When predicting a ship's resistance, what technique involves building a small model and pulling it through water in a specialized tank to measure forces?

The technique involving building a small model and pulling it through water in a specialized tank to measure forces for predicting a ship's resistance is known as towing tank testing or ship model resistance testing. In this process, a ship model, which is a precise, scaled-down replica of the actual ship's hull, is constructed. This model is then moved through a controlled body of water contained within a long, narrow facility called a towing tank. A specialized apparatus known as a towing carriage moves along rails above the tank, pulling the model at various predetermined speeds. As the model is pulled, the resistance, which is the total drag force exerted by the water on the model, is accurately measured by sensitive instruments called dynamometers or force transducers mounted on the towing carriage. To translate these model measurements to the full-size ship, Froude's Law of Similitude is primarily applied. This law ensures that the wave-making resistance, the component of drag associated with creating waves, is correctly scaled between the model and the full-scale ship when tested at corresponding dimensionless speeds called Froude numbers. However, the viscous resistance, which arises from friction between the hull surface and the water, scales differently and is estimated separately for both the model and the full-scale ship using empirical formulations like the ITTC-1957 friction line. The total full-scale ship resistance is then predicted by adding the appropriately scaled wave-making resistance and the estimated full-scale viscous resistance. This entire procedure of measuring model forces and extrapolating them to predict full-scale ship performance is a fundamental method in naval architecture.