Resistance & Propulsion (1) MAR Propeller hull interaction

Transcription

1 Resistance & Propulsion (1) MAR 2010 Propeller hull interaction

2 Propeller hull interaction Propeller operating behind a hull will have different characteristics than the same design operating in open water, due in theory to: 1. Wake gain 2. Thrust deduction 3. Relative rotative efficiency

3 Wake gain Ship speed Mean flow velocity at the propeller plane

4 Thrust deduction V s Thrust force > Towing force V s

5 Relative rotative efficiency P D behind > (same diameter) P D open

6 Propeller hull interaction Interaction reflects on the propulsive efficiency P T P D P E = RV Towed resistance (R) η o = P T P D No interaction η D = η o η D = P E P D Generally η D > η o

7 Propeller hull interaction The 3 main propeller hull interaction effects MAY cause the overall efficiency of the propulsion system to be greater than the efficiency of the propeller.

8 Wake Gain Flow around a propeller is affected by the presence of a hull Potential and viscous nature of the boundary layer contribute to the development of the wake Average speed of the water through the propeller plane is usually different (less) than the hull speed There are 3 contributing factors

9 Wake Gain - Potential wake component Potential flow past a hull causes increased pressure around the stern as the streamlines close. Relative velocity of the flow past the hull is less than the hull speed Appears as a forward wake increasing the wake speed Model based on unbound assumption

10 Wake Gain - Velocity distribution AP FP

11 Wake Gain - Velocity distribution Pressure distribution Velocity distribution AP FP

12 Wake Gain - Frictional wake component Viscous flow causes retardation of the flow inside a ships boundary layer effect increases towards the stern causing a forward velocity component

13 Wake Gain - Velocity distribution Boundary layer Velocity Hull Viscous wake Potential wake

14 Wake Gain - Velocity distribution Mean speed through B.L. is less than the ship speed

15 Wake Gain - Velocity distribution Frictional wake 80 ~ 90% of total wake Single screws mainly operate in the viscous wake (frictional) the effect is important Twin screws operate outside of the viscous wake and the effect is therefore less important

16 Wake Gain - Wave making component Waves generated by the ship have orbital motion Wave crests have forward motion Wave troughs have aft motion

17 Wake Gain - Wave making component Wave component of the wake varies with speed Slow / Medium speed vessel = Crest High speed vessel = trough

18 Total Wake Total wake = Potential wake Viscous + wake + Wavemaking wake Hence Advance speed (Va) is less than the ship speed (V)

19 Total Wake Assuming T (thrust) = R (towed resistance) η o = P T P D = T V a P D η D = P E P D = RV P D V > Va, then RV > T Va η D > η o

20 Wake definition and wake fraction small part of the total wake

21 Wake Gain Text

22 Wake definition and wake fraction Wake in the propeller plane without the action of the propeller is known as the: NOMINAL WAKE metre/sec tunnel speed Axial Velocity (m/s) r 0.51r 0.68r 0.84r 0.92r Radial Position

23 Wake definition and wake fraction

24 Wake definition and wake fraction Wake in the propeller plane with the action of the propeller is known as the: EFFECTIVE WAKE This is difficult to measure

25 Wake definition and wake fraction Wake is defined as a fraction of ship speed or advance velocity at the propeller plane Froude wake fraction w = V V A V a V a = V 1 + w Taylor wake fraction w = V V A V V a = V (1 w)

26 Wake definition and wake fraction Wake fraction depends on length and fulness of the ship and increases with hull roughness A typical moderate speed cargo ship of Cb = 0.70 would expect w = 0.30

27 Thrust Deduction Propeller accelerates flow in front and behind of it resulting in: Increased rate of shear in boundary layer ( + Frictional resistance) Reduced pressure over the rear of the hull (+ Pressure resistance)

28 Thrust Deduction...If separation occurs in the afterbody when towed w/o the propeller, the action of the propeller will supress the separation and reduce the unfavourable pressure gradient...

29 Thrust Deduction The propeller therefore ALTERS the resistance of the hull by an amount proportional to the thrust. The thrust (T) must therefore EXCEED the towed resistance of the hull (R)

30 Thrust Deduction + P AP P Text FP + P 1 Thrust Augment of resistance P 1 R = ( P P 1 )ds R = T R

31 Thrust Deduction + P 1 P 1 Thrust By defining a as a Resistance augmentation factor a = R R = T R R T = R(1 + a) (1+a) is the Resistance augmentation factor

32 Thrust Deduction Augment of resistance terminology defines an increase in resistance. In practice this is viewed as a THRUST DEDUCTION t = T R T R = T (1 t)

33 Thrust Deduction Thrust deduction can be estimated using semi-empirical formulae. It is common to measure it in model scale using stock propellers (appropriate diameter and loading at the design speed). thrust deduction is a function of streamlining, propeller clearances and fullness

34 Thrust Deduction Typical values of t are: Single screw t = 0.6w twin screw t = w Modern single screw t = 0.3 Cb

35 Relative Rotative Efficiency Efficiency of a propeller behind a hull is not the same as a propeller working in open water Turbulence in the flow is low in open water, in the behind condition the flow is turbulent and unsteady In addition the flow at each radii is different to the open water case

36 Relative Rotative Efficiency High turbulence affect the lift and drag of each radial section. Modern propellers are Wake Adapted to take into account this variation in loading and maximise gains

37 Relative Rotative Efficiency Relative rotative efficiency is defined as the ratio of power delivered to a propeller in producing the same thrust in open water and behind conditions η R = P Dopen P D

38 Relative Rotative Efficiency η R = Efficiency behind hull Efficiency in open water = η B η o η R

39 Propulsive Efficiency and Propulsion factors The relationship between QPC can be refined as follows η D = P E P D η D = P E P T P T P Do P Do P D η D = RV P T η o η r

40 Propulsive Efficiency and Propulsion factors The relationship between QPC can be refined as follows η D = t(1 t)v T V (1 w) η o η r η D = (1 t) (1 w) η o η r

41 Propulsive Efficiency and Propulsion factors By denoting the hull efficiency as: (1 t) η h = (1 w) Single screw Twin screw η h η h

42 Propulsive Efficiency and Propulsion factors Thrust Power (P T ) Thrust " # $%&' " # Thrust Power (P T ) " # " $ BEHIND CONDITION Delivered Power (P D ) OPEN WATER CONDITION Delivered Power (P D open ) " #$%&' " # " $ "#$%&'($) *+%,-(-+%./%-0+) " # " $ " # " Forward Speed (V) Brake Power (P B ) Resistance (R) P e = RxV " # " 5)+#/26-7$800-9-$%9:.;5*4 " # " $ </ $%9: " # " $ =$>-%,?</ $%9: " # $%&' " " #$%&' 5)+#$22$)800-9-$%9: " # " $ A>'0(800-9-$%9:

43 End of Presentation