Ship resistance and propulsion

A ship differs from any other large engineering structure in that – in addition to its other functions –it must be designed to move efficiently through the water with a minimum of external force. For thousands of years ship designers and builders of sailing vessels used rules of thumb based on the midship-section area to size the sails for a given vessel. The hull form and sail plan for the clipper ships, for, example evolved from experience, not from theory. It was not until the advent of steam power and the construction of large iron ships in the mid- 1800s that it became clear to ship owners and builders that a more rigorous approach was needed.

Definition

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

Components of resistance

A body in water experiences, which is stationary with respect to water, experiences only hydrostatic pressure. Hydrostatic pressure, always acts to oppose the weight of the body. If the body is in motion, then there are also hydrdynamic pressures that act on the body.

If the body is in non-viscous fluid, fully submerged and far from the water surface, then the body experiences no resistance. This is the d'Alembert's paradox. This happens because the pressure forces at the fore end of the ship opposing the motion are equal in magnitude, opposite in direction as the pressure forces at the aft end of the ship.

kin Friction

In a viscous fluid, a boundary layer is formed. This causes a net drag due to skin friction. Further, because the ideal pressure now acts on the boundary layer, as opposed to the ship, and the boundary layer grows along the length of the ship, the net opposing forces are greater than the net supporting forces. This further adds to the resistance.

Wave-Making Resistance

A ship moving over the surface of undisturbed water sets up waves emanating from the bow and stern of the ship. The waves created by the ship consist of divergent and transverse waves. The divergent wave are observed as the wake of a ship with a series of diagonal or oblique crests moving outwardly from the point of disturbance. These wave were first studies by Lord Kelvin, who found that regardless to the speed of the ship always make a 19 degree angle to the ship. These waves produce little in the way of resistance against the ships forward motion. Transverse wave the appear as troughs and crest down the side of a ship and create the majority of the wave resistant on a ship. The energy associated with a the transverse wave system travels at one half the phase velocity or velocity of propagation of the waves. The prime mover of the vessel must put additional energy into the system in order to make up for this difference. The relationship between the ships velocity and that of the transverse waves can be found by equating the wave celerity and the ship’s velocity.

References

*E. V. Lewis, ed., Principles of Naval Architecture, vol. 2 (1988)
* http://yarchive.net/mil/wave_drag.html

External links

* [http://www.wikiwaves.org/index.php/Ship_Kelvin_Wake Water Waves Wiki]