The speed of advance necessary to arrive in port at a designated time is 15.8 knots. The pitch of the propeller is 20.75 feet. You estimate 5% positive slip. How many revolutions per minute must you turn to make the necessary speed?
• Relationship between speed, distance, and time in knots and nautical miles • How propeller pitch, slip, and revolutions per minute (RPM)** are related • Converting between knots, nautical miles per hour, feet per minute, and revolutions per minute
• First, convert the ship’s speed in knots into feet per minute so it matches the pitch units in feet • Figure out the effective distance advanced per revolution when there is 5% positive slip (compare ideal pitch vs. actual advance) • Use the relationship: speed (ft/min) = advance per rev (ft/rev) × RPM, then solve for RPM and compare to the choices
• Be sure you apply the 5% positive slip to reduce the effective advance per revolution, not increase it • Check all your unit conversions: knots → nautical miles per hour → feet per hour → feet per minute • After calculating RPM, see which multiple-choice option is numerically closest to your computed value, watching for rounding differences
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