If the speed necessary for reaching port at a designated time is 17.8 knots and the pitch of the propeller is 24.7 feet, how many revolutions per minute will the shaft have to turn, assuming a 7% positive slip?

Correct Answer: Option A — 79 RPM

Utilizing the propeller slip formula: RPM = (Speed * 6080) / (Pitch * 60 * (1 - Slip)). Substituting 17.8 knots, 24.7 feet pitch, and 7% slip (0.93), the equation becomes (17.8 * 6080) / (24.7 * 60 * 0.93). The result is approximately 79 RPM, the necessary shaft speed to maintain the required vessel velocity.