There are two primary reasons windvanes are used by most long-distance sailboats. The first as Jon points out has to do with power consumption
. And keep in mind that, as conditions build offshore
, the autopilot
has the same level of steering
force with which to work, so its only option is to work harder (more frequent cycling) which drives up electrical consumption
(and the potential risk for something to overheat or break). Servo-oar systems, the kind of windvane
you normally find on boats, become more powerful in heavier conditions - one of their key assets.
The second reason windvanes are commonly used is reliability
. Beyond chafe to the steering
lines, there is less to break on a windvane
(and arguably, more you can repair when it does) than with the multiple component below-decks autopilots. Personally, I think this is in part because windvanes are accessible and easily inspected while working; problems which are developing can be seen and addressed.
When we crossed to Horta in 2003, I was curious how old-fashioned we were in carrying a windvane but no below-decks a/p (we have a small helm-mounted unit for motoring), so I conducted a daily survey
as I walked past the transient docks on the way to town. Boats are constantly coming and going there, 800 that season from May thru Sep. Each day I counted the number of transoms I could see - big boats/little boats; heavily equipped to spartan - and of those, the number that had windvanes. The lowest percentage I counted was 60+%, the highest 90%, the average I came up with over 5 weeks was 75%. Bigger, more expensive boats had the financial backing to make it easy to add a windvane. Smaller, more spartan boats HAD to have a windvane, due to both financial and electrical
generation limitations. Most boats, independent of size, had smaller crews (2-3 were most common, with some singlehanders) and losing self-steering can be a real downer and present real challenges to small crews, so self-steering reliaibility was a major issue for them.
It was also interesting to see how many vanes were set up to accept a tiller a/p. We'd rigged this up before leaving the States and it 'fixes' the one weak point with windvanes: steady courseholding in light conditions. It also can provide self-steering while motoring, should the a/p go away.