The propeller on a vessel has a diameter of 20.2 feet and a pitch of 19.0 feet. What would be the apparent slip if the vessel cruised 367 miles in a 24 hour day (observed distance) at an average RPM of 84?
• Propeller pitch as theoretical distance advanced per revolution • How to calculate theoretical distance in 24 hours from pitch and RPM • Formula for percent slip and how to interpret a negative sign
• First, convert all distances into the same unit (feet, miles, or nautical miles) and be consistent. What is the observed distance in feet? • How far would the vessel advance in 24 hours with zero slip, using pitch and RPM? Compare this to the observed distance. • When the observed distance is greater than the theoretical pitch distance, what sign (positive/negative) should the slip have?
• Be sure the time (24 hours), RPM, and pitch per revolution are all applied together to get total theoretical distance. • Double‑check whether the problem wants percent slip based on theoretical minus observed, divided by theoretical, or the reverse. • Carefully consider whether the direction of rotation / current / tide could make the slip appear negative, and how that is reflected in the sign of the percentage.
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