Ship Resistance/Introduction

A ship differs from any other large engineering structure in that – in addition to its other functions –it must be designed to move efficiently through water with a minimum of external force.

It is found that the resistance, depends on the velocity of the ship. Therefore, resistance is always specified at a particular velocity. Furthermore, intuitively we understand that resistance will depend on the condition of the sea. We cannot expect that resistance in a rough sea is the same as in a calm sea. Therefore, operating conditions must also be specified. Therefore, Ship resistance is defined as the force required to tow the ship in calm water at a constant velocity.

Why is knowing resistance of a ship so important? The answer comes from knowing that the ship is usually a part of a larger transportation system. For the overall transportation system to be efficient, it is required that ships operate at a specified "optimum speed". This speed is usually communicated to the naval architect, who must design the ship, so that this speed is attained. One way to ensure this is to put a very powerful engine in the ship, so for all possible values of resistance, the ship will be able to run at the optimum speed.

This solution is clearly not the best. Installing an engine that is more powerful than needed, results in higher construction costs, higher operating costs, and higher maintenance costs. The owner will therefore not accept such a design. If we want to minimize costs, but still attain the desired speed, we must know what resistance to expect at the desired speed. We can then use the formula for calculating power, $$P = F \times V$$, to calculate the power of engine required. Here $$P$$ is power in watts, $$F$$ is the force in newtons and $$V$$ is the velocity in meters per second.

Since resistance of a ship is not constant, conditions must be specified. There are usually two types of conditions. Service condition refers to the resistance while the ship operates under real conditions, where there are currents, waves, wind etc. The effect of these factors is not very easy to measure or predict, because these conditions are always changing.

Another condition defined is the trial condition. This is conducted in relatively calm water. This is the time when the ship is put to trial to see if the naval architect or ship builder has met all the obligations as specified in the contract with the ship owner. One of these is also to ensure that the ship is able to attain the optimum speed.

Given that it is important for the naval architect to know how much resistance will have to be overcome before the ship is made, some way to get this figure must be devised. In order to do that, we must understand what are the reasons behind this resistance. We therefore study the components of resistance.