Raymarine ST2000 Tiller Pilot repair

[Nick Orchard]

My 6 year old Raymarine ST2000 Tiller Pilot recently started making strange noises and was somewhat erratic in holding course, so I feared the worst. At best I thought it would be an expensive repair and at worst a new replacement, even more expensive. Having taken it apart to investigate I found that it was a relatively simple repair so I thought others might be interested as it's likely to be a common problem.

Taking it apart is very easy thankfully - 8 screws hold the cover together, unscrew from the back and then lift the top straight off. Remove the 3 screws holding the PCB down and swing it out to the side, no need to unplug any wires. The main motor/gearbox/leadscrew assembly then just lifts out of the bottom case, attached only by the wire to the motor. I suggest wearing rubber gloves when handling the moving parts as the grease is very, very black! 4 screws hold the motor/gear assembly to the drive assembly so remove these, then remove the screw securing the motor earth wire to the end plate and you can separate the two parts. (The two small drive belts and one gear will fall out).

Investigation - the first obvious sign of the problem is a variety of bits of rubber and plastic lying around inside the case. I don't think most of them have a major function, so can probably be ignored (remains to be seen). The main culprit for the failure is a small plastic ring that looks like a washer. This ring is supposed to be held securely in the plastic block (possibly glued in?), and its function is to stop the drive rod and its bearing moving outwards when it's trying to pull the tiller. Bearing in mind that the spec says that the thing can exert up to 77kg it's surprising that it lasts 10 minutes never mind 6 years. When this ring gives up, the result is that the drive rod tries to escape but can only move about 5mm or so before the gear attached to the end of the shaft comes up against the aluminium back plate. This is the point at which it starts making nasty noises, the friction increases dramatically and you have 5-10mm of backlash in the system, hence the erratic course holding.

The fix – even if a replacement ring was available as a spare I don't think I'd bother trying to replace it, as I'd just be waiting for it to happen again. What is needed is a more robust method of holding the bearing in place. Doing the usual YouTube search for people who've done this before turned up two videos. The first was a German who designed a replacement plate in CAD then CNC machined it up before discovering to his surprise that he couldn't get the whole assembly back in the case! The second was an Aussie who took a simpler approach and he just inserted an aluminium shim spacer between the drive gear and the bearing housing.  He didn't show the result of his work as I suspect that it still made a terrible noise with lots of friction because he'd put the shim on the wrong side of the housing. What is needed is something to stop the bearing moving outwards and this can be done relatively simply. Firstly you need to pack out the space between the bearing outer race and the face of the housing, and for this I found a couple of fibre washers in my plumbing box that were pretty much the right size. Then you just need to hold these washers in place, and for that I took a large stainless penny washer and filed it down to fit the shape of the various apertures and used 4 screws to hold it in place. Ideally I'd have put the screws through the washer but there isn't much space available so just filed notches in the edge.

The result – it works hooray! Well so far I've only tested it on a stationary boat in the harbour but it seems to be doing what it's supposed to do quietly and with no backlash, so next step is extended sea trials. If it breaks again I'll let you know.

Caveats – a couple of things to note if you do decide to take yours apart. The first is a warning from the helpful German YouTuber – don't try and take the drive shaft out of the main ball-screw drive bearing. He took his apart from the wrong end (the outer end) and when he took the drive shaft out all the balls fell out and disappeared. The second warning is that although most of the disassembly is very easy you do need to remove the drive gear from the end of the shaft. This is held on with a very small roll-pin, so you need a suitably small drift to get it out, and to get it fully back in again. I used a jeweller's screwdriver with a 2mm a/f hexagon drive.