ST4000 Mk 1 Autopilot fix

I have an ST4000 autopilot (circa 1997), wheel mounted. The drive unit consists of two round pieces, snapped together. One half is bolted to the wheel, and the other (the drive unit half) snaps into the wheel half. The drive half is held motionless (prevented from rotating) by a 2” fixed rod bolted to the pedestal.

Inside the drive half of the unit, there is a belt connected to a clutch handle. Flipping the clutch handle tightens the belt. When properly assembled, this belt wraps around a ring on the wheel half of the unit. Tightening the belt allows the wheel to be moved/controlled. Also inside the drive half are seven rollers spaced evenly around the perimeter. These rollers act as a guide for the rotation of the wheel half.

Recently, while executing a turn, I heard a "crack" noise. As I reached for the clutch to disengage, I heard another. Soon the drive half and the wheel mounted half had separated.

Tossed out the anchor to take the wheel off and investigate.... Upon examination, it seems two of the rollers inside the drive half had sheered off. That is, the plastic (injection molded) barrels onto which the rollers are screwed broke clean off. I assume the unit separated because the rollers had broken off. Best I can tell, it’s only the tension of these rollers on the other half that holds them together.

I do not know what caused this. There was no foreign material found inside the mechanism; looked good except for the broken roller mounts.

Anyway, I brought the broken roller mounts to BoatUS and got some stainless machine bolts that closely matched the size of the original mounting screw. Also got washers and locking nuts to match. The length of the bolt is 1-1/2”, but 1-1/4” would have been a better fit if they had them. 1” is too short.

The plastic knobs that sheered off are supported by a brass barrel around them to hold their shape/strength. This made it easy to hold them with a vice grip clamp (not so tight as to deform the round shape!) and drill out the barrel to match my new bolts. Drilled a matching hole through the plastic drive half.

This allowed me to reassemble the rollers on the mounts, and bolt them in place on the drive half.

Back to the boat and snapped the two halves together, mounted the wheel, and we’re back in business.

For what its worth- the Raymarine help center gave me the following response (5 days later):

“Cause installation maybe, what is the wieght [sic] of your vessel? The part number to repair is N013, and is about $380.00 cheaper to send to repair center here at Raymarine Inc. 22 Cotton Rd Nashua NH 03063”

Lessons learned:

If unit breaks while underway, be careful that valuable parts do not fall out and roll off the back of the boat!

Check your wheel lock nut often to make sure the unit is held securely in place.

Check the mounting of the wheel half of the autopilot unit, make sure it is centered on the wheel; any variation here could cause extra torque on the system (and those rollers).

Parts in hand, the whole fix took 15 minutes, and cost about $0.80.

Hugh and Ali

C36 II #1620