Lesson Nine
Ship Motions,Manoeuvrability
Ship motions
Ship motions are defined by the movements from the equilibrium position of the ship’s centre of gravity along the three axes shown in Figure 1 and by rotations about axes approximately parallel to these,The linear displacements along the horizontal (x),lateral (y),and veritical(z)
Fig,1 Coordinate axes of ship motions (see text)
axes are termed surge,sway,and heave,respectively,The rotations about the corresponding body axes are respectively termed roll,pitch,and yaw (veering off course),Roll,pitch,and heave are oscillatory because hydrodynamic forces and moments oppose them.
Ship motions are important for many reasons,A ship should be able to survive any sea that may be Encountered and,in addition,to behave well and to respond to control,In brief,a ship should respond to the action of the sea in such a manner that the amplitudes of its motions and its position never become dangerous,and so that the accelerations it undergoes are kept within reasonable limits,Propulsive performance,or heaving,Hence these motions are made as small as possible.
Ship motions are excited by waves,whose growth is governed by the wind velocity at the sea surface,the area of water,or distance,over which the wind blows (the,fetch”),and the length of time during which the wind has been blowing (the,duration”),Any seaway is always a complex mixture of waves of different lengths,as wind itself is a complex mixture of gusts,All wave components do not travel in the same direction,but the directions of most of them in a single storm lie within 30°of each other,Regular trains of waves of uniform height and length are rarely,if ever,encountered,Most seas are confused and can be considered as made up of many separate component waves that differ in height and length,Pitching,rolling,and heaving are all excited by the changing pattern of surface waves in relation to the speed and course of the ship,In practice,it is possible to damp one motion only---that of rolling,The fitting of bilge keels (finlike longitudinal projections along the part of the underwater body of a ship between the flat of the bottom and the vertical topsides) has this effect,and still more effective means are the activated for stabilizer (a device along the side of a ship activated by a gyroscope and used to keep the ship steady) and the passive or flume stabilizing tank,filled with water inside the ship.
Manoeuvrability
Increases in the size and speed of ships bring problems of safe operation in congested waters and control at high speed in waves,Therefore,designs necessarily represent a compromise between manoeuvrability and course-keeping ability,Ship operators desire maximum manoeuvrability in port to minimize the need for assistance from tugs and to reduce delays in docking,They also desire a ship that can hold a steady course at sea with the minimum use of helm,These aims,however,are mutually conflicting.
A ship is steered by means of one or more rudders arranged at the stern or,in rare cases,at the bow,There are many types and shapes of rudders,depending upon the type of ship,design of stern,and number of propellers,When a yaw---that is,a change of angle about a vertical axis through the centre of gravity ---is started,a turning moment is set up and the ship swings off course unless the swing is corrected by rudder action,This turning effects arises because the hull′s centre of lateral resistance is much nearer the bow than the ship′s centre of gravity.
Good course keeping demands directional stability,This is aided by design features that bring the centre of lateral resistance nearer to the ship′s centre of gravity,These measures,however,increase the diameter of the ship′s turning circle,requiring a design compromise,In warships,in vessels operating in confined water,and in tugs,a small turning circle is essential,In merchant ships,rapid manoeuvring is required only in port; accordingly,the everyday function of the rudder is to ensure the maintenance of a steady course with the minimum use of helm,In this sense,turning circle properties are of less practical significance than the effect of small rudder angles.
(From,Encyclopedia Britannica”,Vol,16,1980)
Technical Terms
manoeuvrability 操纵性
linear displacement 线性位移
surge 纵荡
sway 横荡
heave 垂荡
veer 变向
oscillatory 振荡
hydrodynamic 流体动力(学)的
Amplitude 振幅
acceleration 加速度
wind velocity 风速
fetch 风区长度，波浪形成区
duration 持续时间
seaway 航路（道）
gusts 阵风(雨)
storm 风暴
regular trains of waves 规则波系
damp 阻尼
bilge keel 舭龙骨
finlike 鳍状
projection 突出体，投影，规则
activated fin stabilitizer 主动式稳定（减摇）鳍
gyroscope(gyro) 陀螺仪，回转仪
steady 稳定
flume 槽
congested waters 拥挤水域
course-keeping 保持航向
tug 拖船
docking 靠码头
helm 操舵，驾驶
swing 摆动
turning circle 回转圈
warship 军舰
confined water 受限制水域
Additional Terms and Expressions
seakeeping 耐波性
seaworthiness 适航性
course 航向
track,path 航迹
drift 横漂
side slip 横移
rudder effect 舵效
sea condition 海况
swell 涌
trochoidal wave 坦谷波
divergent wave 散波
natural period 固有周期
slamming 砰击
turning quality 回转性
turning circle 回转圈
turning circle test 回转试验
stopping test 停船试验
free running model test 自由自航模操纵性试验
rotating arm test 旋臂试验
planar motion mechanism 平面运动机构
Notes to the Text
In brief,a ship should respond to the action of the sea in such a manner that the amplitudes of its motions and its position never become dangerous,and so that the accelerations it undergoes are kept within reasonable limits.
in such a manner that the amplitudes---become dangerous 句为结果状语从句。原一位“以这样的方法，以至于------”，译成中文时可灵活些，例如可把前半句译为“简略说，船舶对海浪的响应方式应使其运动的幅值和所在的位置永远不处于一种危险状态”。
and so that 引出的也是结果状语从句。此句中的it undergoes 为省略了关系代词 that 的定语从句（that 在定语从句中作宾语时，让往被省略），用来修饰 the accelerations.
of each other 中的of表示（相互间的）方位、距离。
The shipyard is within 5km of shanghai,
这个船厂离上海5公里以内。
if ever 为if they are ever encountered 的简化形式。当从句内的谓语动词为to be,有其主语跟主句的主语相同时，从句中的主语和to be 就可省略。这类连接词除if外，还有when,while,once 以及as 等。
Most seas are confused and can be considered as made up of many separate component waves that differ in height and length。
其中的as made up of many separate component waves 是as引导的过去分词短语作为主语补足语。
that 引出的定语从句用来修饰waves.
This turning effect arises because the hull’s centre of lateral resistance is much nearer the bow than the ship’s centre of gravity.
because引出的原因状语从句中包含了一个比较级状语从句，than后面的从句中省略了与主句中相同的部分（is near the bow）,这是科技文章中常见的情况。
These measures,however,increase the diameter of the ship’s turning circle,requiring a design compromise,
此句中的requiring a design compromise 为现在分词短语，作状语(表示结果)用（参见第七课注释
