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How do self-propelled model tests help determine the power a ship needs?

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naval architecture A cargo ship passing the Golden Gate Bridge, near San Francisco.

In naval architecture, self-propelled model tests are essential for predicting the power required for a full-scale ship, as they account for the complex hydrodynamic interactions between the hull and the propulsion system.12

While basic towing tests measure hull resistance by scaling based on the Froude number, they do not capture how a screw propeller operates within the ship's wake or how it influences the hull. Self-propelled tests allow engineers to observe these interactions, specifically:31

Wake Fraction: The propeller operates in water moving at a different velocity than the ship due to the hull's boundary layer, which affects the propeller's efficiency.1
Thrust Deduction: The propeller creates reduced pressure forward of its blades, which increases the ship's total resistance. This "thrust deduction" means the propeller must generate more thrust than the bare hull resistance would suggest.1

By measuring these factors in a controlled environment, naval architects can accurately determine the total propulsive power needed, ensuring the design is efficient before construction begins. These techniques are standardized globally through the International Towing Tank Conference.23