You must average 16.25 knots to reach port at a designated time. Your propeller has a pitch of 21'08", and you estimate 4% negative slip. How many revolutions per minute must you average to arrive on time?
ā¢ Speedādistanceātime relationship for propeller pitch problems ā¢ Converting propeller pitch in feet and inches to feet per revolution ā¢ Effect of negative slip (%) on effective advance per revolution
ā¢ First, find how many feet your vessel should advance for each revolution with zero slip. How do you convert 21 feet 8 inches into feet? ā¢ Next, apply 4% negative slip. Does the vessel advance more or less than the theoretical pitch per revolution? How do you express that as a percentage multiplier? ā¢ Once you know how many feet you advance per revolution, how do you turn the required speed in knots into feet per minute, then divide to find RPM?
ā¢ Be sure 1 knot = 6076 feet per hour (or 6080 depending on your exam convention; use the exam standard youāve been taught). Convert it correctly to feet per minute. ā¢ Convert inches to feet correctly: 8 inches = 8/12 feet. ā¢ Apply negative slip in the correct direction: with 4% negative slip, is the actual advance greater or less than the geometric pitch?
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