Since the tug boat works under various towing and free running conditions, its propeller is to be so designed that in every condition it may develop enough towing force and at the same time enough free running speed. On the other hand the main engine can work neither over a certain maximum torque nor over a maximum revolution. The common rigid pitch propeller cannot make use of the full power of the main engine, restrained either by torque or by revolution except at the designed full power speed. In the case that the type and dimensions of the ship and the power of propulsive engine are known and either the diameter or revolution of propeller is given, we have found that the power is most effectively utilized if we design the propeller so that it can absorb the full power of the engine at the three quarters of the ship speed expected at free running condition. According to this idea propeller for the tug boat by the commonly used systematic screw series diagrams, i.e., √<B_p> - δ design diagram.
