SHIP HYDRODYNAMICS LECTURE NOTES OF PROPULSION PART

Transcription

1 SHIP HYDRODYNAMICS LECTURE NOTES OF PROPULSION PART

2 Course Outline Contents Time Date Week 1. Propulsion Systems a) History and Development of Screw Propeller b) Modern Propulsion Systems i- Fixed pitch propellers ii- Ducted propellers iii- Contra-rotating propellers iv- Overlapping propellers v- Controllable pitch propellers 1 hour th vi- Waterjet propulsion system vii- Cyclodial propellers viii- Paddle wheels ix- Superconducting electric propulsion system x- Azimuth podded propulsion system 2. Propeller Geometry a) Frames of Reference b) Propeller Reference Lines c) Pitch d) Rake and Skew e) Propeller Outlines and Areas f) Blade Sections (NACA Definition of Thickness and Camber) g) Propeller Drawing Example (Wageningen series offset) 3. Hydrodynamic Characteristics of Propellers a) Open Water Characteristics b) Propeller-Hull Interaction-Wake i- Wake field characteristics ii- Wake field definition (nominal wake field, effective wake field) iii- Thrust deduction iv- Relative-rotative efficiency v- Propulsive efficiency and propulsion factors vi- Summary of efficiencies in powering c) Standard Series Propeller Data 4. Basic Outlines of Propeller Design Example (Propeller Design - Stage 1) 1 hour th 2 hours th 2 hours th 2 hours th 5. Propeller Theories a) Momentum Theory b) Blade Element Theory Example (K T, K Q, η 0 and B p -δ) 2 hours th

3 Contents Time Date Week Mid-Term Exam 2 hours th c) Profile Characteristics d) Profile Series e) Lifting Line Theory f) Lifting Surface Theory g) Boundary Element Methods 6. Cavitation a) The basics of cavitation i- Physics of cavitation ii- Historical development iii- Cavitation number (inception) iv- Effect of angle of attack on cavitation b) Types of cavitation experienced by propellers i- Location on a blade ii- Physical appearance of cavitation c) Effects of Cavitation on Propellers i- Performance breakdown ii- Noise iii- Vibration iv- Erosion d) Cavitation Considerations in Design i- Cavitation criteria ii- Cavitation bucket diagrams d) Preventing Cavitation 2 hours th 2 hours th Example (NACA pressure distribution, KT-KQ- RÖ) 1 hour? th 7. Propeller Design a) Propeller Design Basis 1. Resistance and power estimation b) The use of Standard Series Data in Design 2. Determination of optimum RPM and propeller size (diameter) (general case) 3. Engine Selection 4. Prediction of performance in service 5. Determination of blade surface area (BAR) and cavitation control 2 hours th Example (Propeller Design - Stage 2+3, Cavitation Criteria) 2 hours th 8. Propeller Tests a) Open Water Tests b) Self Propulsion Tests c) Cavitation Tests d) Other Tests 2 hours th Example (Self propulsion tests) Lab visit 2 hours th

4 1. PROPULSION SYSTEMS Propulsion is the act or an instance of driving or pushing forward of a body, i.e. ship, by a propeller (in our case a screw propeller). a) History and Development of Screw Propeller Time period Inventor BC Archimedes invented his Archimedean Screw Pumps to irrigate the field of Syracuse in Sicily Leonardo da Vinci had sketches of screw principle to use as a helicopter rotor Toogood and Hayes of Britain claimed patent for using helical surfaces (Archimedean screws) as a propeller 1680 Hooke the English physicist suggested to use Archimedean screw for ship propulsion 1

5 1802/04 C. Steves the American used a kind of screw propeller similar to today s screws to propel a 7.5 m twin screw steamer R. Wilson the Scottish farmer successfully demonstrated the first principles 1836 P. Smith, the English farmer achieved the first practical application. He used single bladed screw of two turns made by wood J. Ericsson, the Swedish engineer developed fore runner of contrarotating propeller(i.e. two wheels of three helicoidal blades rotating in opposite direction) 1839 Smith equipped 237 ton of ship Archimedes with screw props having a great success and this led to Paddle propulsion systems to screw propulsion system Development of steam engines contributed to effective use of screw propellers 1845 Great Britain was the first screw propeller acrossed the Atlantic 1880 Thornycroft designed propellers similar today s propellers Basic shape of props remained unchanged s Fuel crisis and environmental effects (low noise and vibrations) had an impact on propeller shape and stern configurations as well as the developments of unconventional propellers 2

6 b) Modern Propulsion Systems i- Fixed pitch propellers (FPP) This kind of propellers has traditionally formed the basis of propeller production. They cover the majority proportion of propellers and design types and sizes, ranging from propellers for small powerboats to those for large tankers and bulk carriers. It is easy to manufacture. ii- Ducted propellers Ducted propellers consist of two components 1- An annular duct having an aerofoil cross section 2- A propeller inside the duct The presence of duct would reduce the pressure forces induced on the hull This kind of propellers sometimes is referred to as Kort nozzles by way of recognition of the Kort Propulsion Company s initial patents and long association with this type of propeller. Propeller efficiency is increased depending upon the propeller loading. A duct protects propeller against damage. 3

7 iii- Contra-rotating propellers This kind of propellers has two coaxial propellers sited one behind the other and rotating in opposite directions. They have the hydrodynamic advantage of recovering part of the slip stream rotational energy which would otherwise be lost to a conventional single screw system. This leads to an energy saving about 15% in power. Improved efficiency but higher drag and cost It is usually applied to small outboard units operating at around 1500 to 2000 RPM due to the mechanical problems associated with longer line shafting systems of larger vessels. iv- Overlapping propellers Two propellers are not mounted coaxially but are each located on separate shaft systems. The system has rarely been used in practice Although the propulsion efficiency of this system is higher than a single propeller, this system causes vibration and cavitation 4

8 v- Controllable pitch propellers (CPP) The choice of a CPP to a FPP is due to flexibility of its control rather than propulsion efficiency at service condition. CPP provides an extra degree of freedom in its ability to change blade pitch. It is especially used for ferries, tugs, trawlers, and fisheries due to better manoeuvrability than FPPs. Manufacturing cost is very high and it requires more maintenance and repairment. vi- Waterjet propulsion system This system has found considerable application on a wide variety of small high speed craft, although it is also used for larger ships. The operation principle of waterjet is that water is drawn through a ducting system by an internal pump adding energy and the water is expelled aft at high velocity. The unit s thrust is primarily generated as a result of momentum increase given in the water. The system is preferred to a conventional propeller. Because a conventional propeller experiences cavitation at very high speeds (45 knots), but in the waterjet unit the pump should not cavitate. It has a good manoeuvrability 5

9 vii- Cycloidal propellers The system is also called vertical axis propellers which comprise a set of vertically mounted vanes, six or eight in number, rotating on a disc mounted in a horizontal or near horizontal plane. The system has considerable advantages when manoeuvrability or station keeping is an important factor in the ship design. A separate rudder installation on the vessel is not required. The system provides with guards to help protect the propulsor from damage from external sources. Vertical axis propellers are fitted in tugs or other cases where low speed manoeuvrability is desired. 6

10 viii- Paddle wheels It is a predator of screw propulsion system. Used largely on lakes and river services or where limited draughts are encountered. ix- Superconducting electric propulsion The system provides ship propulsion without the aid of either propellers or paddles. The fundamental principal of electromagnetic propulsion is that of interaction between a fixed coil inside the ship and an electric current is passed through the sea water from electrodes in the bottom of the ship. A force is produced orthogonal to the magnetic field and to the current as a result of Fleming s right-hand rule. It provides noise and vibration free hydrodynamic propulsion so that it is found some applications in navy vessels. One of the major problems in this propulsion system is the difficulty to maintain superconducting coil zero resistance property, which is required, to be kept at the temperature of liquid helium (-268 C). 7

11 x- Azimuth podded propulsion system It provides propellers with high manoeuvrability, low fuel consumption, high efficiency, low noise and less cavitation Today, the major users of pod units have been cruise liner operators. The introduction of pod propulsion, which will allow the propulsion unit to be placed without considering any shaft arrangements or space for machinery will, of course, give the naval architect many new opportunities to design the 'ultimate hullform'. 8