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 propelle 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 Power for PTop R Skin Friction+ wave-inaking +Eddy-making Added Tolal Resstance Prone Muliipliedafrapaisine Efficency Fig. I Power for propulsion 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/VL 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 sy stem. 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
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 sy stem 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 esistance,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 hich does not apply to frictional resistance, looks more concise if stated symbolically, namely R provided 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 Populsion 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
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: l L v V provided l L r R t t = = 3 3 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 hp. /ibp 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 horsepowe 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 necess 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 int what is left is a certain amount of the origina which is now delivered at the propeller drags along with it a large 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 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)
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 stance阻力 bil!ekel舭龙骨 2. thrust推力 34. propeller bossing推进器箍 3. propeller推进器 35. streamline流线型 4. skin friction resistance摩擦阻力 36. reciprocating engine往复式发动机 5.wave- making resistance兴波阻力 37 diesel/steam engine柴油/蒸汽机 6.eddy- making resistance漩涡阻力 38. indicator card示功图 7. appendage resistance附体阻力 39. indicated h p.指示马达 8. propulsive efficiency推进效率 40. brake制动 9. hull efficiency船身效率 41. crankshaft coupling曲轴连轴器 10. transmission efficiency轴系效率 42. mechanical efficiency机械效率 1l. speed// ength ratio速长比 43. thrust block推力轴承 12. perfect fluid理想流体 44. gearing齿轮 oughness粗糙度 45. shaft h p.轴马达 14. turbulence紊动 46 brake h p.制动马达 15. boundary layer边界层 47. turbine汽轮机 16. spectacular sights壮观景色 48. gas turbine燃气轮机 17. fluid shear流体剪力 49. stern tube尾轴管 18. fluid viscosity流体粘性 伴流 19. immersed body浸没的船体部分 51. astern向(在)船尾 20. residuary resistance剩余阻力 52. wake gain伴流增益 21.bow船首 推力减额 22. stern船尾 54 effective horse-power(eh p)有效马达 23. divergent分散的 55. screw efficiency in the open( water))螺旋桨趟水 24. submarine潜水艇 效率 25. aircraft飞机 56 imaginary vessel假想船 26. crest波峰 57 mechanism作用原理(过程),机构 27. hollow凹陷,孔隙,波谷 58 proposal建议 28. parallel middle body平行中体 59 statistical统计分析 29. through波谷 60. criterion衡准 0.ship- form characteristics船型特性 61. ship performance prediction船舶性能预报 31. laws of dynamics similarity动力相似定律 62. regression equation回归方程 63. form parameter形状参数 Additional Terms and Expression service speed服务航速 7. fouling污底 design speed设计航速 8. hydrodynamics水动力学 cruising speed巡航速度 9.inow进流 4. trial speed试航速度 10. angle of attack攻角 5. endurance续航力 1l.lit升力 6. admiralty coefficient/constant海军系数 12. circulation环量
Technical Terms 1. resistance 阻力 2. thrust 推力 3. propeller 推进器 4. skin friction resistance 摩擦阻力 5. wave-making resistance 兴波阻力 6. eddy-making resistance 漩涡阻力 7. appendage resistance 附体阻力 8. propulsive efficiency 推进效率 9. hull efficiency 船身效率 10. transmission efficiency 轴系效率 11. speed/length ratio 速长比 12. perfect fluid 理想流体 13. roughness 粗糙度 14. turbulence 紊动 15. boundary layer 边界层 16. spectacular sights 壮观景色 17. fluid shear 流体剪力 18. fluid viscosity 流体粘性 19. immersed body 浸没的船体部分 20. residuary resistance 剩余阻力 21. bow 船首 22. stern 船尾 23. divergent 分散的 24. submarine 潜水艇 25. aircraft 飞机 26. crest 波峰 27. hollow 凹陷，孔隙，波谷 28. parallel middle body 平行中体 29. through 波谷 30. ship-form characteristics 船型特性 31. laws of dynamics similarity 动力相似定律 32. rudder 舵 33. bilge keel 舭龙骨 34. propeller bossing 推进器箍 35. 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 1. service speed 服务航速 2. design speed 设计航速 3. cruising speed 巡航速度 4. trial speed 试航速度 5. endurance 续航力 6. admiralty coefficient/constant 海军系数 7. fouling 污底 8. hydrodynamics 水动力学 9. inflow 进流 10. angle of attack 攻角 11. lift 升力 12. circulation 环量
13. aspect ratio展弦比 23. tandem propeller串列螺旋桨 14. Reynolds number雷诺数 24. jet propeller喷射推进器 15. Froude number傅汝德数 25. paddle wheel明轮 16. momentum theory动量理论 26. ship model experiment tank船模试验水池 17. impulse theory冲量理论 27. ship model towing tank船模拖拽试验水池 18. cavitation空泡现象 28. wind tunnel风洞 19. adjustable- pitch propeller可调螺距螺旋桨 29. cavitation tunnel空泡试验水筒 controllable- pitch propeller可调螺距螺旋桨 30. self propulsion test自航试验 20. reversible propeller可反转螺旋桨 31. scale effect尺度效应 21. coaxial contra- rotating propellers对转螺旋桨 32. naked model裸体模型 2. ducted propeller; shrouded propeller导管螺旋桨 Notes to the Text the family tree of power for propulsion推进马力族类表 2. 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 urface 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 3. 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 4. this is not the case情况并非如此 5. and the like= and such like以及诸如此类 6. The eventual link up with work now being done on the complete definition of hull shape in mathematical
13. aspect ratio 展弦比 14. Reynolds number 雷诺数 15. Froude number 傅汝德数 16. momentum theory 动量理论 17. impulse theory 冲量理论 18. cavitation 空泡现象 19. adjustable-pitch propeller 可调螺距螺旋桨 controllable-pitch propeller 可调螺距螺旋桨 20. reversible propeller 可反转螺旋桨 21. coaxial contra-rotating propellers 对转螺旋桨 22. ducted propeller, shrouded propeller 导管螺旋桨 23. tandem propeller 串列螺旋桨 24. jet propeller 喷射推进器 25. paddle wheel 明轮 26. ship model experiment tank 船模试验水池 27. ship model towing tank 船模拖拽试验水池 28. wind tunnel 风洞 29. cavitation tunnel 空泡试验水筒 30. self propulsion test 自航试验 31. scale effect 尺度效应 32. naked model 裸体模型 Notes to the Text 1. the family tree of power for propulsion 推进马力族类表 2. 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。 3. 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。 4. this is not the case 情况并非如此 5. and the like = and such like 以及诸如此类 6. The eventual link up with work now being done on the complete definition of hull shape in mathematical
