Lesson Eight
Estimating Power Requirements
The power required to propel a new ship is subject to a formidable number of variable items,The family tree of power for propulsion (Fig.1) shows these divided into two main groups,One is concerned with the resistance to motion caused by the interaction of the hull of the ship with the surrounding water and the other concerns the efficiency with which the power developed in the engine itself can be used and converted into thrust at the propeller.
Before considering the methods used for estimating their combined effect on power requirements,it is necessary to take the items in turn and discuss briefly their significance and nature.
Fig.1 Power for propulsion
Ship resistance
Friction at the hull surface in contact with the water is the major part of the resistance of all merchant vessels,Wave-making resistance does not assume prime importance until a speed/length ratio (V/√L) in excess of unity has been reached,The reason for surface friction is that water is far from being a perfect fluid,Its magnitude depends on the length and area of surface in contact and its degree of roughness,and it varies with the speed of the body through the fluid.
By observation and experiment it can be shown that the particles of water in actual contact with the ship adhere to its surface and are carried along by it (it does not seem unreasonable to assume some interlocking of particles),There is no slip,At small distances from the body the velocity imparted to the surrounding fluid is only very small but with a noticeable degree of turbulence,The width of this belt,known as the layer increases somewhat towards the after end of the moving body,Its appearance is one of the most spectacular sights to be seen when a vessel is moving at high speed,from a practical point of view it is assumed that all the fluid shear responsible for skin friction occurs within this belt and also that outside it fluid viscosity can be disregarded,The exact width of the belt is difficult to determine,but an arbitrary assessment is usually accurate enough,If it is now considered that the effective shape of the immersed body is defined by the extremities of the boundary layer,then that body may be assumed to move without friction,However,this does not apply to the transmission of pressure.
Part of the energy necessary to move a ship over the surface of the sea is expended in the form of pressure waves,This form of resistance to motion is known as residual resistance,or wave-making,Three such wave systems are created by the passage of a ship,a bow system,a stern system (both of which are divergent),and a transverse system,They occur only in the case of a body moving through two fluids simultaneously,For instance,the residuary resistance of well formed bodies like aircraft or submarines,wholly immersed,is comparatively small,Because of surface waves formed by a floating body the flow pattern varies considerably with speed,but with an immersed body this flow pattern is the same at all speeds,For this reason the shape of a submarine or aircraft (in consideration of submerged performance only) is more easily related to the constant conditions under which it performs,in the dynamic sense,than is the form of surface vessel.
Returning to a consideration of our three wave systems,it can easily be understood that the bow system is initiated by a crest due to the build-up of pressure necessary to push the water aside and the greater the speed the greater will be the height of the crest and its distance from the bow,Conversely,the stern system is associated with a hollow due to filling-in at the stern,If a ship had a sufficient length of parallel middle body the bow wave system would die out before it reached the stern,but in practice ships are never long enough for this to obtain and interference effects have to be taken into account,The transverse wave system becomes of importance at high speeds and is responsible for the greater part of wave-making resistance,The net effect of the three systems is extremely important from a residuary resistance point of view,and it is necessary to ensure that they do not combine to produce a hollow (a through) at the stern.
Of course,if the energy produced at the bow could be recovered at the stern then there would be no net energy loss,But this is not the case as energy is dissipated laterally in order to maintain a wave pattern,The more developed the wave pattern the more energy is needed to maintain it,Considerations of minimum resistance,therefore,involved a complicated assessment of the interrelation of ship-form characteristics likely to reduce wave causation.
Wave-making resistance follows the laws of dynamic similarity (also known as Froude’s Law of Comparison),which state that the resistances of geometrically ships will vary as the cube of their linear dimensions provided the speeds are in the ratio of the square root of the linear dimensions,Perhaps the law,which does not apply to frictional resistance,looks more concise if stated symbolically,namely:
The most important cause of eddy-making is the ship,There is sometimes a tendency to think of eddy-making as being related only to such appendages as rudders,bilge keel,propeller bossings and the like,While it is perfectly true that badly designed appendages can have eddy-making resistances which are excessive in relation to their size and frictional resistances,the eddy-making of a ship,though relatively small,may be a very large part of the total eddy-making resistance,Eddy making is usually included with the wave-making resistance because it is impracticable to measure the one without the other,However,some distinction is helpful to an understanding of resistance phenomena,In eddy-making it is the stern of the ship which plays the influential part because of the difficulty of maintaining streamline flow even in the most easily shaped body.
Propulsion
It will be obvious that the total resistance of a ship at any speed and the force necessary to propel it must be equal and opposite,The power that the ship’s machinery is capable of developing,however,must be considerably more than this to overcome the various deficiencies inherent in the system,because engines,transmission arrangements and propellers all waste power before it becomes available as thrust,The total efficiency of propulsion therefore involves a consideration of the separate efficiencies of individual items the product of which is expressed in the form of a propulsive coefficient.
The engine efficiency depends upon the type of engine employed and its loading,In the case of a reciprocating engine,either diesel or steam,the power developed in the cylinders can be calculated from the effective pressures recorded on indicator cards,This is known as indicated h.p.,which is naturally more than the horsepower output when measured by means of a brake at the crankshaft coupling,The ratio b.h.p./i.b.p,is,of course the mechanical efficiency of the engine,If the power is measured on the propeller shaft aft of the thrust block and any gearing,then this is known as shaft h.p,and in the case of a turbine is the only place at which it is practicable to measure the power output,There is no such thing as indicated or brake horsepower for a steam or gas turbine,shaft h.p,is almost the same as b.h.p,for a reciprocating engine which drives the propeller directly,but where gearing or special couplings are introduced in the case of high-speed diesel engines or turbines,the transmission losses in these items influence the s.h.p,This is,of course very necessary in order that fair comparisons between the efficiencies of different types of drives can be made,The remainder of the transmission losses are those in the stern tube,When all the engine and transmission losses have been taken into account what is left is a certain amount of the original power which is now delivered at the propeller.
We have already noted that a ship in motion drags along with it a large mass of water,This,wake” as it is known (not the popular interpretation of something that is left astern!) has a forward velocity in which the screw operates,so that the speed of the screw through the wake water is less than the speed of the ship,This is beneficial as it involves a gain in efficiency which is referred to as the wake gain,On the pressure distribution at the stern of the vessel which causes some augment of resistance,It is usual to consider this as a thrust deduction effect,These almost separate effects can be combined to give the effective horse-power required,The screw efficiency in the open,i.e,delivering its thrust to an imaginary vessel,is most important,It is only by considering hull resistance and propeller performance as separate entities that any proper assessment can be made of their effect when combined,The mechanism of hull resistance has been fairly well explored,but the theories of propeller action are still incomplete.
Power estimates
When power estimates are required by a shipbuilder who is tendering for the construction of a new vessel,there is no time to run model tests,nor would the expense normally the justified,The naked e.h.p,is therefore estimated from a published series of methodical tests such as those of Ayre or Taylor,Percentage allowances are made to the naked e.h.p,for appendages and air resistance combined with an estimated lies in the proper selection of the QPC,There are numerous methods of estimating power,but the above is one of the most popular.
Some rapid means of evaluating ship power requirements merely from a lines plan and main technical particulars has long been needed,With increasing productivity,faster construction times and fierce international competition for new orders this has become ever more pressing,Detailed power assessments for ship design proposals are needed frequently well in advance of any firm order,Statistical analysis methods are now being applied to resistance and propulsion problems to peed up the process of ship performance prediction.
Performance criteria are expressed,in terms of equations based on selected parameters of hull shape,dimensions,propeller characteristics and stern conditions,Performance of a design can be assessed from these regression equations which have been derived from a large number of previous model results for the ship type under review,Comparison of a particular result with established data is obtained by minimization of the regression equations,The big advantage of doing things this way is that the coefficients of the regression equations can be fed into a high-speed digital computer,This means that in less than an hour the results of well over a dozen different combinations of hull characteristics can be calculated,This should then lead to an optimum combination of form parameters,The eventual link up with work now being done on the complete definition of hull shape in mathematical terms should take us one step nearer to the soundly based fully automated shipyard.
(From,Background to Ship Design and Shipbuilding Production” by J,Anthony Hind,1965).
Technical Terms
resistance 阻力
thrust 推力
propeller 推进器
skin friction resistance 摩擦阻力
wave-making resistance 兴波阻力
eddy-making resistance 漩涡阻力
appendage resistance 附体阻力
propulsive efficiency 推进效率
hull efficiency 船身效率
transmission efficiency 轴系效率
speed/length ratio 速长比
perfect fluid 理想流体
roughness 粗糙度
turbulence 紊动
boundary layer 边界层
spectacular sights 壮观景色
fluid shear 流体剪力
fluid viscosity 流体粘性
immersed body 浸没的船体部分
residuary resistance 剩余阻力
bow 船首
stern 船尾
divergent 分散的
submarine 潜水艇
aircraft 飞机
crest 波峰
hollow 凹陷，孔隙，波谷
parallel middle body 平行中体
through 波谷
ship-form characteristics 船型特性
laws of dynamics similarity 动力相似定律
rudder 舵
bilge keel 舭龙骨
propeller bossing 推进器箍
streamline 流线型
36．reciprocating engine 往复式发动机
37,diesel/steam engine 柴油/蒸汽机
38,indicator card 示功图
39,indicated h.p,指示马达
40,brake 制动
41,crankshaft coupling 曲轴连轴器
42,mechanical efficiency 机械效率
43,thrust block 推力轴承
44,gearing 齿轮
45,shaft h.p,轴马达
46,brake h.p,制动马达
47,turbine 汽轮机
48,gas turbine 燃气轮机
49,stern tube 尾轴管
50,wake 伴流
51,astern 向（在）船尾
52,wake gain 伴流增益
53,thrust deduction 推力减额
54,effective horse-power (e.h.p.) 有效马达
55,screw efficiency in the open(water) 螺旋桨趟水效率
56,imaginary vessel 假想船
57,mechanism 作用原理（过程），机构
58,proposal 建议
59,statistical 统计分析
60,criterion 衡准
61,ship performance prediction 船舶性能预报
62,regression equation 回归方程
63,form parameter 形状参数
Additional Terms and Expression
service speed 服务航速
design speed 设计航速
cruising speed 巡航速度
trial speed 试航速度
endurance 续航力
admiralty coefficient/constant 海军系数
fouling 污底
hydrodynamics 水动力学
inflow 进流
angle of attack 攻角
lift 升力
circulation 环量
aspect ratio 展弦比
Reynolds number 雷诺数
Froude number 傅汝德数
momentum theory 动量理论
impulse theory 冲量理论
cavitation 空泡现象
adjustable-pitch propeller 可调螺距螺旋桨
controllable-pitch propeller 可调螺距螺旋桨
reversible propeller 可反转螺旋桨
coaxial contra-rotating propellers 对转螺旋桨
ducted propeller,shrouded propeller 导管螺旋桨
tandem propeller 串列螺旋桨
jet propeller 喷射推进器
paddle wheel 明轮
ship model experiment tank 船模试验水池
ship model towing tank 船模拖拽试验水池
wind tunnel 风洞
cavitation tunnel 空泡试验水筒
self propulsion test 自航试验
scale effect 尺度效应
naked model 裸体模型
Notes to the Text
the family tree of power for propulsion 推进马力族类表
For this reason the shape of a submarine or aircraft(in consideration of submerged performance only) is more easily related to the constant conditions under which it performs,in the dynamic sense,than is the form of a surface vessel.
其中的主要句子the shape---is more easily --- than---是一句带有比较状语从句的复合句。在than is the form of a surface vessel 中省略了 easily related to the variable conditions under which it performs,显然，to the constant conditions 和 to the variable conditions 实际上是不同的。严格说，这种省略方法是不正规的，但由于读者能从上下文联系中容易判断出种种不同，为了简便起见，作了省略。在英美科技文章中有此种现象。
the greater the speed the greater will be the height of the crest and its distance from the bow.
The more developed the wave pattern the more energy is needed to maintain it.
这两句都是“the+比较级---the +比较级”结构的句型。
this is not the case 情况并非如此
and the like = and such like 以及诸如此类
The eventual link up with work now being done on the complete definition of hull shape in mathematical
Estimating Power Requirements
The power required to propel a new ship is subject to a formidable number of variable items,The family tree of power for propulsion (Fig.1) shows these divided into two main groups,One is concerned with the resistance to motion caused by the interaction of the hull of the ship with the surrounding water and the other concerns the efficiency with which the power developed in the engine itself can be used and converted into thrust at the propeller.
Before considering the methods used for estimating their combined effect on power requirements,it is necessary to take the items in turn and discuss briefly their significance and nature.
Fig.1 Power for propulsion
Ship resistance
Friction at the hull surface in contact with the water is the major part of the resistance of all merchant vessels,Wave-making resistance does not assume prime importance until a speed/length ratio (V/√L) in excess of unity has been reached,The reason for surface friction is that water is far from being a perfect fluid,Its magnitude depends on the length and area of surface in contact and its degree of roughness,and it varies with the speed of the body through the fluid.
By observation and experiment it can be shown that the particles of water in actual contact with the ship adhere to its surface and are carried along by it (it does not seem unreasonable to assume some interlocking of particles),There is no slip,At small distances from the body the velocity imparted to the surrounding fluid is only very small but with a noticeable degree of turbulence,The width of this belt,known as the layer increases somewhat towards the after end of the moving body,Its appearance is one of the most spectacular sights to be seen when a vessel is moving at high speed,from a practical point of view it is assumed that all the fluid shear responsible for skin friction occurs within this belt and also that outside it fluid viscosity can be disregarded,The exact width of the belt is difficult to determine,but an arbitrary assessment is usually accurate enough,If it is now considered that the effective shape of the immersed body is defined by the extremities of the boundary layer,then that body may be assumed to move without friction,However,this does not apply to the transmission of pressure.
Part of the energy necessary to move a ship over the surface of the sea is expended in the form of pressure waves,This form of resistance to motion is known as residual resistance,or wave-making,Three such wave systems are created by the passage of a ship,a bow system,a stern system (both of which are divergent),and a transverse system,They occur only in the case of a body moving through two fluids simultaneously,For instance,the residuary resistance of well formed bodies like aircraft or submarines,wholly immersed,is comparatively small,Because of surface waves formed by a floating body the flow pattern varies considerably with speed,but with an immersed body this flow pattern is the same at all speeds,For this reason the shape of a submarine or aircraft (in consideration of submerged performance only) is more easily related to the constant conditions under which it performs,in the dynamic sense,than is the form of surface vessel.
Returning to a consideration of our three wave systems,it can easily be understood that the bow system is initiated by a crest due to the build-up of pressure necessary to push the water aside and the greater the speed the greater will be the height of the crest and its distance from the bow,Conversely,the stern system is associated with a hollow due to filling-in at the stern,If a ship had a sufficient length of parallel middle body the bow wave system would die out before it reached the stern,but in practice ships are never long enough for this to obtain and interference effects have to be taken into account,The transverse wave system becomes of importance at high speeds and is responsible for the greater part of wave-making resistance,The net effect of the three systems is extremely important from a residuary resistance point of view,and it is necessary to ensure that they do not combine to produce a hollow (a through) at the stern.
Of course,if the energy produced at the bow could be recovered at the stern then there would be no net energy loss,But this is not the case as energy is dissipated laterally in order to maintain a wave pattern,The more developed the wave pattern the more energy is needed to maintain it,Considerations of minimum resistance,therefore,involved a complicated assessment of the interrelation of ship-form characteristics likely to reduce wave causation.
Wave-making resistance follows the laws of dynamic similarity (also known as Froude’s Law of Comparison),which state that the resistances of geometrically ships will vary as the cube of their linear dimensions provided the speeds are in the ratio of the square root of the linear dimensions,Perhaps the law,which does not apply to frictional resistance,looks more concise if stated symbolically,namely:
The most important cause of eddy-making is the ship,There is sometimes a tendency to think of eddy-making as being related only to such appendages as rudders,bilge keel,propeller bossings and the like,While it is perfectly true that badly designed appendages can have eddy-making resistances which are excessive in relation to their size and frictional resistances,the eddy-making of a ship,though relatively small,may be a very large part of the total eddy-making resistance,Eddy making is usually included with the wave-making resistance because it is impracticable to measure the one without the other,However,some distinction is helpful to an understanding of resistance phenomena,In eddy-making it is the stern of the ship which plays the influential part because of the difficulty of maintaining streamline flow even in the most easily shaped body.
Propulsion
It will be obvious that the total resistance of a ship at any speed and the force necessary to propel it must be equal and opposite,The power that the ship’s machinery is capable of developing,however,must be considerably more than this to overcome the various deficiencies inherent in the system,because engines,transmission arrangements and propellers all waste power before it becomes available as thrust,The total efficiency of propulsion therefore involves a consideration of the separate efficiencies of individual items the product of which is expressed in the form of a propulsive coefficient.
The engine efficiency depends upon the type of engine employed and its loading,In the case of a reciprocating engine,either diesel or steam,the power developed in the cylinders can be calculated from the effective pressures recorded on indicator cards,This is known as indicated h.p.,which is naturally more than the horsepower output when measured by means of a brake at the crankshaft coupling,The ratio b.h.p./i.b.p,is,of course the mechanical efficiency of the engine,If the power is measured on the propeller shaft aft of the thrust block and any gearing,then this is known as shaft h.p,and in the case of a turbine is the only place at which it is practicable to measure the power output,There is no such thing as indicated or brake horsepower for a steam or gas turbine,shaft h.p,is almost the same as b.h.p,for a reciprocating engine which drives the propeller directly,but where gearing or special couplings are introduced in the case of high-speed diesel engines or turbines,the transmission losses in these items influence the s.h.p,This is,of course very necessary in order that fair comparisons between the efficiencies of different types of drives can be made,The remainder of the transmission losses are those in the stern tube,When all the engine and transmission losses have been taken into account what is left is a certain amount of the original power which is now delivered at the propeller.
We have already noted that a ship in motion drags along with it a large mass of water,This,wake” as it is known (not the popular interpretation of something that is left astern!) has a forward velocity in which the screw operates,so that the speed of the screw through the wake water is less than the speed of the ship,This is beneficial as it involves a gain in efficiency which is referred to as the wake gain,On the pressure distribution at the stern of the vessel which causes some augment of resistance,It is usual to consider this as a thrust deduction effect,These almost separate effects can be combined to give the effective horse-power required,The screw efficiency in the open,i.e,delivering its thrust to an imaginary vessel,is most important,It is only by considering hull resistance and propeller performance as separate entities that any proper assessment can be made of their effect when combined,The mechanism of hull resistance has been fairly well explored,but the theories of propeller action are still incomplete.
Power estimates
When power estimates are required by a shipbuilder who is tendering for the construction of a new vessel,there is no time to run model tests,nor would the expense normally the justified,The naked e.h.p,is therefore estimated from a published series of methodical tests such as those of Ayre or Taylor,Percentage allowances are made to the naked e.h.p,for appendages and air resistance combined with an estimated lies in the proper selection of the QPC,There are numerous methods of estimating power,but the above is one of the most popular.
Some rapid means of evaluating ship power requirements merely from a lines plan and main technical particulars has long been needed,With increasing productivity,faster construction times and fierce international competition for new orders this has become ever more pressing,Detailed power assessments for ship design proposals are needed frequently well in advance of any firm order,Statistical analysis methods are now being applied to resistance and propulsion problems to peed up the process of ship performance prediction.
Performance criteria are expressed,in terms of equations based on selected parameters of hull shape,dimensions,propeller characteristics and stern conditions,Performance of a design can be assessed from these regression equations which have been derived from a large number of previous model results for the ship type under review,Comparison of a particular result with established data is obtained by minimization of the regression equations,The big advantage of doing things this way is that the coefficients of the regression equations can be fed into a high-speed digital computer,This means that in less than an hour the results of well over a dozen different combinations of hull characteristics can be calculated,This should then lead to an optimum combination of form parameters,The eventual link up with work now being done on the complete definition of hull shape in mathematical terms should take us one step nearer to the soundly based fully automated shipyard.
(From,Background to Ship Design and Shipbuilding Production” by J,Anthony Hind,1965).
Technical Terms
resistance 阻力
thrust 推力
propeller 推进器
skin friction resistance 摩擦阻力
wave-making resistance 兴波阻力
eddy-making resistance 漩涡阻力
appendage resistance 附体阻力
propulsive efficiency 推进效率
hull efficiency 船身效率
transmission efficiency 轴系效率
speed/length ratio 速长比
perfect fluid 理想流体
roughness 粗糙度
turbulence 紊动
boundary layer 边界层
spectacular sights 壮观景色
fluid shear 流体剪力
fluid viscosity 流体粘性
immersed body 浸没的船体部分
residuary resistance 剩余阻力
bow 船首
stern 船尾
divergent 分散的
submarine 潜水艇
aircraft 飞机
crest 波峰
hollow 凹陷，孔隙，波谷
parallel middle body 平行中体
through 波谷
ship-form characteristics 船型特性
laws of dynamics similarity 动力相似定律
rudder 舵
bilge keel 舭龙骨
propeller bossing 推进器箍
streamline 流线型
36．reciprocating engine 往复式发动机
37,diesel/steam engine 柴油/蒸汽机
38,indicator card 示功图
39,indicated h.p,指示马达
40,brake 制动
41,crankshaft coupling 曲轴连轴器
42,mechanical efficiency 机械效率
43,thrust block 推力轴承
44,gearing 齿轮
45,shaft h.p,轴马达
46,brake h.p,制动马达
47,turbine 汽轮机
48,gas turbine 燃气轮机
49,stern tube 尾轴管
50,wake 伴流
51,astern 向（在）船尾
52,wake gain 伴流增益
53,thrust deduction 推力减额
54,effective horse-power (e.h.p.) 有效马达
55,screw efficiency in the open(water) 螺旋桨趟水效率
56,imaginary vessel 假想船
57,mechanism 作用原理（过程），机构
58,proposal 建议
59,statistical 统计分析
60,criterion 衡准
61,ship performance prediction 船舶性能预报
62,regression equation 回归方程
63,form parameter 形状参数
Additional Terms and Expression
service speed 服务航速
design speed 设计航速
cruising speed 巡航速度
trial speed 试航速度
endurance 续航力
admiralty coefficient/constant 海军系数
fouling 污底
hydrodynamics 水动力学
inflow 进流
angle of attack 攻角
lift 升力
circulation 环量
aspect ratio 展弦比
Reynolds number 雷诺数
Froude number 傅汝德数
momentum theory 动量理论
impulse theory 冲量理论
cavitation 空泡现象
adjustable-pitch propeller 可调螺距螺旋桨
controllable-pitch propeller 可调螺距螺旋桨
reversible propeller 可反转螺旋桨
coaxial contra-rotating propellers 对转螺旋桨
ducted propeller,shrouded propeller 导管螺旋桨
tandem propeller 串列螺旋桨
jet propeller 喷射推进器
paddle wheel 明轮
ship model experiment tank 船模试验水池
ship model towing tank 船模拖拽试验水池
wind tunnel 风洞
cavitation tunnel 空泡试验水筒
self propulsion test 自航试验
scale effect 尺度效应
naked model 裸体模型
Notes to the Text
the family tree of power for propulsion 推进马力族类表
For this reason the shape of a submarine or aircraft(in consideration of submerged performance only) is more easily related to the constant conditions under which it performs,in the dynamic sense,than is the form of a surface vessel.
其中的主要句子the shape---is more easily --- than---是一句带有比较状语从句的复合句。在than is the form of a surface vessel 中省略了 easily related to the variable conditions under which it performs,显然，to the constant conditions 和 to the variable conditions 实际上是不同的。严格说，这种省略方法是不正规的，但由于读者能从上下文联系中容易判断出种种不同，为了简便起见，作了省略。在英美科技文章中有此种现象。
the greater the speed the greater will be the height of the crest and its distance from the bow.
The more developed the wave pattern the more energy is needed to maintain it.
这两句都是“the+比较级---the +比较级”结构的句型。
this is not the case 情况并非如此
and the like = and such like 以及诸如此类
The eventual link up with work now being done on the complete definition of hull shape in mathematical
