Replacing the Rudder Reference Unit (RRU)

For years, my B&G ACP2 autopilot had a difficult time holding a straight heading. The autopilot "brain" has a couple of parameters you can "tune" to get the autopilot to change its course smoothly and without overshooting. Over the course of countless hours playing with it, I explored the entire 2-D parameter space, yet the best I could get is a course that weaved through 6-8 degrees, over 10-12 seconds.

I was finally able to track the problem to a faulty linear Rudder Reference Unit (RRU). This is a long tube that is mounted above the T2 autopilot ram, and is used to indicate the rudder position. Mine had a big flat spot. The rudder could change a couple of degrees, yet the RRU would indicate ... nothing. No wonder it weaved!

I bought a new RRU from Wheelhouse Marketing (954-229-2460), a B&G representative located in Florida, and got it a few days later.

Unfortunately, it was considerably different from my old unit: more slender, and much longer.

I machined up a new mount for the thing. Here's the drawing I used for the HDPE holder, and for the stainless slider holder.

Here's what it looked like when I was done.

Overview of the new RRU

The T-2 ram with the new linear Rudder Reference Unit on top. A piece of HDPE holds the body of the RRU, and a small piece of stainless holds the slider.

Detail of the HDPE that holds the body of the RRU

Detail showing the HDPE that holds the body of the RRU. There's a slight gap (barely visible in the picture) that closes up when you tighten the bolts, firmly holding the RRU.

Detail of the stainless that holds the slider

Detail of the piece of stainless that holds the slider.

Unfortunately, when I put everything back in the boat, I kept getting a "Fault 111." According to the manual, this error means

The current limit circuit for the drive motor (25 amps) or the clutch (2 amps) has tripped.

I measured the current for each circuit. The current flowing through the "clutch" (it's actually a solenoid that bypasses the hydraulic circuit) looked normal (about an amp), but the motor showed spikes well over 20+ amps. My theory was that taking out the ram shook some carbon lose from the brushes, causing it to short out the motor's commutator.

Taking one of these things apart turns out to be surprisingly easy.

Unscrewing 8mm bolts

To get at the electric motor inside the ram, undo the two bolts on the end cap.

Tap out the end cap

Slide the motor casing off the ram, then tap off the end cap using a drift punch.

The casing

The motor casing with the end cap removed.  

The motor inside

Taking off the motor casing reveals the electric motor inside. It's in surprisingly good shape, with very little evidence of corrosion.

Once apart, there was a lot of dust inside the motor, so it's not surprising there could have been a short. Using a utility knife I cleaned the gaps between the segments of the commutator. I also cleaned the windings and inside of the case using compressed air and rags.

Putting the compression spring in the end cap

Don't forget to put the compression spring in before putting the end cap back on!

I put everything back together, then reinstalled the ram. To my amazement, it worked. It's rare for a boat project to go so well!

B&G T2 ram back in its natural habitat

The autopilot ram, back in its natural habitat.

Postscript: April 2019

After four years, the linear reference unit started to fail again. I think the problem is that because it is held rigidly at each end, it has no "degrees of freedom" — the slightest movement puts strain on the internal resistance element. So, it becomes erratic.

I decided to switch to the more conventional "rotary feedback unit," in the hopes that its additional linkage and ball joints would be more forgiving of small misalignment errors. The B&G ACP 2 requires a unit that changes resistance with changing angles, but, unfortunately, these days B&G only offers units with NMEA2000 or SimNet interfaces. Fortunately, the Raymarine M81105 does have a simple resistance interface.

At first, I thought I'd try a 3rd party clone from i3DGear, but my experience with the owner was so unpleasant, I went with the real thing from Raymarine.

I built a bracket out of 1/2" plywood to hold the transducer. Rather than drill a new hole in the rudder quadrant, I attached the other end of the indicator rod directly to the ram.

Rotary rudder reference indicator

New Raymarine M81105 rotary feedback unit with linkage.