Before revision: Resistive force is roughly ½ water density times velocity square times area of ship wet surface: Background Unlike in ground vehicles, the statement of installed horsepower is not sufficient to understand how strong a tug is - this is because the tug operates mainly in very low or zero speeds, thus may not be delivering power (power = force x velocity; so, for zero speeds, the power is also zero), yet still absorbing torque and delivering thrust.
After revision: Resistive force is roughly ½ water density times velocity square times area of ship wet surface: Background Unlike in ground vehicles, the statement of installed horsepower is not sufficient to understand how strong a tug is – this is because the tug operates mainly in very low or zero speeds, thus may not be delivering power (power = force × velocity; so, for zero speeds, the power is also zero), yet still absorbing torque and delivering thrust.