The propeller on a vessel has a diameter of 21.2 feet and a pitch of 20.0 feet. What would be the apparent slip if the vessel cruised 391 miles in a 24 hour day (observed distance) at an average RPM of 88?

Correct Answer: Option D — +6.2%

Engine distance is calculated as (RPM * 60 * 24 * Pitch) / 6076.1. Here, (88 * 1440 * 20) / 6076.1 = 417.2 miles. Slip is (Engine Distance - Observed Distance) / Engine Distance. (417.2 - 391) / 417.2 = +6.28%. Positive slip indicates the vessel is moving slower than the theoretical propeller advance.