Propellers and propulsion

Transcription

1 Propellers and propulsion Kul Introduction of Marine Hydrodynamics Aalto University 25/10/2015 Introduction of Marine Hydrodynamics 1

2 Content of the course Resistance Propulsion Introduction, Momentum theory on propeller action Screw propeller Propeller-hull interaction Early design of a propeller Propeller main engine interaction Stopping, accelerating and backing properties Propeller cavitation Special types of propulsors Afterbody form of a ship Ship dynamics Introduction of Marine Hydrodynamics 2

3 Propeller: selection and early design Additional reading Matusiak J (2010) Laivan propulsio. M-176. Chapter 5 Matusiak J (2008) Short introduction to Ship Resistance and Propulsion. Section 5.8 Lewis E.V., editor (1988) Principles of Naval Architecture, Second revision. Volume II. SNAME. Available in Knovel. Introduction of Marine Hydrodynamics Aalto University 3

4 Propeller: selection and early design General Preliminary design Blade number Blade area ratio Twin screw: direction of the rotation Optimal shaft speed / Optimum diameter / Freedom to choose both diameter and shaft speed Bollard pull Applying systematic propeller series Introduction of Marine Hydrodynamics 4

5 Propeller: selection and early design General Preliminary design Blade number Blade area ratio Twin screw: direction of the rotation Optimal shaft speed / Optimum diameter / Freedom to choose both diameter and shaft speed Bollard pull Applying systematic propeller series Introduction of Marine Hydrodynamics 5

6 General Jurek p. 92 Introduction of Marine Hydrodynamics 6

7 General on the early design Selecting the main dimensions of the propeller What is known? Resistance of the ship R T Wake factor w Thrust deduction coefficient t What is used? Open water characteristics of the propeller series Given as polynomials K T =f 1 (P/D,A E /A 0,J,Z) and K Q =f 2 (P/D,A E /A 0,J,Z) Other geometrical parameters are fixed. After early design Ensure the compatibility of the propeller and the main engine in the off-design conditions where the ship will operate. Ballast condition, bollard pull, accelerating, stopping, backing Introduction of Marine Hydrodynamics 7

8 Propeller: selection and early design General Preliminary design Blade number Blade area ratio Twin screw: direction of the rotation Optimal shaft speed / Optimum diameter / Freedom to choose both diameter and shaft speeds Bollard pull Applying systematic propeller series Introduction of Marine Hydrodynamics 8

9 Number of blades First dimension to be determinated Typically 2-5 blades The optimum diameter decreases when the blade number increases. The blade frequency f b depends on the revolutions n and blade number Z. f b =nz Consider the possibility of resistance! Hull and propeller shaft Introduction of Marine Hydrodynamics 9

10 Blade area ratio Introduction of Marine Hydrodynamics 10

11 Introduction of Marine Hydrodynamics 11

12 Optimum shaft speed thrust known Known: ship velocity V, ship resistance R T, wake fraction w, thrust deduction coefficient t, stern shape Select the propeller diameter that fits the stern Tip clearance % of the propeller diameter Propeller disk above the base line Calculate Introduction of Marine Hydrodynamics 12

13 b Is plotted on top of an open water diagram Intersections of the polynomial and the torque curves: read Advance number J (a) Efficiency η (b) Thrust coefficient (c) Torque coefficient (d) Calculate c a Tabulate d Plot η and n as a function of P/D Introduction of Marine Hydrodynamics 13

14 Optimum shaft speed thrust known The maximum value of the efficiency η max corresponds the optimal revolutions n opt and the pitch ratio (P/D) opt. Introduction of Marine Hydrodynamics 14

15 Optimum shaft speed delivered power is known Known: delivered power P D, ship velocity V, wake fraction w, propeller diameter Aim: Find out revolutions n, pitch ratio P/D, thrust T Plot to open water diagram Introduction of Marine Hydrodynamics 15

16 Optimum shaft speed delivered power is known Calculate Plot Introduction of Marine Hydrodynamics 16

17 Optimum propeller diameter Fixed: revolutions n, delivered power P D, ship velocity V, wake fraction w Aim: Find out the optimum diameter of the propeller that renders the maximum efficiency Calculate Introduction of Marine Hydrodynamics 17

18 Optimum propeller diameter Plot to open water diagram Calculate for intersections Tabulate Plot η and D as a function of P/D Introduction of Marine Hydrodynamics 18

19 Optimum propeller diameter Efficiency curve is usually flat. Small deviation from D opt doesn t affect much The procedure gives a diameter that is too large. Single screw vessel: 5% Twin screw vessel: 2-3 % Reason: the flow is different in open water and behind the ship (recall h B ). Rule of thumb: P+D=const. Decreasing the diameter due to the available space at stern: % Introduction of Marine Hydrodynamics 19

20 Free choice of both revolutions n and diameter D Open water curves: Efficiency η increases with the pitch ratio η max when P/D = 2.2: very large propeller with low revolutions In practice: (P/D) max = When n and D are not fixed Free choice of P/D (e.g. P/D=1.3) Read from open water curves slightly leftwards from the maximum efficiency: Advance number J Torque coefficient K Q Calculate revolutions and diameter from Introduction of Marine Hydrodynamics 20

21 Bollard pull Trawlers, tugboat and ice-going ships The resistance is often much higher than in open water. Extreme case: bollard pull. The resistance is infinitely large and the ship speed is zero. Estimating bollard pull It is not possible to use K Q /J 5 because J = V A /nd = 0. Known: delivered power P D, revolutions n and thrust deduction coefficient t. Vary diameter D and calculate Interpolate to find out the P/D that corresponds K Q. For this P/D, read K T form open water curves. Calculate thrust. Calculate Pollard pull = T(1-t) Introduction of Marine Hydrodynamics 21

22 Propeller: selection and early design General Preliminary design Blade number Blade area ratio Twin screw: direction of the rotation Optimal shaft speed / Optimum diameter / Freedom to choose both diameter and shaft speed Bollard pull Applying systematic propeller series Introduction of Marine Hydrodynamics 22

23 Introduction of Marine Hydrodynamics 23

24 Introduction of Marine Hydrodynamics 24

25 Introduction of Marine Hydrodynamics 25

26 Summary What is relevant in your opinion? How do you select / do the early design of a propeller? Which parameters need to be selected? How to find out optimal / suitable values? How to ensure that there is enough space at the stern? Introduction of Marine Hydrodynamics 26

27 References Matusiak J (2010) Laivan kulkuvastus. M-289. Available in Mycourses Matusiak J (2013) Slides Propulsion ENG 2. Available in Mycourses Introduction of Marine Hydrodynamics 27