The propeller on a vessel has a diameter of 24.0 feet and a pitch of 21.3 feet. What would be the slip if the vessel cruised 510 miles in a 24 hour day (observed distance) at an average RPM of 86?

Correct Answer: Option B — -17.5%

Propeller slip is calculated using the formula: Slip = [(Theoretical Distance - Observed Distance) / Theoretical Distance] * 100. Theoretical distance is RPM times 1440 minutes times pitch divided by 6080. This yields a negative slip of -17.5%, indicating strong following currents.