船舶设计外文翻译---船舶在开敞水域和受限航道的坐底现象

《船舶设计外文翻译---船舶在开敞水域和受限航道的坐底现象》由会员分享，可在线阅读，更多相关《船舶设计外文翻译---船舶在开敞水域和受限航道的坐底现象（6页珍藏版）》请在毕设资料网上搜索。

1、附录二、外文翻译 ship squat in open water andin confined channels What exactly is ship squat? When a ship proceeds through water, she pushes water ahead of her. In order not to leave a hole in the water, this volume of water must return down the sides and under the bottom of the ship. The streamlines of ret

2、urn flow are speeded up under the ship. This causes a drop in pressure, resulting in the ship dropping vertically in the water. As well as dropping vertically, the ship generally trims ford or aft. Ship squat thus is made up of two components, namely mean bodily sinkage plus a trimming effect. If th

3、e ship is on even keel when static, the trimming effect depends on the ship type and bC being considered. The overall decrease in the static underkeel clearance (ukc), ford or aft, is called ship squat. It is not the difference between the draughts when stationary and the draughts when the ship is m

4、oving ahead. If the ship moves forward at too great a speed when she is in shallow water, say where this static even-keel ukc is 1.01.5 m, then grounding due to excessive squat could occur at the bow or at the stern. For full-form ships such as Supertankers or OBO vessels, grounding will occur gener

5、ally at the bow. For fine-form vessels such as Passenger Liners or Container ships the grounding will generally occur at the stern. This is assuming that they are on even keel when stationary. If bC is 0.700, then maximum squat will occur at the bow. If bC is 0.700, then maximum squat will occur at

6、the stern. If bC is very near to 0.700, then maximum squat will occur at the stern,amidships and at the bow. The squat will consist only of mean bodilysinkage, with no trimming effects. It must be generally, because in the last two decades, several ship types have tended to be shorter in length betw

7、een perpendiculars (LBP) and wider in Breadth Moulded (Br. Mld). This has lead to reported groundings due to ship squat at the bilge strakes at or near to amidships when rolling motions have been present. Why has ship squat become so important in the last 40 years? Ship squat has always existed on s

8、maller and slower vessels when under-way. These squats have only been a matter of centimetres and thus have been inconsequential. However, from the mid-1960s to this new millennium, ship size steadily has grown until we have Supertankers of the order of 350 000 tonnes dead-weight (dwt) and above. Th

9、ese Supertankers have almost out-grown the Ports they visit, resulting in small static even-keel ukc of only 1.01.5 m. Alongside this development in ship size has been an increase in service speed on several ships, e.g. Container ships, where speeds have gradually increased from 16 up to about 25 kt

10、. Ship design has seen tremendous changes in the 1980s and 1990s. In Oil Tanker design we have the Jahre Viking with a dwt of 564 739 tonnes and an LBP of 440 m. This is equivalent to the length of five football pitches. In 2002, the biggest Container ship to date, namely the Hong Kong Express came

11、into service. She has a dwt of 82 800 tonnes, a service speed of 25.3 kt, an LBP of 304 m, Br. Mld of 42.8 m and a draft moulded of 13 m. As the static ukc have decreased and as the service speeds have increased, ship squats have gradually increased. They can now be of the order of 1.50-1.75m, which

12、 are of course by no means inconsequential. Department of Transport M notices In the UK, over the last 20 years the UK Department of Transport have shown their concern by issuing four M notices concerning the problems of ship squat and accompanying problems in shallow water. These alert all Mariners

13、 to the associated dangers. Signs that a ship has entered shallow water conditions can be one or more of the following: 1. Wave-making increases, especially at the forward end of the ship. 2. Ship becomes more sluggish to manoeuvre. A pilots quote almost like being in porridge. 3. Draught indicators

14、 on the bridge or echo sounders will indicate changes in the end draughts. 4. Propeller rpm indicator will show a decrease. If the ship is in open water conditions, i.e. without breadth restrictions, this decrease may be up to 15% of the Service rpm in deep water. If the ship is in a confined channe

15、l, this decrease in rpm can be up to 20% of the service rpm. 5. There will be a drop in speed. If the ship is in open water conditions this decrease may be up to 30%. If the ship is in a confined channel such as a river or a canal then this decrease can be up to 60%. 6. The ship may start to vibrate

16、 suddenly. This is because of the entrained water effects causing the natural hull frequency to become resonant with another frequency associated with the vessel. 7. Any rolling, pitching and heaving motions will all be reduced as ship moves from deep water to shallow water conditions. This is becau

17、se of the cushioning effects produced by the narrow layer of water under the bottom shell of the vessel. 8.Turning circle diameter (TCD) increases. TCD in shallow water could increase 100%. 9. Stopping distances and stopping times increase, compared to when a vessel is in deep waters. 10. Rudder is

18、less effective when a ship is in shallow waters. What are the factors governing ship squat? The main factor is ship speed V. Detailed analysis has shown that squat varies as speed to the power of 2.08. However, squat can be said to vary approximately with the speed squared. In other words, we can ta

19、ke as an example that if we have the speed we quarter the squat. Put another way, if we double the speed we quadruple the squat! In this context, speed V is the ships speed relative to the water. Effect of current/tide speed with or against the ship must therefore be taken into account. Another impo

20、rtant factor is the block coefficient CB. Squat varies directly with CB. Oil Tankers will therefore have comparatively more squat than Passenger Liners. Procedures for reducing ship squat 1. Reduce the mean draft of the vessel if possible by the discharge of water ballast. This causes two reductions

21、 in one: (a) At the lower draft, the block coefficient CB will be slightly lower in value, although with Passenger Liners it will not make for a signifi-cant reduction. (b) At the lower draft, for a similar water depth, the H/T will be higher in value. It has been shown that higher H/T values lead t

22、o smaller squat values. 2. Move the vessel into deeper water depths. For a similar mean ship draft, H/T will increase, leading again to a decrease in ship squat. 3. When in a river if possible, avoid interaction effects from nearby moving ships or with adjacent riverbanks. A greater width of water w

23、ill lead to less ship squat unless the vessel is outside her width of influence. 4. The quickest and most effective way to reduce squat is to reduce the speed of the ship. False drafts If a moored ships drafts are read at a quayside when there is an ebb tide of say 4 kt then the draft readings will

24、be false. They will be incorrect because the ebb tide will have caused a mean bodily sinkage and trimming effects. In many respects this is similar to the ship moving forward at a speed of 4 kt. It is actually a case of the squatting of a static ship. It will appear that the ship has more tonnes dis

25、placement than she actually has. If a Marine Draft Survey is carried out at the next Port of Call (with zero tide speed), there will be a deficiency in the displacement constant. Obviously, larger ships such as Supertankers and Passenger Liners will have greater errors in displacement predictions. S

26、ummary In conclusion, it can be stated that if we can predict the maximum ship squat for a given situation then the following advantages can be gained: 1. The ship operator will know which speed to reduce to in order to ensure the safety of his/her vessel. This could save the cost of a very large re

27、pair bill. It has been reported in technical press that the repair bill for the QEII was $13 million plus an estimate for lost Passenger bookings of $50 million! 2. The ship officers could load the ship up an extra few centimetres (except of course where load-line limits would be exceeded). If a 100

28、 000 tonnesdwt Tanker is loaded by an extra 30 cm or an SD14 General Cargo ship is loaded by an extra 20 cm, the effect is an extra 3% onto their dwt. This gives these ships extra earning capacity. 3. If the ship grounds due to excessive squatting in shallow water, then apart from the large repair b

29、ill, there is the time the ship is out of service. Being out of service is indeed very costly because loss of earnings can be as high as 100 000 per day. 4. When a vessel goes aground there is always a possibility of leakage of oil resulting in compensation claims for oil pollution and fees for clean-up operations following the incident. These costs eventually may have to be paid for by the shipowner. 备注 ：Dr C.B.Barrass Ship Design and Performance for Masters and MatesM Butterworth-Heinemann， 2004 148179