Lesson Eight Estimating Power requirements The power required to propel a new ship is subject to a formidable number of variable items. The family tree of power for propulsion( Fig. 1) shows these divided into two main groups. One is concerned with the resistance to motion caused by the interaction of the hull of the ship with the surrounding water and the other concerns the efficiency with which the power developed in the engine itself can be used and converted into thrust at the propelle Before considering the methods used for estimating their combined effect on power requirements, it is necessary to take the items in turn and discuss briefly their significance and nature Power for PTop R Skin Friction+ wave-inaking +Eddy-making Added Tolal Resstance Prone Muliipliedafrapaisine Efficency Fig. I Power for propulsion Friction at the hull surface in contact with the water is the major part of the resistance of all merchant vessels Wave-making resistance does not assume prime importance until a speed/length ratio(V/VL in excess of unity has been reached. The reason for surface friction is that water is far from being a perfect fluid. Its magnitude depends on the length and area of surface in contact and its degree of roughness, and it varies with the speed of the body through the fluid. By observation and experiment it can be shown that the particles of water in actual contact with the ship adhere to its surface and are carried along by it(it does not seem unreasonable to assume some interlocking of particles). There is no slip. At small distances from the body the velocity imparted to the surrounding fluid is only very small but with a noticeable degree of turbulence. The width of this belt known as the layer increases somewhat towards the after end of the moving body. Its appearance is one of the most spectacular sights to be seen when a vessel is moving at high speed from a practical point of view it is assumed that all the fluid shear responsible for skin friction occurs within this belt and also that outside it fluid viscosity can be disregarded. The exact width of the belt is difficult to determine, but an arbitrary assessment is usually accurate enough. If it is now considered that the effective shape of the immersed body is defined by the extremities of the boundary layer, then that body may be assumed to move without friction. However, this does not apply to the transmission of pressure Part of the energy necessary to move a ship over the surface of the sea is expended in the form of pressure waves. This form of resistance to motion is known as residual resistance, or wave-making. three such wave systems are created by the passage of a ship: a bow system, a stern system(both of which are divergent ), and a transverse sy stem. They occur only in the case of a body moving through two fluids simultaneously. For instance, the residuary resistance of well formed bodies like aircraft or submarines, wholly immersed, is comparativelyLesson Eight Estimating Power Requirements The power required to propel a new ship is subject to a formidable number of variable items. The family tree of power for propulsion (Fig.1) shows these divided into two main groups. One is concerned with the resistance to motion caused by the interaction of the hull of the ship with the surrounding water and the other concerns the efficiency with which the power developed in the engine itself can be used and converted into thrust at the propeller. Before considering the methods used for estimating their combined effect on power requirements, it is necessary to take the items in turn and discuss briefly their significance and nature. Fig.1 Power for propulsion Ship resistance Friction at the hull surface in contact with the water is the major part of the resistance of all merchant vessels. Wave-making resistance does not assume prime importance until a speed/length ratio (V/√L) in excess of unity has been reached. The reason for surface friction is that water is far from being a perfect fluid. Its magnitude depends on the length and area of surface in contact and its degree of roughness, and it varies with the speed of the body through the fluid. By observation and experiment it can be shown that the particles of water in actual contact with the ship adhere to its surface and are carried along by it (it does not seem unreasonable to assume some interlocking of particles). There is no slip. At small distances from the body the velocity imparted to the surrounding fluid is only very small but with a noticeable degree of turbulence. The width of this belt, known as the layer increases somewhat towards the after end of the moving body. Its appearance is one of the most spectacular sights to be seen when a vessel is moving at high speed .from a practical point of view it is assumed that all the fluid shear responsible for skin friction occurs within this belt and also that outside it fluid viscosity can be disregarded. The exact width of the belt is difficult to determine, but an arbitrary assessment is usually accurate enough. If it is now considered that the effective shape of the immersed body is defined by the extremities of the boundary layer, then that body may be assumed to move without friction. However, this does not apply to the transmission of pressure. Part of the energy necessary to move a ship over the surface of the sea is expended in the form of pressure waves. This form of resistance to motion is known as residual resistance, or wave-making. Three such wave systems are created by the passage of a ship: a bow system, a stern system (both of which are divergent), and a transverse system. They occur only in the case of a body moving through two fluids simultaneously. For instance, the residuary resistance of well formed bodies like aircraft or submarines, wholly immersed, is comparatively