Here are some definitions -
(1)Pitch is the distance that the vessel will move with one full rotation of the propeller.
Usually measured in feet.
(2)Slip is the percentage (%) of loss or gain of the actual movement of the vessel compared
to the theoretical movement. It is a measure of efficiency.
Positive Slip - is the actual loss when comparing actual movement to the theoretical movement. It is the opposite of what most people would think, it is less than 100%.
It would be like your RPM of 10 knots, in an hour you should go 10 miles. Then you find out that you actually cover 9 miles, then your speed made good would be 9 knots. So you would have a 100% - 90% = 10% positive slip.
Negative Slip - is where the actual gain when comparing actual movement to the theoretical movement. It is greater than 100%. Using same idea, lets say your RPM speed is 10 knots you should go 10 miles in an hour, but you find that you have covered 11 miles. So, you would have a 110% - 100% = 10% negative slip.
(3)RPM means revolutions per minute.
(4)Nautical mile is 6080 feet.
If the speed necessary for reaching port at a designated time is 18.5 knots and the pitch of the propeller is 21.7 feet, how many revolutions per minute will the shaft have to turn, assuming a 4% negative slip?
A. 83
B. 90
C. 97
D. 114
ANSWER ''A''
RPM = Speed x 6080 divided by Pitch x Slip x 60 =
18.5 kts x 6080 divided by 21.7 ft x 1.04 x 60 = 83 rpms
