Question

A ship has a propeller of 5 m pitch rotating at 120 rpm. The ship travels at 8 m/s and the wake fraction is 0.25. The apparent slip ratio and real slip ratio are ________ respectively.

• A. 0.20 and 0.40
• B. 0.40 and 0.20
• C. 0.20 and 0.25
• D. 0.25 and 0.20

Hint:

Apparent slip is based on ship speed; real slip is based on speed of advance, which accounts for the wake fraction.

Answer 1

The Correct Option is A

Step 1: Calculate the speed of advance \( V_A \) \[ V_A = V \times (1 - w) = 8 \times (1 - 0.25) = 6\, {m/s} \] Step 2: Convert RPM to RPS \[ n = \frac{120}{60} = 2\, {rev/s} \] Step 3: Calculate theoretical speed \[ {Theoretical speed} = {Pitch} \times n = 5 \times 2 = 10\, {m/s} \] Step 4: Apparent slip ratio \[ {Apparent Slip} = \frac{10 - 8}{10} = 0.20 \] Step 5: Real slip ratio \[ {Real Slip} = \frac{10 - 6}{10} = 0.40 \] Conclusion: Apparent slip = 0.20, Real slip = 0.40