As a slight variation/alternative to the standard tillerpilot:

  On my catamaran I have an autohelm S1 (same as the old ST4000+) tillerpilot that has the control head, compass, and electronic mounted inside the port hull.  The control unit is theoretically waterproof, but I like keeping it under cover anyway.  The pilot is just a beefy cylinder, sans display or electronics, that plugs into a waterproof connection on the hull.  I keep it inside a hull when not using it.

  I believe the S1 is good for boats up to 13,000 lb displacement, with a max thrust of around180 lb (depends upon model), and a current draw of just under an amp at a 25% duty cycle. 

  I don't know what the specs are on the newest S1's, but even 180 lb thrust is probably overkill.   The boat should be sailing close to balanced most of the time, and if you're like most people, it's likely you've never had to apply that much force to a tiller.

  Generally I control the pilot with a wireless waterproof smartcontroller, though sometimes I do use the control head. 

  It's also hooked into the GPS, chartplotters, and TackTick instruments, so it can navigate to a waypoint or series of waypoints, to a compass setting, or to the wind direction (roughly one-minute average).  It will happily tack a catamaran, but there's no way it will shunt a proa -- that kind of programmability is generally reserved for the large/expensive/heavy hydraulic systems.

  If you wanted to get fancy, you could get two control pistons, one for each tiller, and then flick a switch to determine which is connected to the control unit inside.  That would also give you a spare if one breaks.  I keep a spare piston in the boat, and also a spare remote that's continuously charging.  (a lightning strike required a new system, but left me with working backups of these two components).  I don't have a backup control head, but since it's both waterproof and inside the hull, it's not much of an issue.

  Besides, the boat is a daysailer, so all of this is over the top.

  If you did have a switch that alternated between two pistons, the same switch could also connect alternate sets of wind and water speed instruments, so that the pilot would always be working with sensors that agree with the current tiller being used.

        - Mike

 

Paul Wilson wrote:

 

>>>what usually goes wrong with electronic pilots? the mechanical part or the electronics?

Depends on the type of pilot....if it is a wheel pilot, the electronics are normally permanently mounted, hopefully sheltered, and should be reliable.   Early motor drive circuits were unreliable but the new ones should be OK.  Nevertheless, a motor that completely shorts out can still overload and burn out the drive circuits. The motors and drive belts fail regularly on the wheel pilots.  The ram type or hydraulic type are much more robust.

On the tiller pilots, failure (assuming it is not overloaded) is usually by water ingress.  That is why I recommend keeping them bagged up at all times.   I have used the same tiller pilot for over 15 years but I have sealed the boards inside and covered all the external seams in "GOOP".  Without doing this, it would have lasted less than a year.  Water has come in through the buttons a couple of times.  Newer models have better seals but I still wouldn't trust them.

On both tillerpilot and wheel pilots they use flux gate sensors.  These are tiny gimballed coils that flex with heel and have wires not much larger than the diameter of hair.   They eventually fail due to the constant flexing of the wires but if you treat your autopilot carefully they should last quite a few years. If you move a tiller pilot around gently you will hear a knocking sound.  This is the gimbals hitting the stops.  Always treat them like eggs.

http://en.wikipedia.org/wiki/Fluxgate_compass

I used to repair autopilots in aircraft.  The poor build and quality of most marine autopilots are shocking.  They are the typical mix of great ideas (the electronics-sexy fancy stuff) combined with poor execution (the hardware- knobbly boring bits).  It reminds me of $25 million helicopter I worked on that was regularly grounded for a cracked door hinge.   It only takes one weak link to let you down.

Cheers, Paul