Autohelm ST4000 Won't Hold Course

[bullweber]

I recently acquired a 1985 Catalina 36 which has an Autohelm ST4000 mounted on the starboard side of the cockpit, near the helmsman's right shin. I've noted this installation location on other Catalina 36s, so I believe this is a factory installation. It appears that everything is functioning from a mechanical perspective - when I turn on the unit, set it to "auto" to maintain my current heading and engage the wheel lock lever, the wheel begins to make slight corrections as if intending to hold the course. But after about 15 seconds it begins to veer 90 or more off course, almost as though it's attempting to tack! Any advice about how to fix this would be appreciated. Thank you!

 

[jgwmaxwell]

If anyone has any ideas I'd love to echo this as a problem! Holds just fine for some time and then suddenly seems to want to explore a different course. Doesn't seem to be any slippage in the wheel lock, steering works just fine without autopilot enabled, works as expected when initially enabled, compass on the unit matches the analog compass on the boat. This time it's a Dehler Delanta 80, but I doubt that is relevant here! Is it just too old?

 

[KeelsonGraham]

I have a 2006 Catalina 34. The original owner had installed this system. The sensor was mounted aft, under the bed, exactly on the centreline. We noticed that the Autohelm could wander by +- 25 degrees off the set heading. I always suspected the sensor unit, because this is an electromechanical device that is very likely to degrade over time. We have a rudder position sensor which probably reduced the issue. Everything was completely resolved by upgrading to an Evolution system, with the solid state sensor. £££ but a MUCH better system.

 

[Graham376]

Metal or electronic objects inadvertently stowed near the fluxgate compass can have this effect, as we had happen with a mobile phone one time.

 

[Kwik Decision]

Is it turning the wheel the correct way? If the boat is tracking nicely, it may not be apparent that the autohelm is making small steering corrections the wrong way for a while, and then at some point it will go wildly wrong.

On our boat, (bought 8 yrs old with an ST4000 tiller pilot fitted from new) this was precisely the problem, and swapping two wires round in the control box sorted the problem.

 

[Stemar]

I had a German army knife - like Swiss, but bigger blades, and green that I carried everywhere. One day, our tiller pilot started doing odd things, and I realise that it would hold a perfectly good course when I was a few metres away, but got mischievous if I got too close. I turned out that that knife in my pocket had become quite a strong magnet - no idea how - and banishing it from the boat solved the problem.

 

[Slycat]

I have one of these and notice the course holding is not great, but I noticed that when I hooked it up to my (B&G) chart plotter and used the track function instead of the built in 'auto' it works a lot better. My symptoms not quite as severe as yours but it seems using a source with a 'better' compass does help in my case!

(NB: Since owning the boat I havent gotten round to fine tuning all the settings yet)