Shafting Systems

Transmission of engine power to the propeller can be made in several ways. VIZ

(i) Simple, direct connected propeller shaft.
(ii) “V” Drives (iii) Z Drives
(iv) Hydraulic Drives

Intensive study of methods (ii)-(iv) shows they cannot be recommended in an ocean going vessel due to their very poor propulsive efficiencies and the extra complexity they introduce to the propulsion system.

The argument for their use is that they free up hull space, by allowing an engine room to be located in the aft part of the vessel, thus providing for more spacious accommodations. However, these engine room spaces are always cramped and seldom provide adequate space for the main engine, an absolutely essential “get-home” and generator set along with all the other essential systems which subsequently become spread throughout the vessel, usually in locations with poor access. Ultimately such arrangements become maintenance nightmares.

After more than 60 years of power boat design and engine room detailing we have determined that by far the best arrangement from every standpoint is a centrally located engine space and a simple direct coupled propeller shaft assembly.

The options for propeller shaft assemblies are generally of one of the following types.

(1) Water Lubricated
(2) Oil Lubricated
(3) Grease Lubricated

1. The Water Lubricated

type is the most common and generally the simplest arrangement. They are comprised of a corrosion resistance propeller shaft; stainless steel, monel or bronze running in a sterntube.

The propeller shaft is supported in bearings at the propeller and at the stern gland. These bearings are usually of rubber or one of the propriety plastics type. The stern gland may be of the old fashioned stuffing box type or more usually nowadays a modern spring loaded face seal. The great advantage of the modern face seal is that they are more or less drip free. So no water enters the engine room. They do however require a continuous supply of flushing water to the seal faces.

A water lubricated system can be expected to run trouble free for many thousands of hours. The only disadvantage is if the vessel is operated in silt laden waters which can result in accelerated wear of the stern tube bearings.

2. Oil Lubricated

systems are sometimes used in the higher power installations. In this system the propeller shaft runs in a bath of oil with special oil seals at both ends of the stern tube. The propeller shaft is usually made from medium tensile steel running in white metal or bronze bearings. An oil bath system can be expected to last the life of the ship and require little or no maintenance over that time. The vast majority of the world’s merchant shipping fleet has this type of system.

3. Grease Lubricated

systems are rarely used nowadays being only suitable for low power and low rpm installations. In this system the stern tube is packed with grease with special seals at both ends of the stern tube.

Of the choices of systems above Pacific Motoryachts have considerable experience in the installation of both water and oil bath types. With today’s modern seals, some of which have Lloyds Register approval, we favour the water lubricated type, specifically for the ability to isolate the main engine/s by the means of “soft” engine mounts thereby limiting the transfer of noise and vibration; essential for long distance cruising.

Last Updated (Wednesday, 08 September 2010 03:56)