Every single ship possesses certain characteristics. They can be either common or individual depending on whether the ship is considered as a floating vessel, which is exposed to physical laws when in water, or looked upon as a complex machine with a specific purpose. Speaking in technical terms, these are defined as sea-faring and operational qualities respectively.
1. Main Parts of the Ship
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General Structure of the Ship |
2. Sea-faring Qualities
a) Floatability - To understand this term, some background knowledge may be helpful:
After the ship is loaded with freight, she is immersed to a particular level in water. This way the load waterline is established and the values of the ships draft are measured. These are individual characteristics because some ships may carry heavier cargo and others - lighter materials and thereby displacing differing volumes of water. Floatability is the ability of the ship to sail safely with a given amount of freight by being immersed to her load waterline. In case of damage, when water leaks in the hull, the draft of the ship increases. The ship can only continue to sail safely if she has free waterproof space between the load waterline and the deck line, known as "reserve floatability" which is measured by the height of the freeboard.
Some merchants use this reserve space as an additional space for cargo because carrying more goods in one ship will bring greater profit. But this way they are certainly playing with human lives and safety at sea. That is why organizations have introduced strict safety codes.
b) Stability - the ability of the ship to regain her balance after being inclined under the influence of external forces such as moving of freight and the effect of strong waves or winds. . In simple words, the ship is stable if she can resist great inclinations without turning over and sinking.
Longitudinal stability of the ship is called the stability she has when the inclinations are bow-stern direction. Its values are very high because practically there is no danger of the ship sinking in a vertical position with her bow or stern down. The more important characteristic is the cross stability - i.e. the ability of the ship to resist inclinations from left to right (or right to left). It shows to what extent sailing is safe.
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Unsuccessful attempt of cross stability!! |
c) Steerability - characterized by the ability of the ship to change her course in whatever direction the pilot wants and to maintain this course. This is done by the ships wheel.
d) Unsinkability - the ability of the ship to stay stable and floatable when one or more compartments of the vessel are flooded. The reason why the ship sinks, is that water intake has the effect of additional freight loaded on the ship which increases her draft. The major steps taken in order to improve this quality of the ship are by increasing the height of the freeboard and preventing the water from leaking in all compartments of the vessel at one time. The latter is achieved by dividing the hull into many waterproof sections, separated with bulkheads. Each of them is on its own and in case of damage, the water floods section after section instead of the whole ship at once. Unfortunately, these are not a guarantee that the ship will survive.
e) Movability - the ability of the ship to sail at a given speed with a given capacity of the main engine. Operational speed is a most important characteristic of the vessel because it determines how fast the transportation of people and cargo will be. The speed is measured in knots (one knot is equal to one nautical mile or 1.852 km/h).
3. Operational Qualities
Normally, when someone asks you about the size of an object, for example - a picture on the wall, you tell him its length and width. However, when you are asked about the size of a ship, you should show your competence by replying in tonnage. Tonnage measures ship size.
Displacement Tonnage
To better explain this kind of tonnage, we offer you the following list of items:
A) the weight of hull and its entire equipment - systems and devices, furniture for living quarters
B) the weight of machinery - main engines, generators, propeller-shafts, etc. plus all liquids in them
C) the weight of fuel - reserves of fuel, water for boilers, fresh water for cooling systems, lubricating oil
D) the weight of supply - crew, passengers with their luggage, food storage, fresh water for washing and drinking, other consumer supplies
E) the weight of carried freight - either on the deck or in the holds (E is also known as pure carrying capacity)
Now here is some algebra:
A+B = light displacement which includes permanent items only (this is the weight of the empty ship)
A+B+C+D+E = load displacement which shows the weight of the ship when it is loaded to her designed capacity
Register Tonnage - This measure is used when determining the c a p a c i t y of all the compartments in the ship. As the name implies, the measured unit is called register ton which is equal to 2.83 cubic meters (100 cubic feet). The name of "ton" dates back from the era of sailing ships when goods were loaded in wooden casks (in French - "tonneau"). People counted these tasks and determined how much freight the hold could keep. Register tonnage can be either gross or net tonnage. Every single ship receives a certificate indicating her register tonnage which is prepared according to national and international rules. All the taxes paid by a ship for a service (be it passing through a canal or using a port) are estimated on the basis of her register tonnage.
Deadweight Tonnage - the full carrying capacity of the ship, equal to C+D+E. In other words, deadweight includes all of the changeable items mentioned above or the total weight of the entire cargo when the ship is immersed to her maximum draft. It is measured in metric tons. Deadweight tonnage is the most frequently used measurement of all.
4. Classification Societies
Insurance marine agencies are a natural consequence of seafaring because it is not impossible for ships to suffer a severe damage no matter what safety precautions are taken. However, in order to insure the vessel, these agencies require detailed information on the ship qualities and the conditions under which people and cargo are transported. They obtain such information from specific classification societies, known as ship registers.
The most popular of all is the English Lloyds Register of Shipping. The English tend to say that this organization was born over a cup of coffee Åc Its history is really interesting.
It all began in the 17th century in a London coffeehouse where sailors, captains, shipbuilders and merchants would meet over a cup of coffee and would have a long chat about marine things. The keeper of the coffeehouse - Edward Lloyd had never been to sea before but he would enjoy these conversations. He tried to remember everything said about the ship characteristics and became so knowledgeable that he could tell which captain was experienced and which not. Lloyds knowledge turned out to be very valuable. Many merchants who were in doubt about a ship and her captain would go to Lloyd and pay to receive the information they needed.
Soon Lloyd decided to keep a record of all his research about ships. Eventually, he founded a ship agency which provided helpful data about ships worldwide. Now we know it under the name of Lloyds Register of Shipping.
