What is Meant by the Term Resistance in the Context of Ships?
Introduction
Whenever a body moves through a fluid (eg: water or air ), there is a force which opposes the motion of the body. This force is known as resistance. This is quite a common definition but do you know what is meant by the term resistance in context of ship motion? Let us see what all is happening when a ship is moving through water.
We know that the ship is moving through water as well as through air (some part of ship). So it experiences both the forces due to water as well as air.The water may itself be in motion due to the water currents and air in motion due to the winds.Their magnitudes and directions will be of course different, only the resistance due to water will be considered unless the winds are strong enough.
Resistance is measured so as to calculate the power of the engine. The resistance determines the thrust required to be produced by the propulsion device. In other words, it is the force required to pull the ship in calm water.
TYPES OF RESISTANCE:
- The total resistance may have a number of components namely
- Wave making resistance,
- Frictional resistance,
- Form resistance,
- Air resistance
- Appendage resistance.
WAVE MAKING RESISTANCE:
Consider a body moving on an undisturbed water surface. The effect of it on water is it will produce a wave system.Generally three different waves are produced namely, stem divergent, transverse divergent, and bow divergent.This wave system arises from the pressure field that is around the ship. The energy possessed by the pressure field is derived from the ship.
This energy transfer itself is a force opposing the forward motion is termed as wave making resistance. click here for fig:1<img src="/themes/brighthub/images/camera.gif" alt="Click to view image." title="Click to view image.">
FRICTIONAL RESISTANCE:
Whenever a body is moving through a fluid, a thin layer of fluid will stick to the surface of the body and moves along the ship.The change in velocity of the fluid is close to body but it reduces with increase in distance from the body.This region which is subjected to change in velocity is called as boundary layer. The thickness of boundary layer increases from forward to aft.The body experiences a resistance called frictional resistance.
click here for fig:2 <img src="/themes/brighthub/images/camera.gif" alt="Click to view image." title="Click to view image.">
FORM RESISTANCE:
The water particle in the streamline which passes the ship cannot always follow the ships form. Because of this some water particles in the streamline breaks away and hence eddies are formed.This eddies will absorb energy and hence it causes a resistance to the motion.This resistance is called as form resistance.
AIR RESISTANCE:
Air being a fluid, it will resist the passage of the exposed part of the ship. For example consider the following situation at full speed of:
NO WIND: When there is no wind the air resistance is considered as 2-4% of the total water resistance.
SEVERE WEATHER CONDITION: Consider a situation where the weather condition is severe. This may even slow down the ships speed.
APPENDAGE RESISTANCE:
Appendages are rudder, propeller, shaft brackets, bilge keel, bossing, stabilizers, and so on. The resistance due to these appendages are termed as appendage resistance.The resistance is mostly in the order of say 10% of that of hull.
- In actual practise the resistance is classified into two: 1. Frictional resistance, 2. Residuary resistance
- The resistance such as wave making resistance, appendage resistance, form resistance are collectively termed as residuary resistance.
- The value of frictional resistance Rf depends upon the following factors:
- Wetted surface area of the ship,
- Speed of the ship,
- Degree of roughness of the hull,
- Length of the ship.
These are about the resistance and the types of resistance.
References for figure: Ships and Naval Architecture, Munro. N. Smith
