Power to the water !

Part two: The reduction gear

Authored by: Martin Leduc, October 2002

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On these pages we will explore and dissect the many crucial parts in getting the power generated by the prime mover ie: diesel engine - to do work. Check out the many different setups which have evolve to achieve this, by visiting this page. Learn about the prime movers - steam, diesel engine, gas turbines.  We start at the flywheel, and work our way back to the propeller. Check out the previous page on clutches.


The propeller is match to the vessel, what is it going to take to overcome the resistance of the vessels in the water due to its design. The engine is then matched to the propeller. What power requirements are needed to turn that propeller? At this stage, reduction gears come into play. The reduction gear allows the naval architect to have the most efficient propeller operation while having options on the prime mover.

Diesel engines usually have three general speed in which they are classified in. The slow speeds diesel, 50-300 rpm, rarely need gears as they are already suited for optimum propeller speed. Ships with medium speed, 300-900 rpm, and high speed diesels, 900 + rpm, generally need reduction gears as do gas turbine and steam turbines. By slowing down the propeller we minimizes slippage (waste) which makes the whole power plant more efficient.A reduction gear, it doesn't get much simpler.

Reduction gears are rather simple, one small gear, driven by the engine, driving a large gear which is connected to the propeller shaft. The ratio of teeth between the smaller and larger gears is the ratio of reduction.  Example: take a small gear with 25 teeth and make it turn a big gear with 100 teeth. One revolution of the big gear means that the small gear has turned four times which gives us a 4:1 mechanical advantage, also know in gears as 4:1 reduction.

The larger reduction gears are usually as simple as two gears, with one input and one output. With the increase use of controllable pitch propellers, which allows the prime mover to rotate at a given and steady speed, some features have been added to the larger marine reduction gears. Taking advantage of the power already being developed by the main engine. This allows the whole operation of the ship to be more efficient and perhaps have less equipment to accomplish the same work as before. Hydraulics system on large fishing boats are a good example, also shaft generators are another.

On smaller reduction gears, like those found on tugs, yachts, fishing vessels, etc. they always had more features than just two gears. For example many small gears will feature a built in clutch, they also have reverse and the thrust bearing is built into the gear. In contrast, a coastal freighter may have an independent clutch, reduction gear, thrust bearing, and obtain reverse by using a controllable pitch system.


Gears are generally helical cut gears. This allows for high power to be transmitted but still maintain “low” noise levels. Gears are usually trouble free in their operations as long as their lubricating oil is sufficient, cool and appropriately suited for the function of the gear. Some problems can arise when the “built in” function such as thrust bearing and clutch packs deteriorate and their waste contaminates the oil. Deterioration of the oil cooler - heat exchanger - can also be a source of oil contamination. These problems can be mitigated by scheduled preventive maintenance such as daily checks and oil sampling. Surveys are generally carried out every four years depending on Classification society.