Ship's speed plays a very important role in various environmental as well economic factors for the operator of the vessel.In this article we will talk about how ship Speed is measured, what are the variaus factors affecting ship speed ,What are the various design Speeds of Cargo Ships and why it is important know and regulate ships Speed?
How ship Speed is measured
The unit for Sship Speed is Knots (Unit) which is equivalent to one nautical mile per hour.The ISO standard Symbol for the knot is "Kn"
Ancient seafarers used to measure the Speed of the Ship by "chip log" .The Chip log Consisted of a log allached to a rope with a number of knots at equal intervals. The log was dropped in the water at the aftar of the ship and one Sailor watched the Sand emply through 30 - Second Sandglass.while others Counted the number of knots being passed. The number of knots passed multiplied by the distance between the knots and divided by 30 Second stated that 1 knot is equal to 1.852 kilometer per hour or 1 knot is equal to 1 nautical mile per hour.
This is how 'knots' originated and become o metric System to measure Ships Speed.
A nautrcal mile is based on the Circuference of the earth and is equal to one miunte of latitude.Therefore, it is used by ships in charting and navigation. These days most ships use GPS to measure the Speed of the ship.The GPS System Consists of a transmitter, receiver and Satellite system.
This is the most accurate method of measuring the Speed of the vessel. Multiple satellites orbiting the earth constantly. After receiving Signals from different Satellites, the receiver can, calculate its position. The intermal electronics tracks that position in time and calculates ship Speed.
Other methods of measuring Speed are using doppler shift and correlation velocity log or CVL . Doppler shift uses the Doppler effect, which is the relative Change in frequency of a wave when the Source and the observer are in relative motion
The Sound pulse generated by the Ship's motion is reflected off the ocean floor and the Same is measured by Sonar instruments on Ship. The CVL uses a Combination of multiple transmitters and receivers. Being one the expensive methods of measuring Speed, Correlation is generated between Signals from different transmitters at different locations and the Speed of the ship is calculated.
Now we move to the our main topic.
Why Ships are Slow?
If we Compare a Sea going ship with other transport medium in terms of power, Ships are fitted with massive powerful engines.A mega Container ship engine generates power between 80, 000 hp to 109,000 hp .The biggest marine engine ever made is wartsila- Sulzer RTA96-C which generates whopping 109, 000 hp and it's fitted in Emma maersk, One of the worlds largest Ship. The most Successful Commercial airplane- Boeing 747 Can generate power of up to 90, 000 hp with one engine.
weight of the ship and the Cargo which can be in the range of 150, 000 to 180, 000 Tonnes. Apart from the load the engine needs to bear, ships also need to over Come different types to resistance for land tronsportation, the vehicle moves on 4 Wheels on a tar rood which offers resistance against the motion of vehicle.
How ever,the contact areas of the tyres are very Small hence the resistance acting on the tyres is easily over Come by the engine power. When it comes to Ship the large part of the enormous Ship's hull is inside water which resist the motion of the ship. The motion of a Ship through water requires energy to over come this resistance, i. e. the force working against the movement. when the ship is Sailing thorugh calm waters, it faces three types of opposing resistance . The firctional resistance works on the wetted surface of the ship ie the hull part Contacting the waters.This is the biggest factor among the 3 types of resistance and Can take upto 6o- 80%.of the total resistance for Ships.
Big Trailer truck engine Can generate power between 300 - 600 hp and a normal hatch back car engines are availble in 100-130 hp power range . How ever, when we Compare the graph of these transportation mode in terms of Speed, we will see a different visuals.
The ships tends to be the slowest amongst all even after running with a powerful engine. To understand the reason behind the ships Slow speed, we first need to understand two important Concept, the power to weight ratio and the ship resistance. It is used as a measurement of performance of a ship as a whole, with the engine's power out put being divided by the weight of the Ship, to give a metric that is independent of the vehicle's Size .In Simple terms, the engine has to over Come the total weight of the ship, including cargo and passenger onboard, to move the Ship ahead. for a car engine of 100hp it needs to over Come the weight of the car which is around 1000kg. However for a Ship engine of 90000 hp, it needs to over Come the massive.
like Bulkers and Tankers which are technically- low Speed ships. For Ships Such as container, RORO Ships, passenger ships etc. which has highr Speed, the frictional resistance Comprises of 40/60% of the total resistance.The residual resistance Come in to play when the ship generates wave by loosing its energy. AS the ship Cuts through the water, the hull looses its energy in developing pressure to generates wave and losn can be up to 10-30% for low Speed ship and 40-65 %. for high Speed Commercial ships. The air resistance plays a Small role amongst the three howerer it Can be Singnificant if a loaded ship is heading against the wind.
Factors affectinc Ship Speed
*Apart from the previously mentioned reasoning on Ship resistance the Speed of a ship depends on various foctors Such as, Displacement of the ship is the amount of water displaced by the Ship to get accommodated in the water to float. For an empty ship which has no cargo loaded Displacement is lesshen the draft will be less there is less resistance as the lesser Surface of the hull is in Contact with water and as there is not much load on the main engine. it gives more Speed at less power in Simple terms, the more Cargo the ship carries, the more will be the displacement and less wll be the ship speed if the hull, propeller and rudder are not optimised.
If the Sea State is rough and the Ship heading aganist the tide, the engine will have to work harder to over Come the force and the over all load will increase to maintain the Speed.If the engine is not able to maintain the ship Speed will decrease.Hull condition plays a vital role in maintaining the ship Speed. A clean hull with appropriate paint will have less resistance as compared to a fouled hull. The main engine is the reason behind the ship saling in the seas. If it is not maintained properly or not efficiently run, it will not" produced the required speed for the vessel.
Regualting Ship Speed
At the design level, bigger container Ships with less Speed are being Constructed to obtain less CO2 emissions per Container. At the operatronal level, the practice of slow Steaming i. e. reducing Speed as a response to depressed market Conditiong and high ful prices is being used in almost all Commercial Shipping sector.
Have a nice day until you find another article like this....
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