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Tiller auto pilots
I have had a Navico tillerpilot on my Beneteau 23 since 1996. Unfortunately, I don''t recall the exact model number, but this unit was the "second from the bottom" of their product line. Here''s a few things I remember thinking about as I bought the unit and some of my experience with it. This is a long post, but I hope this info will help you avoid reinventing a couple of the wheels that I had to...
The lowest model was marginal on thrust and had no interface capability. The model I bought has more thrust that my boat should ever need, and has performed beautifully in any conditions that I have used it in. Interface capability has had unexpected benefits. Here''s more on both subjects.
Thrust -- the pin that connects the tillerpilot pushrod to the tiller is located much closer to the rudder than the point where a human helmsman holds the tiller. Consequently, the tillerpilot has much less leverage on the rudder than you do with your arm and has to have more "strength" than a human helmsman to do the same job. While you might read tillerpilot specs on thrust and say "my boat would never put that load on an autopilot", think about what it would be like to hold the helm with a tiller only about a third as long as what you''re used to -- that''s what my Navico has to do.
Interfacing -- my tillerpilot has a "nav lock" mode, where it alters set course as necessary to stay on the track programmed into the GPS. While this offers the possibility of automatically correcting for set and drift, I refuse to run an autopilot in "nav lock" mode, because I consider it hazardous for a sailboat to potentially change course on its own initiative.
However, if my pilot is connected to the NMEA bus, it will use the speed info coming off the GPS to help it determine how much helm to apply, much as a person would. It steers the course I set, but can handle the helm more intelligently if it knows how fast the boat is moving.
Also, interface capable pilots usually can be used with a remote control. I use my remote a lot when motoring as a way to dodge obstacles and such. I can steer the boat from the bow this way for better visibility, from the companionway to get out of the sun, etc.
Hope this helps....