91.NA February 2020 Q.10(b)

The following information relates to a model propeller of 400 mm pitch.

Rev/min                           400                  450                500                     550                   600
Thrust N                           175                 260                 365                     480                   610
Torque N-m                      16.8                22.4               28.2                    34.3                  40.5

a) Plot curves of thrust and torque against rev / min
b) When the speed of advance of model is 150 m/min and slip 0.20, Calculate the efficiency.

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GIVEN

Va = 150 m/min
P = 0.4 m
s = 0.20

TO FIND

a) curves of thrust and torque against rev / min
b) Efficiency

SOLUTION

a)

b) We know that Va = 0.8 x Vt (As slip is 20%)
                          150 = 0.8 x Vt
                            Vt = 187.5 m/min

       Vt = P x N
  187.5 = 0.4 x N
        N = 469 rpm

  From the graph for N = 469 rpm corresponding
  Thrust = 298 N ,  Torque = 24.6 N-m

  Thrust power tp = Thrust x Speed of advance
                             = 298 x (150/60)
                             = 745 W

  Delivered power dp = 2 x pi x N x Torque
                                   = 2 x 3.14 x (469/60) x 24.6
                                   = 1208 W

  Propeller efficiency = tp / dp
                                   = 745 / 1208
                                   = 61.67 %