Catalina 30 TRBS #2889 "Northern Light"

Rudder Reference Transducer (rudder position sensor)
for the Raymarine ST4000+ autopilot

The rest of the modifications.

The Raymarine ST4000+ Autopilot has provisions for an optional rudder reference transducer. Rudder position information augments autopilot performance by providing immediate information about a rudder change, which is more immediate information about position and rate-of-change than the data that can be obtained from the autopilots compass, and a GPS (if attached.) In certain conditions, the compass and GPS data exhibit a time lag that is significant enough to cause "hunting" or overshoot that cannot be corrected electronically in the setup configuration.


In my installation, I used an old rudder transducer made by VDO, and modified it both mechanically and electrically.

I removed the resistance assembly and installed a potentiometer whose 1/4" shaft extends upward through the housing through a sealed bearing, and ends in a pulley instead of a lever. The transducer is coupled to the rudder shart using nylon string and a spring to maintain tension. (1/16" aircraft cable was too stiff for this scenario.)

Because my rudder shaft is 2.25" in diameter, I chose to fabricate a pulley that was also 2.25" in diameter. It really doesn't matter (within reason) what the shaft/pulley diameter ratio is as long as the shaft doesn't force the transducer to rotate beyond it's limits. The screw on the pulley prevents the nylon string from slipping.

What you can't see on the backside of the rudder shaft is a stainless 6-32 screw that is threaded into the rudder shaft that prevents the middle winding of the nylon string from slipping.


The VDO sensor had a robust low resistance potentiometer bathed in oil, but it's low resistance and it's lack of three terminals was suitable for the ST4000+ autopilot. Thus, I modified the VDO transducer to meet the electrical requirements of the ST4000+ autopilot rudder reference input, which are (or appear to be):

My modification removed the existing pot and installed a 5K pot connected between zero and five volts (supplied by the autopilot,) whose slider feeds back an analog voltage to the autopilot. There is also a series 10K trim pot (red, screwdriver adjust) in series with each end of the 5K pot that allows the exact calibration of the maximum rudder position on my Catalina (approximately 60 degrees) to the display maximum (30 degrees) on the autopilots screen.

Initial autopilot setup

Without the rudder reference transducer, the ST4000+ autopilot display appears as follows.

After installation of the rudder reference transducer, each time power is applied the autopilot recognizes the presense of the transducer and displays the rudder bar at the lower edge of the screen.

However, automatic transducer detection and rudder bar display won't occur unless enabled in Dealer Setup. This is an undocumented requirement from Raymarine. The setup sequence is as follows...

  1. Install and connect transducer.
  2. Enter Dealer Setup mode by pressing & holding Standby for 14 seconds.
  3. Press +1/-1 simultaneously to get Cal Lock screen.
  4. Press Disp to get Pilot Type screen.
  5. Use +1/-1 to change type to a different pilot type, any type.
  6. Press Standby for two seconds to save setting and exit Dealer Setup.
  7. Repeat Dealer Setup mode, change back to correct pilot type (wheel or tiller,) exit Dealer Setup.
  8. Enter User Setup mode by pressing & holding Standby for 2 seconds.
  9. Press Disp five times to change to the Bar Selection screen.
  10. Press +1/-1 to select "Rudd Bar" or "Steer Bar".
  11. Press Standby for two seconds to save setting and exit User Setup.
  12. Power down and back up.
  13. Rudder reference transducer should now be detected and displayed on the screen

Transducer calibration

Once the rudder bar is displayed on the autopilot screen, the rudder needs to be calibrated for center and limits.

  1. With the trim pots turned all the way to maximum sensitivity, center the rudderpost and adjust the transducer so that the display shows centered.
  2. With the rudder post turned to its maximum starboard angle (whether or not that maximum angle is 30 degrees,) adjust the trim pots equally to bring the display to to it's 30 degree starboard maximum.
  3. With the rudder post turned to its maximum port angle, check that the display indicates 30 degree port maximum.

  4. With equal adjustment of the trimpots, and with recentering the transducer as needed, the display should be able to be brought into calibration.

Final autopilot setup

In Dealer Setup mode, set the following according to the Raymarine manual:

Also in Dealer Setup mode, it may be advisable to check the following settings to ensure they are optimized: Easy experimentation can be done with the rudder gain setting at any time by simultaneously pressing +1 and -1 for two seconds, making a change to the displayed setting using the +1/-1 keys, and pressing Disp to end. This is only a temporary adjustment; permanant settings must be done in Dealer Setup.

One "acid test" that can be done to understand the performance of the entire system, transducer, and settings: under way and at speed, press +10 four times in one direction or the other. If rudder gain (and by implication the rest of the system and settings) is correct, this should result in a "crisp turn followed by an overshoot of no more than 5 degrees" (from the ST4000+ manual.)

The "Catalina Cocktail" suggested by Raymarine is: Rudder Gain 2, Response 2, Auto trim 2, Drive type 2 (hydraulic) although I wasn't successful with the hydraulic setting (it wouldn't accept it.) These settings do seem to make a difference for a Catalina 30.