How is optimal ship engine power determined, given hull drag, propeller efficiency, and sea conditions?

This course covers the process of determining a ship’s engine power, explaining how to apply data from a scale model to a full-scale ship, and how to select the optimal engine considering sea conditions and emergencies.

 

A ship is a structure designed to float on water while carrying cargo or people. Throughout human history, ships have been used for a variety of purposes, including trade, exploration, and warfare, and they remain an important form of transportation today. In order for these ships to operate safely and efficiently, they require a number of technical elements. Among them, the ship’s engine is the main engine that allows the ship to move, and if the engine is underpowered or overpowered, problems will arise. If the power of the engine is small, it will be impossible to operate the ship, and if it is large, the size of the engine will decrease the cargo space, and the price of the ship will increase, and the cost of fuel and maintenance will increase. Therefore, when building a ship, it is very important to obtain the correct power of the engine for the ship.
On the other hand, it is very difficult to determine the engine power of a new ship based on existing ships, as ships are customized according to the type of ship, purpose, conditions of the terrain they are sailing in, and the needs of the owner. So how do you determine the engine power of a new ship? It is a complex process that takes into account many variables and conditions, and requires a scientific and systematic approach to make accurate predictions.
First, a mock-up is needed to determine the engine power of the ship. Due to the large size of ships, it is not possible to build a full-size model, so a small scale model ship is used. In this process, modeling plays an important role in predicting the performance of the ship. Model tests do not simply test a scaled-down ship, but simulate the actual sailing conditions of the ship to evaluate its performance in various environments. This can lead to performance optimization from the design stage, which can help avoid unexpected problems later on.
The tests are carried out in a calm water tank, and the test conditions are determined by taking into account the scale ratio of the model ship to the actual ship. The tests consist of a resistance test to determine the resistance of the hull to the speed required by the shipowner, a propeller-only test to determine the propeller’s own performance, and a self-drag test to determine the relationship between the hull and the propeller. These experiments allow you to analyze various aspects of a ship’s performance, and the resistance test in particular plays an important role in evaluating the various physical resistances that a ship will encounter during a voyage.
The resistance of the model ship can be obtained from this test. Multiplying the model ship’s resistance value by the test speed gives the Effective Horse Power, which is the power that the model ship has to overcome the resistance and produce the required speed. Effective horsepower is the power the propeller puts out, so the propeller must receive more power than this. To consider how much power must be delivered to the propeller to produce effective horsepower, we find the efficiency of the propeller and use it to find Delivered Horse Power, which is the horsepower the propeller receives.
Since the power up to this point is for a model ship, we need to know the model ship-to-solid ship correlation factor, which is the coefficient for scaling to a solid ship. The model-track correlation coefficient is calculated by considering Reynolds number, which is the ratio of inertial force to viscous force, surface roughness, and characteristics of the test tank, and multiplying it by the model horse power to get the horse power of the track. After the transmission horsepower of the solid line is obtained, the amount of power lost from the propeller shaft (conduction efficiency) is obtained through experiments, and then the brake horsepower, which is the output of the engine, is obtained.
The brake horsepower obtained through this process is based on idealized conditions in calm water. However, in real-world sailing, various external environmental factors such as wind, waves, and currents affect the performance of the vessel. Therefore, this brake horsepower alone is not sufficient, and a margin of power is required to account for the various variables in sea conditions. The Normal Continuous Rating is calculated by adding about 15% of the brake horsepower to the spare power, and the Maximum Continuous Rating is calculated by adding about 10% of the spare power for unexpected emergencies. These final power values are used to compare different engine options and select the optimal engine.
To summarize, the power of the model ship is obtained through experiments and then scaled up to a full-scale ship to determine the engine that will be installed on the actual ship, taking into account sea conditions and emergency situations. This process is about more than just determining engine power. It’s a comprehensive design process that takes into account the safety, efficiency, and economy of the ship, and is an important step in laying the foundation for the ship’s successful operation at sea. We hope that each shipyard will ensure that the ship is equipped with an engine of the right power for the ship, resulting in a safe and efficient ship, and we hope that this article has sparked your interest in ships and marine engineering.