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Lect 9 ship propellers (new)
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  1. 1.  www.tanhatarin109.javanblog.com
  2. 2. www.tanhatarin109.javanblog.com
  3. 3. Frames of Reference  10th International Towing Tank Committee (ITTC) initiated the preparation of a dictionary and nomenclature of ship hydrodynamic terms and this work was completed in 1975.  The global reference frame proposed by the ITTC is a right-handed rectangular Cartesian system. www.tanhatarin109.javanblog.com
  4. 4.  For propeller geometry it is convenient to define a local reference frame having a common axis such that OX and Ox are coincident but Oy and Oz rotate relative to the OY and OZ fixed global frame. www.tanhatarin109.javanblog.com
  5. 5.  The line normal to the shaft axis is called either propeller reference line or directrix. www.tanhatarin109.javanblog.com
  6. 6.  Generator line: The line formed by intersection of the pitch helices and the plane containing the shaft axis and propeller reference line. www.tanhatarin109.javanblog.com
  7. 7.  The aerofoil sections which together comprise the blade of a propeller are defined on the surfaces of cylinders whose axes are concentric with the shaft axis. www.tanhatarin109.javanblog.com
  8. 8.  Face: The side of a propeller blade which faces downstream during ahead motion is called face or pressure side (when viewed from aft of a ship to the bow the seen side of a propeller blade is called face or pressure side). www.tanhatarin109.javanblog.com
  9. 9.  Back: The side of a propeller blade which faces generally direction of ahead motion is called back or suction side (when viewed from aft of a ship to the bow the unseen side of a propeller blade is called back or suction side). www.tanhatarin109.javanblog.com
  10. 10.  Leading Edge: When the propeller rotating the edge piercing water is called leading edge. www.tanhatarin109.javanblog.com
  11. 11.  Trailing Edge: When the propeller rotating the edge trailing the leading edge is called trailing edge. www.tanhatarin109.javanblog.com
  12. 12. www.tanhatarin109.javanblog.com
  13. 13.  Consider a point P lying on the surface of a cylinder of radius r which is at some initial point P0 and moves as to from a helix over the surface of a cylinder.  The propeller moves forward as to rotate and this movement creates a helix. www.tanhatarin109.javanblog.com
  14. 14.  When the point P has completed one revolution of helix that means the angle of rotation 360 deg. www.tanhatarin109.javanblog.com
  15. 15.  In the projection one revolution of the helix around the cylinder measured normal to the OX axis is equal to 2πr.  The distance moved forward by the helical line during this revolution is p and the helix angle is given by: www.tanhatarin109.javanblog.com
  16. 16.  There are several pitch definitions: www.tanhatarin109.javanblog.com
  17. 17.  Nose-tail pitch: The straight line connecting the extremities of the mean line or nose and tail of a propeller blade is called nose- tail pitch line The section angles of attack are defined to the nose-tail line. www.tanhatarin109.javanblog.com
  18. 18.  Face pitch: The face pitch line is basically a tangent to section’s pressure side surface and you can draw so many lines to the pressure side. Therefore its definition is not clear. It is rarely used but it can be seen in older drawings like Wageningen B series. www.tanhatarin109.javanblog.com
  19. 19.  Effective or no-lift pitch: It is the pitch line of the section corresponding to aerodynamic no-lift line which results zero lift. www.tanhatarin109.javanblog.com
  20. 20.  Hydrodynamic pitch: The hydrodynamic pitch angle (βi) is the pitch angle at which the incident flow encounters the blade section.  Pitch values at different radii are called radial pitch distribution. www.tanhatarin109.javanblog.com
  21. 21.  Slip & Slip Ratio www.tanhatarin109.javanblog.com
  22. 22.  Skew  It is the angle between the mid-chord position of a section and the directrix (θs).  The propeller skew angle (θsp) is defined as the greatest angle measured at the shaft centre line which can be drawn between lines passing from the shaft centreline through the mid chord position of any two sections. www.tanhatarin109.javanblog.com
  23. 23.  Skew www.tanhatarin109.javanblog.com
  24. 24.  The skew can be classified into two types:  i- Balanced skew: Directrix intersects with the mid-chord line at least twice.  ii- Biased skew: Mid-chord locus crosses the directrix not more than once normally in the inner sections. www.tanhatarin109.javanblog.com
  25. 25.  The displacement from the propeller plane to the generator line in the direction of the shaft axis is called rake. The propeller rake is divided into two components: generator line rake and skew induced rake. www.tanhatarin109.javanblog.com
  26. 26.  Propeller Outlines and Areas www.tanhatarin109.javanblog.com
  27. 27.  There are five different outlines and associated areas of propeller in use. These are:  1. Disc outline (area) (A0)  2. Projected outline (Ap)  3. Developed outline (AD)  4. Expanded outline (AE)  5. Swept outline (AS) www.tanhatarin109.javanblog.com
  28. 28.  Swept Outline: This outline is swept by the leading and trailing edges when the propeller is rotating.  In general, the developed area is greater than the projected area and slightly less than the expanded area. www.tanhatarin109.javanblog.com
  29. 29.  Calculation of AE www.tanhatarin109.javanblog.com
  30. 30.  Propeller drawing methods www.tanhatarin109.javanblog.com
  31. 31.  Propeller drawing methods www.tanhatarin109.javanblog.com
  32. 32.  Propeller section definition www.tanhatarin109.javanblog.com
  33. 33.  Propeller series www.tanhatarin109.javanblog.com
  34. 34.  Model ship & propeller tests  1- Resistance Test  2. Open water tests  3. Self propulsion tests  4. Cavitation tests  5. Others (wake surveys, hull pressure tests, noise measurements, etc.) www.tanhatarin109.javanblog.com
  35. 35.  1- Resistance Test  In the resistance test the ship model is towed by the carriage and the total longitudinal force acting on the model is measured for various speeds. The breadth and depth of the towing tank essentially governs the size of the model that can be used. Todd’s original criterion that the immersed cross-section of the vessel should not exceed 1 per cent of the tank’s cross sectional area was placed in doubt after the famous Lucy Ashton experiment. This showed that to avoid boundary interference from the tank walls and bottom this proportion should be reduced to the order of 0.4 per cent. www.tanhatarin109.javanblog.com
  36. 36.  The model, constructed from paraffin wax, wood or glass- reinforced plastic, requires to be manufactured to a high degree of finish and turbulence simulators placed at the bow of the model in order to stimulate the transition from a laminar into a turbulent boundary layer over the hull. The model is positioned under the carriage and towed in such a way that it is free to heave and pitch, and ballasted to the required draught and trim. www.tanhatarin109.javanblog.com
  37. 37.  In general there are two kinds of resistance tests:  the naked hull and the appended resistance test. If appendages are present local turbulence tripping is applied in order to prevent the occurrence of uncontrolled laminar flow over the appendages. Also the propeller should be replaced by a streamlined cone to prevent flow separation in this area. www.tanhatarin109.javanblog.com
  38. 38. www.tanhatarin109.javanblog.com
  39. 39. www.tanhatarin109.javanblog.com
  40. 40. www.tanhatarin109.javanblog.com
  41. 41.  2. Open water tests  Open water tests of propellers can be performed either in a cavitation tunnel or in a towing tank. Although the test procedure applied to obtain open water characteristics of a propeller in a cavitation tunnel is different from those in a towing tank, these characteristics are the same used in the analysis of the Propulsion Tests and the estimation of the required power. www.tanhatarin109.javanblog.com
  42. 42. www.tanhatarin109.javanblog.com
  43. 43. www.tanhatarin109.javanblog.com
  44. 44. www.tanhatarin109.javanblog.com
  45. 45.  2.1 Open water test in Towing tank  In a towing the model propeller is run without any hull ahead of it, as shown in the figure. www.tanhatarin109.javanblog.com
  46. 46. www.tanhatarin109.javanblog.com
  47. 47.  The drive shaft housing should not be too close to the model propeller blades. A distance of not less than 1.5 D to 2.0 D is recommended, where D is the propeller diameter. The drive shaft should be arranged parallel to the calm water surface and the carriage rails. A typical set up is shown in the above figure.  The propeller immersion has to be selected such that air drawing from the water surface is avoided at any test condition. As a guideline, an immersion of the propeller shaft of at least 1.5 D is recommended. www.tanhatarin109.javanblog.com
  48. 48.  The test procedure is as follows:  1. The propeller advances through undisturbed water with a known forward speed, V, which is the speed of the towing carriage. Values of thrust, T and torque, Q are taken from the dynamometer, and rate of rotation, n is recorded using a tachometer.  2. Usually measurements are taken during series of runs for T & Q at varying J numbers so that n is kept constant and V is varied from zero speed (i.e. J=0) to a high value ( ≈ J=1).  3. The results are analised and coefficients are derived as similar to those in cavitation tunnel. www.tanhatarin109.javanblog.com
  49. 49.  3. Self propulsion tests  Self propulsion tests are carried out to estimate ship power at various speeds and to derive propulsion factors, w, t, ηR.  The hull model is equipped with an electric motor mounted inside the hull and respective devices (dynamometer) fitted inboard to enable the measurement of thrust, torque and rate of revolutions of the model propeller(s). Appendages, such as rudders, single brackets or A-brackets, propeller shafts, shaft protection tubes, short bossings or bossings, extracted stabilisers and openings in the hull such as for bow thrusters, should be in the same condition as for the resistance experiment. www.tanhatarin109.javanblog.com
  50. 50.  3. Self propulsion tests www.tanhatarin109.javanblog.com
  51. 51.  The size of the propeller/propulsion unit model for propulsion tests is determined automatically by the size of the ship model and its scale ratio; this in turn means that the size of the model propeller, or say a stock propeller, is also to be taken into consideration when the scale for a ship model is selected.  During a self propulsion test, an external tow force in propulsion experiments should be applied along the same line of action as the tow force in the resistance experiment.  This external force comes from the skin friction correction such that the skin friction coefficient of the model CFm is greater than skin friction coefficient of the ship, CFs. www.tanhatarin109.javanblog.com
  52. 52. www.tanhatarin109.javanblog.com
  53. 53. www.tanhatarin109.javanblog.com
  54. 54. www.tanhatarin109.javanblog.com
  55. 55. www.tanhatarin109.javanblog.com
  56. 56. www.tanhatarin109.javanblog.com