【文章內(nèi)容簡介】 量為了取得新的均衡就要比靜止時多下沉一些與此同時由于船體首尾線的不同必然引起縱傾的變化其實在深水中航行也會有下沉和縱傾的變化但這種變化改變的程度在淺水中就變得更加激烈在商船速度范圍內(nèi)淺水中低速時就出現(xiàn)船體下沉隨著船速的增高下沉的增加率也很快首上浮時機較早而且越是水淺達到最大首縱傾和開始變?yōu)槲矁A所需的船速越低因此船舶通過淺水時對船體下沉及縱傾現(xiàn)象必須引起重視并應(yīng)根據(jù)估計求出剩余水深以防船舶擱淺或觸底 文獻 [7]作者 分析了船舶由海水進入淡水港或航道是吃水增加的問題指出隨船型尺度排水量越來越大等因素影響導(dǎo)致的船舶縱傾變化已不可忽視分析了與這種縱傾變化的大小相關(guān)的因素通過模擬計算定量的反映了縱傾變化的大小并提出了預(yù)防造成的嚴重后果的對策 文獻 [1]和文獻 [12]作者通過對淺水域中航行船舶吃水增加而產(chǎn)生下沉現(xiàn)象分析包括船舶在臨界亞臨界超臨界速度段相應(yīng)阻力縱傾以及下沉的探討給出了下沉量確定的幾種方法對于計算船舶的首位下沉量具有積極的意義 文獻 [13]作者主要討論了為減小淺水效應(yīng)對船舶的影響而需采取的一些實際措施做了闡述描述了淺 水的界定大型船舶淺水效應(yīng)的具體表現(xiàn)及相應(yīng)的注意問題對于船舶縱傾的變化作者從理論上進行了說明相對水深 hd 越小首尾下沉量越大而大型船舶吃水大進出港口或淺灘時受實際水深的制約因而往往需要通過調(diào)節(jié)吃水差盡可能保持平吃水以滿足最大裝貨量的需要 文獻 [3]中作者對淺水航道的定義進行了理論上的討論并結(jié)合長江口錢吃水航道情況進行了總結(jié)文獻 [2]作者通過對船舶阻力的分析和計算根據(jù)影響阻力的主要因素航速水深與船舶吃水比斷面系數(shù)由換算系數(shù)來界定深水淺水與限制性航道文獻 [14]中也提出了淺水影響的判別兩者對淺水的概念進行了理論上的 說明使得何為淺水域更加的明晰 文獻 [10]作者根據(jù)船舶在淺水區(qū)航行時的限制航速的原則建立了限速數(shù)學(xué)模型分析了船速與船舶下沉量的關(guān)系為駕駛員在淺水區(qū)航行時提供了參考依據(jù)為進一步研究航行船舶在淺水中的縱傾變化提供了一種思路 文獻 [15]中作者分析了限制航道中出現(xiàn)的多種不安全現(xiàn)象對船舶操縱性的影響提出了安全防范措施對于淺水引起的船舶下沉和縱傾變化作者只從理論層面上進行了解釋并提出了相應(yīng)的防范措施 文獻 [16]中作者對于由于淺水效應(yīng)引起的船舶尾傾在理論上進行了推導(dǎo)得出了相應(yīng)的結(jié)論平均下沉量隨著船速的增加而增加隨著水 深的減少而增加這位計算船體下沉量提供了借鑒 文獻 [11]中作者使用別爾舍茨方法對縱傾對艦船操縱性的影響做了更精確的定量計算得出其量化規(guī)律為船舶駕駛提供了參考 四．ship squat in open water and in 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 return 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 ponents namely mean bodily sinkage plus a trimming effect If the ship is on even keel when static the trimming effect depends on the ship type andbeing 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 moving ahead If the ship moves forward at too great a speed when she is in shallow water say where this static evenkeel ukc is 10– 15 m then grounding due to excessive squat could occur at the bow or at the stern For fullform ships such as Supertankers or OBO vessels grounding will occur generally at the bow For fineform 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 is 0700 then imum squat will occur at the bow If is 0700 then imum squat will occur at the stern If is very near to 0700 then imum squat will occur at the stern amidships and at the bow The squat will consist only of mean bodily sinkage with no trimming effects It must be generally because in the last two decades several ship types have tended to be shorter in length between 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 bee so important in the last 40 years Ship squat has always existed on smaller and slower vessels when underway These squats have only been a matter of centimetres and thus have been inconsequential However from the mid1960s to this new millennium ship size steadily has grown until we have Supertankers of the order of 350 000 tonnes deadweight dwt and above These Supertankers have almost outgrown the Ports they visit resulting in small static evenkeel ukc of only 10– 15 m Alongside this development in ship size has been an increase in service speed on several ships eg Container ships where speeds have gradually increased from 16 up to about 25 kt 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 2020 the biggest Container ship to date namely the Hong Kong Express came into service She has a dwt of 82 800 tonnes a service speed of 253 kt an LBP of 304 m Br Mld of 428 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 150175m which 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 acpanying problems in shallow water These alert all Mariners to the associated dangers Signs that a ship has entered shallow water conditions can be one or more of the following 1 Wavemaking increases especially at the forward end of the ship 2 Ship bees more sluggish to manoeuvre A pilots quote almost like being in porridge 3 Draught indicators 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 ie without breadth restrictions this decrease may be up to 15 of the Service rpm in deep water If the ship is in a confined channel 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