19 August 2008

[4S/yim233] Basic Principles of Ship Propulsion

Ship propulsion normally occurs with the help of a propeller, which is the term most widely used in English, although the word “screw” is sometimes seen, inter alia in combinations such as a “twin-screw” propulsion plant.
This paper is divided into three chapters which, in principle, may be considered as three separate papers but which also, with advantage, may be read in close connection to each other.

  • Chapter 1, describes the most elementary terms used to define ship sizes and hull forms such as, for example, the ship’s displacement, deadweight,design draught, length between perpendiculars, block coefficient, etc.Other ship terms described include the effective towing resistance, consisting of frictional, residual and air resistance, and the influence of these resistances in service.
  • Chapter 2, deals with ship propulsion and the flow conditions around the propeller(s). In this connection, the wake fraction coefficient and thrust deduction coefficient, etc. are mentioned. The total power needed for the propeller is found based on the above effective towing resistance and various propeller and hull dependent efficiencies which are also described. The operating conditions of a propeller according to the propeller law valid for a propeller with fixed pitch are described for free sailing in calm weather, and followed up by the relative heavy/light running conditions which apply when the ship is sailing and subject to different types of extra resistance, like fouling, heavy sea against, etc.
  • Chapter 3, elucidates the importance of choosing the correct specified MCR and optimising point of the main engine, and thereby the engine’s load diagram in consideration to the propeller’s design point. The construction of the relevant load diagram lines is described in detail by means of several examples (i.e a ship with fixed pitch propeller)

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