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.

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

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. If the body is in motion, then there are also hydrodynamic pressures that act on the body.

When testing ship models and then comparing the results to actual ships, the models tend to over predict the resistance of the ship.

Froude had observed that when a ship or model was at its so called Hull speed the wave pattern of the transverse waves (the waves along the hull) 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

${\displaystyle V=1.34{\sqrt {L}}}$

Observing this, Froude realized that the ship resistance problem had to be broken into two different parts: residuary resistance (mainly wave making 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.

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Wikipedia