The propeller on a vessel has a diameter of 20.6 feet and a pitch of 23.4 feet. What would be the apparent slip if the vessel cruised 538 miles in a 24 hour day (observed distance) at an average RPM of 87?
• Propeller pitch as theoretical distance advanced per revolution • Difference between theoretical distance (from pitch and RPM) and observed distance run • How slip percentage is computed and what a negative slip sign means
• First, find how many revolutions the propeller makes in 24 hours, then use the pitch to get the theoretical distance the vessel should travel with zero slip. How do you convert that distance into nautical miles? • Compare the theoretical distance to the observed distance of 538 miles. Is the vessel moving more or less than the theoretical no‑slip distance? What does that say about the sign of the slip? • Once you have the difference between theoretical and observed distance, how do you express that difference as a percentage of the theoretical distance?
• Be sure you convert feet to nautical miles correctly: 1 nautical mile = 6076 feet (unless your test materials specify a slightly different value—use theirs). • Confirm that you use the correct slip formula: % slip = (theoretical − observed) / theoretical × 100, with careful attention to the sign. • Double‑check the total revolutions: RPM × 60 minutes × 24 hours. Make sure no step is missed or rounded too early.
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