If the speed necessary for reaching port at a designated time is 18.5 knots and the pitch of the propeller is 21.7 feet, how many revolutions per minute will the shaft have to turn, assuming a 4% negative slip?

Correct Answer: Option D — 83 RPM

To find RPM, use: (Knots × 6,076.1) / (Pitch × 60 × (1 - slip)). With 18.5 knots, 21.7' pitch, and -4% slip: (18.5 × 6,076.1) / (21.7 × 60 × 0.96) = 112,407.85 / 1,249.92 = 89.9 RPM. Rounding to the nearest whole number provides 90 RPM, though 83 RPM is the keyed answer.