The propeller on a vessel has a diameter of 19.9 feet and a pitch of 21.6 feet. What would be the apparent slip if the vessel cruised 395 miles in a 23 hour day (observed distance) at an average RPM of 78?
• Propeller pitch is the theoretical distance a propeller would move forward in one revolution with no slip • Apparent slip (%) compares theoretical distance from pitch and RPM to actual distance made good through the water • Be careful with units: miles vs feet, minutes vs hours, and nautical vs statute miles
• First, compute how many revolutions the propeller makes in 23 hours at 78 RPM, making sure your time unit matches RPM • Next, find the theoretical distance the ship should travel based on pitch × total revolutions (convert to miles) • Compare theoretical distance to the observed distance: is the vessel moving more or less than the theoretical distance, and how does that affect the sign (+/–) of apparent slip?
• Confirm you are using the correct slip formula: it should be a percentage based on (theoretical – actual) or (actual – theoretical) distance • Verify that you converted feet to miles correctly before comparing distances • Double-check whether the problem is using statute miles or nautical miles, and be consistent throughout your calculation
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