PPT:-Components of Ship Resistance

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PT:-Components of Ship Resistance

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COMPONENTS OF RESISTANCE

By Anoop Kumar M.A

Introduction
• Ship must be designed to move efficiently through water with a minimum of external force. • Resistance, depends on the velocity of the ship. • It is always specified at a particular velocity. • Resistance will depend on the condition of the sea.(rough sea,calm sea etc…..)

Definition
• Ship resistance is defined as the force required to tow the ship in calm water at a constant velocity.

Components of Resistance
• A ship moves on the surface of water. Water is normally taken to be an incompressible fluid, with low viscosity. • Let us begin by assuming that the ship is submerged in an ideal fluid. Body in Ideal Fluid • The ship is submerged in an infinitely large ideal fluid. • The forces acting on the ship in static equilibrium are the weight of the ship acting at the centre of gravity, and the pressure forces acting all along the surface, normal to the surface.

Body in Viscous Fluid
• The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress. • Viscosity is due to friction between neighboring parcels of the fluid that are moving at different velocities. • Due to viscosity, the tangential velocity of water relative to the ship is zero at all points on the surface. • As we move away from the surface, the velocity gradually increases until it reaches the ideal fluid velocity at some distance away from the body. • This layer of high velocity gradients is called the boundary layer. • The viscous force is directly proportional to the velocity gradients. • This force which acts on the surface of the body is called drag.

OTHER COMPONENTS OF RESISTANCE

1. Induced resistance 2. Appendage resistance

3. Blockage Effect
4. Air resistance 5. Added resistance

Induced resistance
• If a vessel moves with a leeway( the amount of drift motion to leeward of an object floating in the water caused by the component of the wind vector that is perpendicular to the object’s forward motion.), like in turn or when there is a wind force component sidewards, a lift force (directed sidewards) is developed. • Especially for sailing yachts and vessels. • When the hull moves slightly sidewards a high pressure is developed on one side (leeward) and a low pressure on the other (windward). • The pressure difference gives rise to a flow from the high to the low pressure, normally under the bottom or tip of the keel and rudder, and longitudinal vortices are generated. • Vortices contain energy left behind and are thus associated with a resistance component, the Induced Resistance.

Appendage resistance
• This resistance is mainly the viscous resistance. • The principal appendages in ships are the bilge keels, rudders, bossings or open shafts and struts. • For rudders this can also be calculated from a knowledge of their shape, using drag coefficients for airfoils of similar characteristics and Reynolds numbers appropriate to their speed and length. • The appendage resistance is expressed as percent of bare hull resistance.

Blockage Effect
• In restricted waters the flow around the hull and the wave making are influenced by the presence of the confining surface. • This could be the seabed in shallow water or the banks of a canal. • All resistance components may be influenced. • Often the effect is modeled as an additional resistance component due to the Blockage Effect of the confining wall.

Air resistance

• • • An additional resistance, which may be considerable, for instance for fully loaded container ships is the wind

resistance. The frontal area facing the relative wind on board the ship can be large and the containers do not have an aerodynamic shape, so large forces may be generated in strong winds. Even in still air, there is a resistance component.
This component, the Air Resistance, is considered in the model-ship extrapolation procedure

.

Added resistance
• • • A seaway will cause an additional resistance on the vessel. Mainly due to the generation of waves by the hull when set in motion by waves. Also due to wave reflection in short waves.

REFERENCE

Google Wikipedia Ship resistance text

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