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Main Propulsion Engines


The most striking feature of the modern diesel engine is their incredible durability. Our observations are that if an engine is properly installed, used with the correct lubricating oils, run with clean fuel and cooled strictly to the manufacturers requirements and operated at or below its continuous rating a 10,000 hour trouble free life can be expected, and 20,000 hours is not unheard of.

Even when trouble does occur it is very rarely caused by a failure of a major engine component. External engine accessories are usually the first to wear out, but because the vessels spare parts cover these eventualities these features seldom leave the vessel dead in the water.

All the common makers of diesel engines have machines suitable for trawler yachts. Furthermore most have worldwide dealerships. In a purely technical sense there is little to choose between makes, so often the choice of machine is for price or personal preference reasons.

The choice of engine can be broadly divided into 2 main types:

1. Medium Speed Engines
These used to be common in a power range up to 400 BHP @ 900 to 1500 rpm. These machines have high mechanical efficiency and simplicity but suffer from the disadvantage of great size and weight.

An example is the GARDNER 8L3B diesel rated at 230 BHP @ 1150 rpm and 250 BHP @ 1250 rpm respectively. We have fitted a number of these in the past. While these certainly are a wonderful engine, they are 12 feet long and weight nearly 4 tons so are only suitable for larger motor yachts.

2. High Speed Engines

These engines have operating speeds of 1800 to 2500 rpm and there are makes and models suitable for all trawler yachts. Typically each engine series is available in 3 aspirations:

1. Naturally aspirated
2. Turbo charged
3. Turbo charged and intercooled

And 3 service ratings:
(1) Pleasure rating
(2) Medium Continuous Rating
(3) Continuous Rating

With the first two of these ratings there is a limit on the daily full power usage of the engine, in the case of the Medium Continuous Rating it is for 6 hours out of every 12 hours, along with a maximum annual usage as well. This makes these two ratings unsuitable for an ocean cruising vessel. So the Continuous Rating should always be chosen. With a Continuous Rating there is no limit on the daily or annual usage times.

Most new engine installations carry an overall warranty of 12 months and an extended warranty for the internal parts of the engine i.e. crankshaft, connection rods and cylinder block. Typically this extended warranty is for 3 years or 1,000 hours which ever comes first.


For conventional propeller shaft drives the engine speed is far too high for a direct drive system so a reduction gear must be fitted. Typically at least a 3 to 1 or higher speed reducing gear is used incorporating both ahead and astern clutches.

Once again these gears must be continuous rated for ocean cruising vessels. These gearboxes are always direct coupled to the engine.

They are simple devices with rugged spur reducing gears and hydraulically operated multi plate clutches.

Like the engines themselves they have an amazing durability. Most have get-home features where in the event of oil pressure failure the “ahead” clutch can be locked up manually.

Last Updated (Saturday, 29 August 2020 03:16)