Main Components of Ship Resistance

When a vessel moves through water, it experiences total resistance, which is the combined effect of several opposing forces acting against its forward motion. These forces arise due to water viscosity, pressure distribution around the hull, wave formation, and external environmental conditions. For analysis, total resistance is divided into the following main components:

1. Frictional Resistance

Frictional resistance is caused by the viscosity of water acting along the ship’s wetted surface as the vessel moves forward. As water flows over the hull, a thin boundary layer forms, and shear stresses develop between the hull surface and adjacent water particles. This shear stress produces resistance.

Frictional resistance depends primarily on:

• Wetted surface area – A larger underwater hull area increases the contact between hull and water, thereby increasing resistance.
• Ship speed – Resistance increases rapidly with speed because higher velocity intensifies boundary layer shear forces.
• Surface roughness of the hull – Fouling, corrosion, or rough coatings increase turbulence and significantly raise resistance.

At normal service speeds, frictional resistance forms the largest portion of total resistance, especially for slow and medium-speed vessels. It increases approximately with the square of the ship’s speed, making hull maintenance crucial for fuel efficiency.

2. Residual Resistance

Residual resistance is the portion of resistance remaining after subtracting frictional resistance from total resistance. It mainly arises from pressure effects and wave formation around the hull. Residual resistance consists of the following components:

(a) Wave-Making Resistance

Wave-making resistance is caused by the energy expended in generating surface waves at the bow and stern as the ship moves. When a vessel travels through water, it disturbs the free surface and creates a wave system that carries energy away from the ship.

This resistance becomes particularly significant at higher speeds because wave height and wave energy increase rapidly with speed.

Wave-making resistance depends on:

• Hull form – Fuller hull shapes generally create larger waves.
• Speed–length ratio (Froude number) – As the Froude number increases, wave-making resistance rises sharply.

At high speeds, wave-making resistance can become a dominant component of total resistance.

(b) Eddy-Making Resistance

Eddy-making resistance is caused by flow separation and the formation of vortices (eddies) around certain parts of the hull. When water flow cannot smoothly follow the hull contour, it separates and creates turbulent regions.

This commonly occurs around:

• The stern region
• Appendages
• Areas with sudden changes in hull form

These turbulent eddies consume energy and increase resistance. Proper streamlining of the hull and stern design can significantly reduce eddy-making resistance.

3. Air Resistance

Air resistance is the force exerted by air on the portion of the ship above the waterline. As the ship moves, it must also overcome aerodynamic drag.

Air resistance depends on:

• Wind speed and direction
• Projected area above water
• Shape of the superstructure

Although usually small compared to water resistance, it becomes significant for:

• Container ships
• Ro-Ro vessels
• Ships operating in strong headwinds

For vessels with large exposed areas, air resistance can noticeably affect fuel consumption.

4. Appendage Resistance

Appendage resistance is caused by external fittings attached to the hull, such as:

• Rudders
• Bilge keels
• Shaft brackets
• Propeller bossings

These appendages increase the wetted surface area and disturb smooth water flow, thereby increasing both frictional and pressure resistance. In resistance calculations, appendage resistance is generally included as a separate correction added to frictional resistance.

5. Added (Special) Resistance

Added resistance refers to the additional resistance experienced in real sea conditions that is not present in calm-water trials.

It occurs due to:

• Waves and swell, which cause pitching and heaving motions
• Steering and yawing motions, which disturb steady flow around the hull

Although not considered in calm-water resistance analysis, added resistance is highly important in practical operations because it significantly affects power requirements and fuel consumption in rough weather.