The speed necessary to reach port at a designated time is 18.7 knots. The propeller pitch is 24'03", and you estimate 3% positive slip. How many revolutions per minute will the shaft have to turn?

Correct Answer: Option C — 81 RPM

To find RPM, rearrange the slip formula: RPM = (Speed × 6076.1) / (Pitch × 60 × (1 - Slip)). Using 18.7 knots, 24.25-foot pitch, and 3% positive slip (0.97), the required engine speed is 81 RPM. This ensures the vessel maintains the necessary speed of advance to meet the schedule.