Ship resistance and propulsion
A ship 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-19th century 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 which is stationary with respect to water, experiences only hydrostatic pressure. Hydrostatic pressure always acts to oppose the weight of the body. The total force due to this buoyancy is equal to the weight of the displaced water. If the body is in motion, then there are also hydrodynamic pressures that act on the body. For a displacement vessel, that is the usual type of ship, three main types of resistance are considered: that due to wave-making, that due to the pressure of the moving water on the form, often not calculated or measured separately, and that due to friction of moving water on the wetted surface of the hull. These can be split up into more components:Froude's experiments
Froude's method for extrapolating the results of model tests to ships was adopted in the 1870s. Another method created by Hughes introduced in the 1950s and later adopted by the International Towing Tank Conference. Froude's method tends to overestimate the power for very large ships.Froude had observed that when a ship or model was at its so-called Hull speed the wave pattern of the transverse waves have a wavelength equal to the length of the waterline. This means that the ship's bow was riding on one wave crest and so was its stern. This is often called the hull speed and is a function of the length of the ship
where constant should be taken as: 2.43 for velocity in kn and length in metres or, 1.34 for velocity in kn and length in feet.
Observing this, Froude realized that the ship resistance problem had to be broken into two different parts: residuary resistance and frictional resistance. To get the proper residuary resistance, it was necessary to recreate the wave train created by the ship in the model tests. He found for any ship and geometrically similar model towed at the suitable speed that:
There is a frictional drag that is given by the shear due to the viscosity. This can result in 50% of the total resistance in fast ship designs and 80% of the total resistance in slower ship designs.
To account for the frictional resistance Froude decided to tow a series of flat plates and measure the resistance of these plates, which were of the same wetted surface area and length as the model ship, and subtract this frictional resistance from the total resistance and get the remainder as the residuary resistance.