary disadvantages were higher initial cost, slightly higher maintenance cost, and relatively low transmission efficiency when compared to reverse reduction gear drives. Diesel-electric drive systems are still used for certain applications that require extremely precise vessel control, or where large electric power requirements for other shipboard machinery make it advantageous to have all engines interchangeable for either propulsion or power generating service.

Above: Cleveland Diesel 16-498 Diesel Engine With Electric Drive

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Reversing Reduction Gear Drives

The Reversing Reduction Gear became popular in marine transportation applications in the 1930s, and today is the predominant propulsion system used in diesel tug applications. The gearbox accomplishes several functions. It provides the means to engage and disengage the diesel engine from the driveline through the use of air or hydraulic clutches, which is necessary because the diesel engine cannot idle below a certain minimum speed. It also provides for the reversal of the propeller shaft rotation while the engine operates in the same direction of rotation all the time. Another function is to provide a numerical reduction in drive speed between the engine and the propeller shaft. The speed reduction allows modern high speed engines to be used to drive a propeller shaft at a much lower speed where the propeller efficiency is greater. The system costs less and is more efficient than Diesel-Electric drive. However, it has the disadvantage that there is a minimum propeller speed when clutched "in" that is dictated by the lowest operating speed of the engine and the reduction ratio of the gear box (unless a slip clutch system is applied). One other disadvantage is that the rapid application of load when reversing can sometimes stall engines. In some installations this dictates the need for shaft brakes, to stop the shaft from freewheeling while doing a reversal.

Above: EMD Model 12-567C Diesel Engine With A Reversing Reduction Gear (for a Pennsylvania RR Tug)

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