If the speed necessary for reaching port at a designated time is 16.4 knots and the pitch of the propeller is 23.8 feet, how many revolutions per minute will the shaft have to turn, assuming a 6% positive slip?
• Relationship between speed, pitch, and RPM** for a propeller** • Definition of positive slip and how it affects effective advance per revolution • Unit conversions between knots, feet per minute, and revolutions per minute (RPM)
• First convert the ships speed in knots to feet per minute so it matches the pitch units in feet • Think about how far ahead the ship actually moves for each revolution when there is positive slip is it more or less than the pitch? • Once you know the effective advance per revolution in feet, ask: how many of those advances are needed each minute to equal the ships speed in feet per minute?
• Be sure you apply the 6% positive slip in the correct direction (does effective advance increase or decrease?) • Confirm all units match: speed in ft/min and pitch in ft/rev before solving for RPM • After calculating RPM, compare it to each choice and see which is closest to your computed value
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