About good autopilots and mechanical steering systems--just this week we've been in touch with WH Autopilot and AutoNav (owned by Paul Wagner) to come up with a system for our boat. We'll choose whichever works best for our situation. Both companies will work with owners for owner installation of a custom system. Both companies have excellent reputations in the world of commercial fishing in the north Pacific where a solid autopilot that works in all conditions is a key thing. We expect differences between the two proposed systems will be the variety of user interface, nav inputs, and cost.
We have a wonderful Edison Robinson double arm steering gear that is pretty much indestructible but unfortunately not exactly compatible with a hydraulic autopilot. The particular worm gear is way oversized for our boat, we're 29 T and 46' waterline with a 19 sf keel hung rudder. The particular worm gear is usually installed in boats of twice our displacement and waterline so it's "stout."
I'm writing about "our" situation because most people will tell you a worm gear precludes the use of a hydraulic pilot and most will tell you that the worm gear must be removed entirely from the boat to install the hydraulic pilot.
Its not true.
The worm gear does have to be disconnected from the rudder shaft unless one wishes to drive the rudder with autopilot using a chain and sprocket on the helm (this really slows down the responsiveness of the autopilot, takes more energy, and isn't recommended). This disconnect means that you're operating with a hydraulic steering system instead of using the worm gear. But, having the worm gear as a backup steering system is a great thing (much better than a tiller...). It gives me confidence to know that we'll have two different and reliable steering systems--the first being hydraulic and the second being the trusty worm gear if something goes wrong with the autopilot.
Having the hydraulic arm directly attached to the rudder with a keyed collar is the desired way to go if one has access to a spot on the rudder shaft to do this. This leaves the top of the rudder shaft available for an emergency tiller arm, or in our case, for reconnecting the worm gear to the rudder.
In our case (and in most wormgear cases I suspect...) the worm gear attaches to the rudder shaft with a flange that is bolted with 4 thru bolts to a flange keyed to the topmost end of the rudder shaft. The disconnect involves taking out the 4 bolts. Its possible to reconnect the worm gear by putting the bolts back in place. This will be done in an emergency by employing a collar made up with 4 aligned thru-bolts for a quick re-connect. (I could just see trying to align and insert each of those 4 bolts individually in an emergency....they'd all end up in the bilge for sure
In our case, the rudder shaft is 2.5" diameter and 10' 1/2" long and we have 16" of open rudder shaft above the rudder bearing post (It probably has another name...but its function is bearing and alignment of the keel hung rudder.) The weight of the rudder is on a gudgeon attached to the keel. The weight of the worm gear is taken by attachment to the hull. The bearing is a 36" long tube and runs up through the hull to a point sufficiently above the water line to not have water ingress via it. This 16" access gives us plenty of room to key a collar for the hydraulic autopilot's direct connection to the rudder shaft and allows us to choose a height above the waterline that lines up with a good place to attach a steel plate to the hull to absorb the forces involved with using the hydraulic drive arm.
The chain/sprocket on the helm (that previously held an electric motor driven autopilot) will be used for a manual hydraulic pump so the helm can be used for manual steering. We also have a chart house with inside steering station--we expect to use a manual hydraulic pump in that location as well; even though both WH Autopilot and AutoNav offer handheld remotes for the purpose of manual steering via the autopilot. We like the idea of being able to do manual steering from inside. Previously, the boat had a really rinky dink mechanical system for this purpose.
We're a bit anxious about the cost of the autopilot and steering re-fit, but believe, behind the rigging, that this is the most important system on the boat.
If anyone on the forum also has a worm gear and is facing similar re-fit, please feel free to keep in touch as we go through this process. If anyone's done this or had a boat with a similar set up with worm gear, we'd love to hear about it, too.