Froude's Experiments
When testing ship models and then comparing the results to actual ships, the models tended 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 hull speed (the theoretical top speed of the ship) is a function of the length of the ship
Observing this, Froude realized that the ship resistance problem had to be broken in to 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.
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.
Read more about this topic: Ship Resistance And Propulsion
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