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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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