The propeller on a vessel has a diameter of 24.6 feet and a pitch of 26.1 feet. What would be the apparent slip if the vessel cruised 462 miles in a 24 hour day (observed distance) at an average RPM of 72?
• Propeller pitch as theoretical distance advanced per revolution • Relationship between RPM, pitch, and theoretical distance traveled in a given time • Formula for propeller slip percentage using observed and theoretical distance
• First, if the propeller makes 72 revolutions per minute for 24 hours, how many total revolutions does it make? • Given the pitch in feet and the number of revolutions, what is the theoretical distance the vessel would travel in nautical miles with zero slip? • How do you compare the observed distance to the theoretical distance to determine if the slip is positive or negative, and then convert that to a percentage?
• Be sure you convert feet to nautical miles correctly (6080 ft per nautical mile is commonly used on license exams—verify what your exam uses). • Check whether apparent slip is based on (theoretical − observed) or (observed − theoretical); the sign will determine whether your answer is positive or negative. • After computing the slip percentage, compare the magnitude and sign to the answer choices to rule out clearly incorrect options.
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