003 sm415-propulsion engineering-01-introduction

1. Piri Reis University
Faculty of Maritime – Department of Marine Engineering
SM415-Propulsion Engineering
01-Introduction
1
INTRODUCTION
Ships are objects that are intended to navigate in water. One of the first achievements of the
Neolithic man was the invention of the “ship”- a wooden, floating structure (actually, a log of
wood) that was moved in water - initially by oars and then by the action of a special wind-
power device, called “sail”. Although the first hulls were very primitive with the conception
of present day, the advantages of using seas and waterways as a transportation environment
were recognized widely. Therefore, various hull forms for various purposes and for various
seas were developed during the course of history. Development of ships and marine
transportation has been one of the determinants of the political history of the following
periods - most important cities are also the major ports, wars and world politics were mostly
aimed to the acquisition of superiority on the waterways. The discovery and colonization of
the new countries “New World” have been possible only after the availability of ships that
were able to negotiate open seas and seamanship techniques to handle those ships.
The advantage of transportation of cargoes or payloads by the sea is that the weight of the
ship and the weights of cargo for the case of merchant ships, passengers and their effects for
the case of passenger vessels and payloads (weapons, sensors and combat systems, together
with their operating personnel) for the case of warships is carried by the hydrostatic buoyancy
forces, totally for displacement hulls and partially for planning hulls. The entire task of the
propulsor is then to provide the forward thrust force to propel the vessel in water, with
powering demands much less than other modes of transportation.
Ships (with the exception of submarines that move totally under water) move both in water
and in air. The forward motion of the vessel is opposed by resistance forces from the air and
the water. These forces, called “drag forces” require the ship to exert a force directed forward
(“thrust” force) to overcome them in order to make the ship cruise in water and in air. The
devices that impart the thrust forces on the hull are called the “propulsors”. The most widely
used propulsor is the screw propeller or one of its variants. It is of utmost importance for the
ship designer to determine the components of ship resistance accurately and reduce them as
much as possible, and also to select the optimum propulsor to provide the necessary thrust for
a given ship speed. Therefore, the science of ship propulsion is involved with the
investigation of the sources of those drag forces and their relations with ship speed, hull
geometry, hull condition, relative sea state, etc. and with the design of ship propulsors that are

2. Piri Reis University
Faculty of Maritime – Department of Marine Engineering
SM415-Propulsion Engineering
01-Introduction
2
required to impart opposing forces to overcome them with the minimum loss of energy
possible.
An important issue is the selection of an ideal propeller and the ideal propeller rotational
speed (revolutions per minute) for a given hull and engine. Except the case of low speed
Diesel engines, where the propeller revolutions are equal to engine crankshaft revolutions, the
designer has to install a reduction gear between the propeller shaft and the engine. The
selection of this reduction gear to provide a reduction in shaft rotational speed to match the
engine and propeller is also an important issue of the engineer.
The use of windpower for the propulsion of ships has always been an important topic in naval
architecture. With the development of high-power engine systems and the availability of
fuels, the usage of sails has seen a considerably decrease during the 19th
-20th
centuries, except
for the yachting and pleasure craft industry. However, with the increasing costs of fuels, re-
consideration of windpower as a viable supplementary propulsion system has gained
importance in the recent years. In addition to classical sails, modern kite-sails, rigid-sails,
auto-gyro type sails, wind turbine-propeller combinations, blown rotors and Flettner-rotor
type propulsion systems are being studied.