Ship resistance refers to the force that opposes the motion of a ship as it moves through water. Understanding and accurately predicting ship resistance is crucial in ship design and operation, as it directly impacts factors such as fuel consumption, speed, and overall performance.
Introduction to Ship Resistance
The effective power required to propel a ship through water at a given speed ( V ) is determined by the total resistance ( RT ) encountered by the ship. This total resistance consists of three main components: frictional resistance ( RF ), residual resistance ( RR ), and air resistance ( RA ). The effective power ( PE ) is calculated as the product of the total resistance and the ship’s speed ( V ):
PE=RT×V
Where:
- PE is the effective power (power required to overcome resistance) in the propulsion of the ship,
- RT is the total resistance encountered by the ship, and
- V is the speed of the ship through the water.
RT=RF +RR+RA
By understanding and accurately calculating the components of resistance, ship designers and operators can optimize propulsion systems and hull designs to minimize resistance and achieve efficient propulsion, thereby reducing fuel consumption and operational costs.
Frictional resistance is primarily influenced by the thin layer of water viscosity that forms around the hull surface as the ship moves through waves. This layer creates resistance as the ship progresses. Residual resistance, on the other hand, accounts for energy lost due to the formation of waves, eddies, and viscous pressure resistance, all of which are influenced by the shape of the hull.
For slower-moving vessels like tankers and bulk carriers, frictional resistance tends to have the greatest impact, often accounting for 70% to 90% of the total resistance. However, for faster ships such as Panamax container carriers, frictional resistance may make up only half of the total resistance.
Achieving a perfectly streamlined design above the waterline is challenging due to fabrication difficulties and the limited reduction in resistance it offers. Discontinuities in a ship’s superstructure and broken streamlines contribute to the formation of eddies, further increasing resistance.
Air resistance typically represents a small portion, around 2%, of the total resistance. However, for ships with large superstructures like container ships with stacked containers on deck, air resistance can increase significantly, reaching up to approximately 10%, especially when considering wind resistance.
Various methods, such as those proposed by Holtrop & Mennen and Taylor & Getler, are used to measure hull resistance. However, each method has its limitations and is only valid for certain speeds and hull forms. In regions where the sea freezes over, such as in polar areas, ice resistance becomes dominant and is a significant consideration for icebreakers.