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Measurements of Rudder and Wheel Torque at Speed 

by , s/v Beatrix (KP44 #276, 1980)


Although it is possible to calculate angular rudder torque from the square feet of the rudder, I felt an empirical method was more accurate.  This data was gathered aboard Beatrix by putting an automotive torque wrench on the steering wheel nut. "Spokes" were translated to degrees of wheel turn and rudder angles. This data was used in evaluating the stroke (or "throw length") and power required when using a linear actuator for a below-decks steering system. The experiment was done at 6 knots.  The hull speed data is an extrapolation.

 

Most actuators have a stroke of about 12".  The Octopus unit used aboard Beatrix has a stroke of 300mm (or 11.8" ) and 400kg (or 882 pounds) of thrust.  Once the data below was gathered I could determine what rudder angle and tiller length was best for this unit. A longer tiller arm gives more leverage, but restricts the allowable rudder angle.  Refer to the table below.  Using a 7" tiller will allow full rudder travel, but raises the thrust (and power) required for turns at speed.  The intersection of the 10" Tiller column with the 37.5 degree rudder angle row in the spreadsheet shows the required thrust to be 660 pounds, about 75% of the Octopus' rated thrust.  The 10" tiller is more appropriate for the KP44, but requires reducing the factory rudder limit from 45 to 37.5 on either side of center.  Reducing the rudder angle also improves safety in following seas or when backing down hard.  I found no noticeable handling differences docking the boat with the lower rudder angle.  Also keep in mind that an electric autopilot should not normally be required to "hard-over" the helm at hull speed.  

To see how the rudder angle restriction was done on Beatrix click here.  

For more information on the Octopus and the installation aboard Beatrix, click here.

Wheel to Rudder Turn Ratio 11
Rudder Quadrant Degrees 90
Steering Wheel Diameter 30
Speed during test (knots) 6
Hull Speed Correction 1.927469
AT 6 KNOTS AT HULL SPEED
6-Spoke Wheel Turns Wheel Degrees Rudder Angle in Degrees Measured Wheel Torque in Ft-Lb Rudder Post Torque Ft-Lb 10" Tiller (pounds) 7" Tiller (pounds) Rudder Post Torque Ft-Lb 10" Tiller (pounds) 7" Tiller (pounds)
1 turn 2.25 spokes 495 45.0 78 858 1030 1471 1654 1985 2835   Hard Over Limit
1 turn 1.79 spokes 468 42.5              
1 turn 1.33 spokes 440 40.0              
1 turn 0.88 spokes 413 37.5 50 550 660 943 1060 1272 1817   New Hard Over Limit
1 turn 0.42 spokes 385 35.0              
5.96 spokes 358 32.5              
5.5 spokes 330 30.0 40 440 528 754 848 1018 1454
5.04 spokes 303 27.5              
4.58 spokes 275 25.0              
4.13 spokes 248 22.5              
3.67 spokes 220 20.0 30 330 396 566 636 763 1090
3.21 spokes 193 17.5              
2.75 spokes 165 15.0              
2.29 spokes 138 12.5              
1.83 spokes 110 10.0 10 110 132 189 212 254 363
1.38 spokes 83 7.5              
0.92 spokes 55 5.0              
0.46 spokes 28 2.5              
0 spokes 0 0.0 0 0 0 0 0 0 0


 
Last modified: January 01 2016 05:12