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Diesel Tug Propulsion
Machinery
Direct Reversing Diesel
Engines |
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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
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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
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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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