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?
• Propeller pitch as theoretical distance advanced per revolution (in feet) • Converting RPM, time, and pitch into a theoretical distance traveled • Formula for apparent slip %: difference between theoretical and observed distance, divided by theoretical distance
• First, find the theoretical distance the ship should travel in 23 hours with a 24.5 ft pitch at 78 RPM. How do you convert feet per revolution into miles per day? • Compare your theoretical distance with the observed distance of 458 miles. Is the vessel traveling more or less than the theoretical distance? • Once you know whether it’s more or less, what sign (positive or negative) should the slip have?
• Confirm your unit conversions: feet to miles and minutes/hours to revolutions • Check that you used pitch only (not diameter) to find theoretical distance • Verify that you computed slip % = (theoretical − observed) / theoretical × 100 and kept track of the sign.
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