Lesson Two
Definitions,Principal Dimensions
Before studying in detail the various technical branches of naval architecture it is important to define chapters,The purpose of this chapter is to explain these terms and to familiarise the reader with them,In the first place the dimensions by which the size of a ship is measured will be considered; they are referred to as ‘principal dimensions’,The ship,like any solid body,requires three dimensions to define its size,and these are a length,a breadth and a depth,Each of these will be considered in turn.
Principal dimensions
Length
There are various ways of defining the length of a ship,but first the length between perpendiculars will be considered,The length between perpendiculars is the distance measured parallel to the base at the level of the summer load waterline from the after perpendicular to the forward perpendicular,The after perpendicular is taken as the after side of the rudder post where there is such a post,and the forward perpendicular is the vertical line drawn through the intersection of the stem with summer load waterline,In ships where there is no rudder post the after perpendicular is taken as the line passing through the centre line of the rudder pintals,The perpendiculars and the length between perpendiculars are shown in Figure 1.
The length between perpendiculars (LBP) is used for calculation purposes as will be seen later,but it will be obvious from Figure 1 that this does not represent the greatest length of the ship,For many purposes,such as the docking of a ship,it is necessary to know what the greatest length of the ship is,This length is known as the length of the extreme point at the after end to a similar point at the forward end,This can be clearly seen by referring again to Figure 1,In most ships the length overall will exceed by a considerable amount the length between perpendiculars,The excess will include the overhang of the stern and also that of the stem where the stem is raked forward,In modern ships having large bulbous bows the length overall LOA may have to be measured to the extreme point of the bulb.
A third length which is often used,particularly when dealing with ship resistance,is the length on the waterline LWL,This is the distance measured on the waterline at which the ship is floating from the intersection of the stern with the waterline to the length is not a fixed quantity for a particular ship,as it will depend upon the waterline at which the ship is floating and upon the trim of the ship,This length is also shown in Figure 1,
Breadth
The mid point of the length between perpendiculars is called ‘amidships’and the ship is usually broadest at this point,The breadth is measured at this position and the breadth most commonly used is called the ‘breadth moulded’,It may be defined simply as the distance from the inside of plating on one side to a similar point on the other side measured at the broadest part of the ship.
As is the case in the length between perpendiculars,the breadth moulded dose not represent the greatest breadth the breadth extreme is required (see Figure 2 ),In many ships the breadth extreme is the breadth moulded plus the thickness of the shell plating where the strakes of shell plating were overlapped the breadth extreme was equal to the breadth moulded plus four thicknesses of shell plating,but in the case of modern welded ships the extra breadth consists of two thicknesses of shell plating only.
The breadth extreme may be much greater than this in some ships,since it is the distance from the extreme overhang on one side of the ship to a similar point on the other side,This distance would include the overhang of decks,a feature which is sometimes found in passenger ships in order to provide additional deck area,It would be measured over fenders,which are sometimes fitted to ships such as cross channel vessels which have to operate in and out of port under their own power and have fenders provided to protect the sides of the ships when coming alongside quays.
Depth
The third principal dimension is depth,which varies along the length of the ship but is usually measured ant amidships,This depth is known as the ‘depth moulded and is measured from the underside of the plating of the deck at side amidships to the base line,It is shown in Figure 2(a),It is sometimes quoted as a ‘depth moulded to upper deck’ or ‘depth moulded to second deck’,etc,Where no deck is specified it can be taken the depth is measured to the uppermost continuous deck,In some modern ships there is a rounded gunwale as shown in Figure 2(b),In such cases the depth moulded is measured from the intersection of the deck line continued with the breadth moulded line.
Other features
The three principal dimensions give a general idea of the size of a ship but there are several other features which have to be considered and which could be different in two ships having the same length,breadth and depth,The more important of these will now be defined.
Sheer
Sheer is the height of the deck at side above a line drawn parallel to the base and tangent to the length of the ship and is usually greatest at the ends,In modern ships the deck line at side often has a variety of shapes,it may be flat with zero sheer over some distance on either side of amidships and then rise as a straight line towards the ends; on the other hand there may be no sheer at all on the deck,which will then be parallel to the base over the entire length,In older ships the deck at side line was parabolic in profile and the sheer was quoted as its value on the forward and after perpendiculars as shown in Figure 1,So called ‘standard’ sheer was given by the formulae:
Sheer forward (in) =0.2Lft+20
Sheer aft (in) =0.1Lft+10
These two formulae in terms of metric units would give:
Sheer forward (cm) =1.666Lm+50.8
Sheer aft (cm) =0.833Lm+25.4
It will be seen that the sheer forward is twice as much as the sheer aft in these standard formulae,It was often the case,however,that considerable variation was made from these standard values,Sometimes the sheer forward was increased while the sheer after was reduced,Occasionally the lowest point of the upper deck was some distance aft of amidships and sometimes departures were made from the parabolic sheer profile,The value of sheer and particularly the sheer forward was to increase the height of the deck above water (the ‘height of platform’ as it was called ) and this helped to prevent water being shipped when the vessel was moving through rough sea,The reason for the abolition of sheer in some modern ships is that their depths are so great that additional height of the deck above water at the fore end is unnecessary from a seakeeping point of view.
Deletion of sheer also tends to make the ship easier to construct,but on the other hand it could be said that the appearance of the ship suffers in consequence.
Camber
Camber or round of beam is beam is defined as the rise of the deck of the ship in going from the side to the centre as shown in Figure 3(a),The camber curve used to be parabolic but here again often nowadays straight line camber curves are used or there may be no camber at all on decks,Camber is useful on the weather deck of a ship from a drainage point of view,but this may not be very important since the ship is very rarely upright and at rest,Often,if the weather deck of a ship is cambered,the lower decks particularly in passenger ships may have no camber at all,as this makes for horizontal decks in accommodation which is an advantage.
Camber is usually stated as its value on the moulded breadth of the ship and standard camber was taken as one-fiftieth of the breadth,The camber on the deck diminishes towards the ends of the ship as the deck breadths become smaller.
Bilge radius
An outline of the midship section of a ship is shown in Figure 3(a),In many ‘full’ cargo ships the section is virtually a rectangle with the lower corners rounded off,This part of the section is referred to as the ‘bilge’ and the shape is often circular at this position,The radius of the circular arc forming the bilge is called the ‘bilge radius’,Some designers prefer to make the section some curve other than a circle in way of the bilge,The curve would have a radius of curvature which increases as it approaches the straight parts of the section with which it has to link up.
Rise of floor
The bottom of a ship at amidships is usually flat but is not necessarily horizontal,If the line of the flat bottom is continued outwards it will intersect the breadth moulded line as shown in Figure 3(a),The height of this intersection above base is called the ‘rise of floor ’,The rise of floor is very much dependent on the ship form,In ships of full form such as cargo ships the rise of floor may only be a few centimeters or may be eliminated altogether,In fine form ships much bigger rise of floor would be adopted in association with a larger bilge radius.
Flat of keel
A feature which was common in the days of riveted ships what was known as ‘flat of keel ’ or ‘flat of bottom ’,Where there is no rise of floor,of course,the bottom is flat from the centre line to the point where the curve of the bilge starts,If there was a rise of floor it was customary for the line of the bottom to intersect the base line some distance from the centre line so that on either side of the centre line there was a small portion of the bottom which was horizontal,as shown in Figure 3(a),this was known as the ‘flat of bottom’ and its value lay in the fact that a rightangle connection could be made between the flat plate keel and the vertical centre girder and this connection could be accomplished without having to bevel the connecting angle bars.
Tumble home
Another feature of the midship section of a ship which was at one time quite common but has now almost completely disappeared is what was called ‘tumble home’,This is the amount which the side of the ship falls in from the breadth moulded line,as shown in Figure 3(b),Tumble home was a usual feature in sailing ships and often appeared in steel merchant ships before World War II,Ships of the present day rarely employ this feature since its elimination makes for ease of production and it is of doubtful value.
Rake of stem
In ships which have straight stems formed by a stem bar or a plate the inclination of the stem to the vertical is called the ‘rake’,It may be defined either by the angle to the vertical or the distance between the intersection of the stem produced with the base line and the forward perpendicular,When ships have curved stems in profile,and especially where they also have bulbous bows,stem rake cannot be simply defined and it would be necessary to define the stem profile by a number of ordinates at different waterlines.
In the case of a simple straight stem the stem line is usually joined up with the base line by a circular are,but sometimes a curve of some other form is used,in which case several ordinates are required to define its shape.
Draught and trim
The draught at which a ship floats is simply the distance from the bottom of the ship to the waterline,If the waterline is parallel to the keel the ship is said to be floating on an even keel,but if the waterline is not parallel then the ship is said to be trimmed,If the draught at the after end is greater than that at the fore end the ship is trimmed by the stern and if the converse is the case it is trimmed by the bow or by the head,The draught can be measured in two ways,either as a moulded draught which is the distance from the base line to the waterline,or as an extreme draught which is the distance from the bottom of the ship to the waterline,In the modern welded merchant ship to the waterline,In the modern welded merchant ship these two draughts differ only by one thickness of plating,but in certain types of ships where,say,a bar keel is fitted the extreme draught would be measured to the underside of the keel and may exceed the moulded draught of by 15-23cm (6-9in),It is important to know the draught of a ship,or how much water the ship is ‘drawing’,and so that the draught may be readily obtained draught marks are cut in the stem and the stern,These are 6 in high with a space of 6in between the top of one figure and the bottom of the next one,When the water level is up to the bottom of a particular figure the draught in feet has the value of that figure,If metric units are used then the figures would probably be 10 cm high with a 10 cm spacing.
In many large vessels the structure bends in the longitudinal vertical plane even in still water,with the result that the base line or the keel does not remain a straight line,The mean draught at which the vessel is floating is not then simply obtained by taking half the sum of the forward and after draughts,To ascertain how much the vessel is hogging or sagging a set of draught marks is placed amidships so that if da,d and df are the draughts at the after end amidships and the forward end respectively then
Hog or sag=- d
When use is made of amidship draughts it is necessary to measure the draught on both sides of the ship and take the mean of the two readings in case the ship should be heeled one side or the other.
The difference between the forward and after draughts of s ship is called the ‘trim’,so that trim T=da- df,and as previously stated the ship will the said to be trimming by the stern or the bow according as the draught aft or the draught forward is in excess,For a given total load on the ship the draught will have its least value when the ship is on an even keel,This is an important point when a ship is navigating in restricted depth of water or when entering a dry dock,Usually a ship should be designed to float on an even keel in the fully loaded condition,and if this is not attainable a small trim by the stern is aimed at,Trim by the bow is not considered desirable and should be avoided as it reduces the ‘height of platform’ forward and increases the liability to take water on board in rough seas.
Freeboard
Freeboard may be defined as the distance which the ship projects above the surface of the water or the distance measured downwards from the deck to the waterline,The freeboard to the weather deck,for example,will vary along the length of the ship because of the sheer of the deck and will also be affected by the trim,if any,Usually the freeboard will be a minimum at amidships and will increase towards the ends.
Freeboard has an important influence on the seaworthiness of a ship,The greater the freeboard the greater is the above water volume,and this volume provides reserve buoyancy,assisting the ship to rise when it goes through waves,The above water volume can also help the ship to remain afloat in the event of damage,It will be seen later that freeboard has an important influence on the range of stability,Minimum freeboards are laid down for ships under International Law in the form of Load Line Regulations.
(from,Naval Architecture for Marine Engineers” by W.Muckle,1975)
Technical Terms
principal dimension 主要尺度
naval architecture 造船（工程）学造船工程（设计）师
length between perpendiculars (LBP) 垂线间长
summer load waterline 夏季载重水线
forward/after perpendicular 首/尾垂线
rudder post 尾柱
stem 首柱
rudder pintle 舵销
length over all (LOA) 总长
overhang （水线以上）悬伸部分
bulbous bow 球鼻艏
length on the waterline （LWL）水线长
amidship 船中
breath moulded 型宽
breath extreme 最大船宽
shell plating 船壳板
rivet 铆接
weld 焊接
strake （船壳板）列板
fender 护舷木
deck area 甲板面积（区域）
cross channel vessel 海峡船
port 港口，船的左舷
side 舷侧（边）
quay 码头
depth moulded 型深
plating of deck 甲板板
base line 基线
upper deck 上甲板
second deck 第二甲板
the uppermost continuous deck 最上层连续甲板
rounded gunwale 圆弧舷边顶部
moulded line 型线
sheer 舷弧
36,ends 船端
37,deck line at side 甲板边线
deck at side line 甲板边线
deck at side 甲板边线
38,profile 纵剖面（图），轮廓
39,sheer forward/aft 首/尾舷
40,platform 平台
41,rough sea 强浪，汹涛海面
42,seakeeping 耐波性
43,appearance 外形（观），出现
44,camber 梁拱
round of beam 梁拱
45,weather deck 露天甲板
46,drainage 排水
47,upright 正浮，直立
48,at rest 在静水中
49,accommodation 居住舱，适应
50,bilge radius 舭（部）半径
5.1 midship section 船中剖面
52,bilge 舭（部）
53,rise of floor 船底升高
54,flat of keel 龙骨宽
55,flat plate keel 平板龙骨
56,vertical center girder 中桁材
57,bevel 折射角，将直角钢改为斜角
58,connecting angle 联接角钢
59,tumble home 内倾
60,sailing ship 帆船
61,steel merchant ship 钢质商船
62,bar 棒，巴（气压单位）
63,rake 倾斜
64,draught 吃水，草图，通风
65,even keel 等吃水，正浮
66,trimmed by the stern/bow 尾/首倾
67,moulded draught 型吃水
68,extreme draught 最大吃水
69,bar keel 棒龙骨
70,”drawing”“吃水”
71.draught marks 吃水标志
72,imperial unit 英制单位
73,metric unit 公制单位
74,spacing 间距
75,hogging 中拱
76,sagging 中垂
77,heel 横倾
78,dry dock 干船坞
79,fully loaded condition 满载标志
80,freeboard 干舷
81,seaworthiness 适航性
82,reserve buoyancy 储备浮力
83,range of stability 稳性范围
84.Load Line Regulations 载重线规范
Additional Terms and Expressions
form coefficients 船型系数
block coefficient 方型系数
prismatic coefficient 棱型系数
midship area coefficient 船中横剖面面积系数
waterplane area coefficient 水线面面积系数
vertical prismatic coefficient 竖向棱型系数
body section of U-form U形横剖面
V-shaped section V形横剖面
geometrically similar ships 几何相似船
base plane 基平面
center plane 中线面
midstation plane 中站面
moulded base line 基线
length breadth ratio 长度比
cruiser stern 巡洋舰型尾
principal coordinate planes 主坐标面
transom 方尾
soft chine 圆舭
hard chine 尖舭
counter 尾伸部
forefoot 首踵
aftfoot 尾踵
deadwood 尾鳍（呆木）
Notes to the Text
as will be seen later 和as is the case in the length between perpendiculars 中as 引出的从句为非限制性定语从句。关系代词as代替整个主句，并在从主语中作主语。as 也可在从句中作宾语，表语用。
A third length 序数字前面，一般用定冠词“the”，但当作者心目中对事物总数还不明确，或还不足以形成一个明确的序列时，序数字前面用不定冠词“a”。例：
will they have to modify the design a fourth time? (它们的设计究竟要修改多少次，心中无数，但依次下来已是第四次，所以用不定冠词“a”。)
This is the distance measured on the waterline at which the ship is floating from the intersection of the stern with the waterline to the intersection of the stem with the waterline.
这是一个符合据。其中at which the ship is floating 为定语从句，修饰the waterline,from the intersection of the stern (with the waterline为intersection 所要求的介词短语)to the intersection of the stern(with the waterline 为第二个intersection 所要求的介词短语）都属于介词短语，作状语用，说明测量的范围。
参见第一课注3.中的第二部分说明
quay 一般指与海岸平行的码头
pier 系指与海岸或呈直角面突出的码头
wharf 一般用于的码头
the deck line continued 和the stern produced 为过去分词作后置定语，分别修饰“the deck line 和the stern,都可译成“延长时”。
considerable variation was made from these standard values 和departures were made from the parabolic sheer profile 和（when）use is made of amidship draughts 这三句都属于主语的成分被位于动词隔离成两部分。这是英语句子结构平衡的需要中带有这种情况，阅读和翻译时需加以注意。
7,considerable variation was made from these standard values 和departures were made from the parabolic sheer profile 和（when）use is made of amidship draughts 这三句都属于主语的成分被位于动词隔离成两部分。这是英语句子结构平衡的需要中带有这种情况，阅读和翻译时需加以注意。
Definitions,Principal Dimensions
Before studying in detail the various technical branches of naval architecture it is important to define chapters,The purpose of this chapter is to explain these terms and to familiarise the reader with them,In the first place the dimensions by which the size of a ship is measured will be considered; they are referred to as ‘principal dimensions’,The ship,like any solid body,requires three dimensions to define its size,and these are a length,a breadth and a depth,Each of these will be considered in turn.
Principal dimensions
Length
There are various ways of defining the length of a ship,but first the length between perpendiculars will be considered,The length between perpendiculars is the distance measured parallel to the base at the level of the summer load waterline from the after perpendicular to the forward perpendicular,The after perpendicular is taken as the after side of the rudder post where there is such a post,and the forward perpendicular is the vertical line drawn through the intersection of the stem with summer load waterline,In ships where there is no rudder post the after perpendicular is taken as the line passing through the centre line of the rudder pintals,The perpendiculars and the length between perpendiculars are shown in Figure 1.
The length between perpendiculars (LBP) is used for calculation purposes as will be seen later,but it will be obvious from Figure 1 that this does not represent the greatest length of the ship,For many purposes,such as the docking of a ship,it is necessary to know what the greatest length of the ship is,This length is known as the length of the extreme point at the after end to a similar point at the forward end,This can be clearly seen by referring again to Figure 1,In most ships the length overall will exceed by a considerable amount the length between perpendiculars,The excess will include the overhang of the stern and also that of the stem where the stem is raked forward,In modern ships having large bulbous bows the length overall LOA may have to be measured to the extreme point of the bulb.
A third length which is often used,particularly when dealing with ship resistance,is the length on the waterline LWL,This is the distance measured on the waterline at which the ship is floating from the intersection of the stern with the waterline to the length is not a fixed quantity for a particular ship,as it will depend upon the waterline at which the ship is floating and upon the trim of the ship,This length is also shown in Figure 1,
Breadth
The mid point of the length between perpendiculars is called ‘amidships’and the ship is usually broadest at this point,The breadth is measured at this position and the breadth most commonly used is called the ‘breadth moulded’,It may be defined simply as the distance from the inside of plating on one side to a similar point on the other side measured at the broadest part of the ship.
As is the case in the length between perpendiculars,the breadth moulded dose not represent the greatest breadth the breadth extreme is required (see Figure 2 ),In many ships the breadth extreme is the breadth moulded plus the thickness of the shell plating where the strakes of shell plating were overlapped the breadth extreme was equal to the breadth moulded plus four thicknesses of shell plating,but in the case of modern welded ships the extra breadth consists of two thicknesses of shell plating only.
The breadth extreme may be much greater than this in some ships,since it is the distance from the extreme overhang on one side of the ship to a similar point on the other side,This distance would include the overhang of decks,a feature which is sometimes found in passenger ships in order to provide additional deck area,It would be measured over fenders,which are sometimes fitted to ships such as cross channel vessels which have to operate in and out of port under their own power and have fenders provided to protect the sides of the ships when coming alongside quays.
Depth
The third principal dimension is depth,which varies along the length of the ship but is usually measured ant amidships,This depth is known as the ‘depth moulded and is measured from the underside of the plating of the deck at side amidships to the base line,It is shown in Figure 2(a),It is sometimes quoted as a ‘depth moulded to upper deck’ or ‘depth moulded to second deck’,etc,Where no deck is specified it can be taken the depth is measured to the uppermost continuous deck,In some modern ships there is a rounded gunwale as shown in Figure 2(b),In such cases the depth moulded is measured from the intersection of the deck line continued with the breadth moulded line.
Other features
The three principal dimensions give a general idea of the size of a ship but there are several other features which have to be considered and which could be different in two ships having the same length,breadth and depth,The more important of these will now be defined.
Sheer
Sheer is the height of the deck at side above a line drawn parallel to the base and tangent to the length of the ship and is usually greatest at the ends,In modern ships the deck line at side often has a variety of shapes,it may be flat with zero sheer over some distance on either side of amidships and then rise as a straight line towards the ends; on the other hand there may be no sheer at all on the deck,which will then be parallel to the base over the entire length,In older ships the deck at side line was parabolic in profile and the sheer was quoted as its value on the forward and after perpendiculars as shown in Figure 1,So called ‘standard’ sheer was given by the formulae:
Sheer forward (in) =0.2Lft+20
Sheer aft (in) =0.1Lft+10
These two formulae in terms of metric units would give:
Sheer forward (cm) =1.666Lm+50.8
Sheer aft (cm) =0.833Lm+25.4
It will be seen that the sheer forward is twice as much as the sheer aft in these standard formulae,It was often the case,however,that considerable variation was made from these standard values,Sometimes the sheer forward was increased while the sheer after was reduced,Occasionally the lowest point of the upper deck was some distance aft of amidships and sometimes departures were made from the parabolic sheer profile,The value of sheer and particularly the sheer forward was to increase the height of the deck above water (the ‘height of platform’ as it was called ) and this helped to prevent water being shipped when the vessel was moving through rough sea,The reason for the abolition of sheer in some modern ships is that their depths are so great that additional height of the deck above water at the fore end is unnecessary from a seakeeping point of view.
Deletion of sheer also tends to make the ship easier to construct,but on the other hand it could be said that the appearance of the ship suffers in consequence.
Camber
Camber or round of beam is beam is defined as the rise of the deck of the ship in going from the side to the centre as shown in Figure 3(a),The camber curve used to be parabolic but here again often nowadays straight line camber curves are used or there may be no camber at all on decks,Camber is useful on the weather deck of a ship from a drainage point of view,but this may not be very important since the ship is very rarely upright and at rest,Often,if the weather deck of a ship is cambered,the lower decks particularly in passenger ships may have no camber at all,as this makes for horizontal decks in accommodation which is an advantage.
Camber is usually stated as its value on the moulded breadth of the ship and standard camber was taken as one-fiftieth of the breadth,The camber on the deck diminishes towards the ends of the ship as the deck breadths become smaller.
Bilge radius
An outline of the midship section of a ship is shown in Figure 3(a),In many ‘full’ cargo ships the section is virtually a rectangle with the lower corners rounded off,This part of the section is referred to as the ‘bilge’ and the shape is often circular at this position,The radius of the circular arc forming the bilge is called the ‘bilge radius’,Some designers prefer to make the section some curve other than a circle in way of the bilge,The curve would have a radius of curvature which increases as it approaches the straight parts of the section with which it has to link up.
Rise of floor
The bottom of a ship at amidships is usually flat but is not necessarily horizontal,If the line of the flat bottom is continued outwards it will intersect the breadth moulded line as shown in Figure 3(a),The height of this intersection above base is called the ‘rise of floor ’,The rise of floor is very much dependent on the ship form,In ships of full form such as cargo ships the rise of floor may only be a few centimeters or may be eliminated altogether,In fine form ships much bigger rise of floor would be adopted in association with a larger bilge radius.
Flat of keel
A feature which was common in the days of riveted ships what was known as ‘flat of keel ’ or ‘flat of bottom ’,Where there is no rise of floor,of course,the bottom is flat from the centre line to the point where the curve of the bilge starts,If there was a rise of floor it was customary for the line of the bottom to intersect the base line some distance from the centre line so that on either side of the centre line there was a small portion of the bottom which was horizontal,as shown in Figure 3(a),this was known as the ‘flat of bottom’ and its value lay in the fact that a rightangle connection could be made between the flat plate keel and the vertical centre girder and this connection could be accomplished without having to bevel the connecting angle bars.
Tumble home
Another feature of the midship section of a ship which was at one time quite common but has now almost completely disappeared is what was called ‘tumble home’,This is the amount which the side of the ship falls in from the breadth moulded line,as shown in Figure 3(b),Tumble home was a usual feature in sailing ships and often appeared in steel merchant ships before World War II,Ships of the present day rarely employ this feature since its elimination makes for ease of production and it is of doubtful value.
Rake of stem
In ships which have straight stems formed by a stem bar or a plate the inclination of the stem to the vertical is called the ‘rake’,It may be defined either by the angle to the vertical or the distance between the intersection of the stem produced with the base line and the forward perpendicular,When ships have curved stems in profile,and especially where they also have bulbous bows,stem rake cannot be simply defined and it would be necessary to define the stem profile by a number of ordinates at different waterlines.
In the case of a simple straight stem the stem line is usually joined up with the base line by a circular are,but sometimes a curve of some other form is used,in which case several ordinates are required to define its shape.
Draught and trim
The draught at which a ship floats is simply the distance from the bottom of the ship to the waterline,If the waterline is parallel to the keel the ship is said to be floating on an even keel,but if the waterline is not parallel then the ship is said to be trimmed,If the draught at the after end is greater than that at the fore end the ship is trimmed by the stern and if the converse is the case it is trimmed by the bow or by the head,The draught can be measured in two ways,either as a moulded draught which is the distance from the base line to the waterline,or as an extreme draught which is the distance from the bottom of the ship to the waterline,In the modern welded merchant ship to the waterline,In the modern welded merchant ship these two draughts differ only by one thickness of plating,but in certain types of ships where,say,a bar keel is fitted the extreme draught would be measured to the underside of the keel and may exceed the moulded draught of by 15-23cm (6-9in),It is important to know the draught of a ship,or how much water the ship is ‘drawing’,and so that the draught may be readily obtained draught marks are cut in the stem and the stern,These are 6 in high with a space of 6in between the top of one figure and the bottom of the next one,When the water level is up to the bottom of a particular figure the draught in feet has the value of that figure,If metric units are used then the figures would probably be 10 cm high with a 10 cm spacing.
In many large vessels the structure bends in the longitudinal vertical plane even in still water,with the result that the base line or the keel does not remain a straight line,The mean draught at which the vessel is floating is not then simply obtained by taking half the sum of the forward and after draughts,To ascertain how much the vessel is hogging or sagging a set of draught marks is placed amidships so that if da,d and df are the draughts at the after end amidships and the forward end respectively then
Hog or sag=- d
When use is made of amidship draughts it is necessary to measure the draught on both sides of the ship and take the mean of the two readings in case the ship should be heeled one side or the other.
The difference between the forward and after draughts of s ship is called the ‘trim’,so that trim T=da- df,and as previously stated the ship will the said to be trimming by the stern or the bow according as the draught aft or the draught forward is in excess,For a given total load on the ship the draught will have its least value when the ship is on an even keel,This is an important point when a ship is navigating in restricted depth of water or when entering a dry dock,Usually a ship should be designed to float on an even keel in the fully loaded condition,and if this is not attainable a small trim by the stern is aimed at,Trim by the bow is not considered desirable and should be avoided as it reduces the ‘height of platform’ forward and increases the liability to take water on board in rough seas.
Freeboard
Freeboard may be defined as the distance which the ship projects above the surface of the water or the distance measured downwards from the deck to the waterline,The freeboard to the weather deck,for example,will vary along the length of the ship because of the sheer of the deck and will also be affected by the trim,if any,Usually the freeboard will be a minimum at amidships and will increase towards the ends.
Freeboard has an important influence on the seaworthiness of a ship,The greater the freeboard the greater is the above water volume,and this volume provides reserve buoyancy,assisting the ship to rise when it goes through waves,The above water volume can also help the ship to remain afloat in the event of damage,It will be seen later that freeboard has an important influence on the range of stability,Minimum freeboards are laid down for ships under International Law in the form of Load Line Regulations.
(from,Naval Architecture for Marine Engineers” by W.Muckle,1975)
Technical Terms
principal dimension 主要尺度
naval architecture 造船（工程）学造船工程（设计）师
length between perpendiculars (LBP) 垂线间长
summer load waterline 夏季载重水线
forward/after perpendicular 首/尾垂线
rudder post 尾柱
stem 首柱
rudder pintle 舵销
length over all (LOA) 总长
overhang （水线以上）悬伸部分
bulbous bow 球鼻艏
length on the waterline （LWL）水线长
amidship 船中
breath moulded 型宽
breath extreme 最大船宽
shell plating 船壳板
rivet 铆接
weld 焊接
strake （船壳板）列板
fender 护舷木
deck area 甲板面积（区域）
cross channel vessel 海峡船
port 港口，船的左舷
side 舷侧（边）
quay 码头
depth moulded 型深
plating of deck 甲板板
base line 基线
upper deck 上甲板
second deck 第二甲板
the uppermost continuous deck 最上层连续甲板
rounded gunwale 圆弧舷边顶部
moulded line 型线
sheer 舷弧
36,ends 船端
37,deck line at side 甲板边线
deck at side line 甲板边线
deck at side 甲板边线
38,profile 纵剖面（图），轮廓
39,sheer forward/aft 首/尾舷
40,platform 平台
41,rough sea 强浪，汹涛海面
42,seakeeping 耐波性
43,appearance 外形（观），出现
44,camber 梁拱
round of beam 梁拱
45,weather deck 露天甲板
46,drainage 排水
47,upright 正浮，直立
48,at rest 在静水中
49,accommodation 居住舱，适应
50,bilge radius 舭（部）半径
5.1 midship section 船中剖面
52,bilge 舭（部）
53,rise of floor 船底升高
54,flat of keel 龙骨宽
55,flat plate keel 平板龙骨
56,vertical center girder 中桁材
57,bevel 折射角，将直角钢改为斜角
58,connecting angle 联接角钢
59,tumble home 内倾
60,sailing ship 帆船
61,steel merchant ship 钢质商船
62,bar 棒，巴（气压单位）
63,rake 倾斜
64,draught 吃水，草图，通风
65,even keel 等吃水，正浮
66,trimmed by the stern/bow 尾/首倾
67,moulded draught 型吃水
68,extreme draught 最大吃水
69,bar keel 棒龙骨
70,”drawing”“吃水”
71.draught marks 吃水标志
72,imperial unit 英制单位
73,metric unit 公制单位
74,spacing 间距
75,hogging 中拱
76,sagging 中垂
77,heel 横倾
78,dry dock 干船坞
79,fully loaded condition 满载标志
80,freeboard 干舷
81,seaworthiness 适航性
82,reserve buoyancy 储备浮力
83,range of stability 稳性范围
84.Load Line Regulations 载重线规范
Additional Terms and Expressions
form coefficients 船型系数
block coefficient 方型系数
prismatic coefficient 棱型系数
midship area coefficient 船中横剖面面积系数
waterplane area coefficient 水线面面积系数
vertical prismatic coefficient 竖向棱型系数
body section of U-form U形横剖面
V-shaped section V形横剖面
geometrically similar ships 几何相似船
base plane 基平面
center plane 中线面
midstation plane 中站面
moulded base line 基线
length breadth ratio 长度比
cruiser stern 巡洋舰型尾
principal coordinate planes 主坐标面
transom 方尾
soft chine 圆舭
hard chine 尖舭
counter 尾伸部
forefoot 首踵
aftfoot 尾踵
deadwood 尾鳍（呆木）
Notes to the Text
as will be seen later 和as is the case in the length between perpendiculars 中as 引出的从句为非限制性定语从句。关系代词as代替整个主句，并在从主语中作主语。as 也可在从句中作宾语，表语用。
A third length 序数字前面，一般用定冠词“the”，但当作者心目中对事物总数还不明确，或还不足以形成一个明确的序列时，序数字前面用不定冠词“a”。例：
will they have to modify the design a fourth time? (它们的设计究竟要修改多少次，心中无数，但依次下来已是第四次，所以用不定冠词“a”。)
This is the distance measured on the waterline at which the ship is floating from the intersection of the stern with the waterline to the intersection of the stem with the waterline.
这是一个符合据。其中at which the ship is floating 为定语从句，修饰the waterline,from the intersection of the stern (with the waterline为intersection 所要求的介词短语)to the intersection of the stern(with the waterline 为第二个intersection 所要求的介词短语）都属于介词短语，作状语用，说明测量的范围。
参见第一课注3.中的第二部分说明
quay 一般指与海岸平行的码头
pier 系指与海岸或呈直角面突出的码头
wharf 一般用于的码头
the deck line continued 和the stern produced 为过去分词作后置定语，分别修饰“the deck line 和the stern,都可译成“延长时”。
considerable variation was made from these standard values 和departures were made from the parabolic sheer profile 和（when）use is made of amidship draughts 这三句都属于主语的成分被位于动词隔离成两部分。这是英语句子结构平衡的需要中带有这种情况，阅读和翻译时需加以注意。
7,considerable variation was made from these standard values 和departures were made from the parabolic sheer profile 和（when）use is made of amidship draughts 这三句都属于主语的成分被位于动词隔离成两部分。这是英语句子结构平衡的需要中带有这种情况，阅读和翻译时需加以注意。
