The propeller on a vessel has a diameter of 21.5 feet and a pitch of 24.5 feet. What would be the apparent slip if the vessel cruised 458 miles in a 23 hour day (observed distance) at an average RPM of 78?

Correct Answer: Option D — -5.6%

Calculate engine speed: (RPM × 60 × Pitch) / 6076.1. Here, (78 × 60 × 24.5) / 6076.1 = 18.86 knots. Observed speed is 458 / 23 = 19.91 knots. Apparent slip = (Engine Speed - Observed Speed) / Engine Speed. (18.86 - 19.91) / 18.86 = -0.0556, or -5.6%.