When a ship moves forward through the
water at a constant velocity, V. Its forward motion is going to generate:
dynamic pressure on the hull, producing a resultant force in the longitudinal
direction and opposite to the advancing direction; tangential stresses on the
immersed (or wetted) surface due to the viscosity; their resultant force is
also opposite to the ship’s moving direction.
This total resistance is made up of a number of different components, which are caused by a variety of factors and which interact one with the other in an extremely complicated way. In order to deal with the question more simply, it is usual to consider the total calm water resistance as being made up of four main components:
1. Frictional resistance
2. Wave-making resistance
3. Eddy resistance
4. Air resistance
1. Frictional Resistance
Frictional resistance is created due to the motion of the hull through a viscous fluid (water). As the water passes, friction slows the water molecules next to the hull, creating a layer of water that is carried along with the hull. This layer, called the boundary layer, is initially quite thin and the flow within it is laminar. As it progresses along the surface, variable pressures cause turbulence. The layer gradually increases in thickness: near the stern, it breaks away into eddies. It is within this layer that friction is generated between the water molecules and between the water and the surface. The major factors affecting the frictional coefficient are the smoothness of the hull, velocity and length/ wetted surface area of the hull. Frictional resistance accounts for nearly 80% of total resistance in slow speed ships and as much as 50% in high speed ships.
2. Wave Making Resistance
As a ship advances in water, it creates a moving pressure field around it. This pressure field, which moves together with the ship, causes the free surface of the sea to be disturbed. This disturbance of free water surface propagates away from the ship in form of a wave system. The energy expended for the creation of waves is accounted as the source of wave making resistance. In other words, The wave making resistance is due to the energy that must be continuously supplied by the ship to the wave system created on the water surface.
As ship speed increases, the height of the waves produced by the ship increases and therefore the energy required to produce these waves also increases. This lost energy is referred to as wave making resistance and often becomes a limiting factor in the speed of a ship.
3. Eddy-Making Resistance
Eddy-making resistance is contributed from normal pressure applied on a hull. Due to the viscosity of the fluid, the flow separates from the surface of a hull and eddies (vortices) are formed. These eddies induce the changes in the velocity field and thus changes the normal pressures on a hull. The changes in the pressure field around a ship result in the eddy-making resistance. So Eddy resistance is basically due to the energy carried away by the eddies shed from hull or appendages.
4. Air Resistance
The part of ship resistance that is due to the resistance of the air to above-water portion of the ship moving through it is called air resistance. The level of air resistance will depend on the size and shape of the superstructure and on ship speed. Air resistance normally represents about 2% of the total resistance, however, for loaded container ships in head wind, the air resistance can be as much as 10%.
The principle factors affecting ship resistance are the friction and viscous effects of water acting on the hull, the energy required to create and maintain the ship’s characteristic bow and stern waves, and the resistance that air provides to ship motion. In mathematical terms, total resistance can be written as:
RT = RV + RW + RAA
Where: RT = total hull resistance
RV = viscous (friction) resistance
RW = wave making resistance
RAA = air resistance caused by ship moving through calm air