The purpose of most machinery on board is to generate propulsion making it possible for the ship to move from A to B. A way to measure the efficiency of this process is by measuring the torque generated by the engine and propeller. Combining this measurement with the fuel consumption will tell you how efficient the fuel is converted into propulsion. If you want to measure hull and propeller performance directly you need to measure thrust as well. This measurement can provide data on both the propellers ability to push the vessel and the drag caused by the hull.