Slide 18 of 24
Notes:
REFERENCES: CRENSHAW; MSI STUDENT GUIDE
THE RUDDER IS MOST EFFECTIVE WHEN PLACED DIRECTLY ASTERN OF THE PROPELLER IN THE SCREW CURRENT AND WHEN GOING AHEAD. WHEN GOING ASTERN, THE DISCHARGE CURRENT IS NOT DIRECTED AGAINST THE RUDDER AND EFFECTIVENESS IS DEPENDENT ENTIRELY ON THE SHIP’S STERNWARD VELOCITY TO CREATE FLOW ACROSS THE RUDDER. IN ADDITION WHEN ENGINES ARE INITIALLY BACKED, THE PIVOT POINT MOVES AFT, REDUCING THE LEVER ARM OF FORCES APPLIED TO THE STERN.
SCREW CURRENT CAN BE USED TO “PUMP TO THE RUDDER” TO SWING THE HEAD OVER. A “KICK” AHEAD ON THE ENGINE WITH THE RUDDER OVER WILL SWING THE HEAD IN THE DIRECTION OF THE RUDDER WITHOUT CREATING SIGNIFICANT AHEAD MOTION. THIS CAN BE USED DURING “BACK AND FILL” MANEUVER.
ONCE STERNWAY IS ACHIEVED, THE LOW PRESSURE SIDE OF THE RUDDER IS REVERSED. LEFT RUDDER WILL FORCE THE STERN TO PORT, RIGHT RUDDER WILL TRY TO TURN THE STERN TO STBD. FOR A SINGLE SCREW SHIP, FLOW ACROSS THE RUDDER HAS TO BE SUFFICIENT TO OVERCOME THE SIDE FORCE WALKING THE STERN TO PORT. TO BACK A SHIP TO STARBOARD REQUIRES STERNWAY AND LARGE AMOUNT OF RIGHT RUDDER, AND WILL STILL NOT BE VERY EFFECTIVE.
AT ALL STOP THE ONLY FLOW ACROSS THE RUDDER IS CREATED BY THE SHIP’S MOTION THROUGH THE WATER. THERE IS NO SIDE FORCE CREATED BY THE WALK OF THE PROPELLER. (EXCEPT CPP).
