I'm backtracking a little here to the question about autopilot power consumption
It's obvious that trimming the boat for minimum helm
force is important, but there's another issue: tuning your autopilot. I use a Simrad
TP22 on my 28' 3.3 ton boat, and I find that adjusting the gain and "seastate" (ie deadband) has a huge impact on its performance. Not only on how well autohelm
can control the boat, but also on the cycling of the motor
(ie power consumption).
For running, you usually need fast response to cope with the effect of following or quartering seas (high gain/small deadband) and so it uses a fair bit of power. However for reaching, where the boat is almost able to steer itself, low gain/larger deadband settings mean the autopilot doesn't have to work as much. This also makes life mechanically much easier for the autopilot, hopefully extending its life.
My TP22 has a self-tune setting for the deadband, but I get far better results setting it manually. Of course if you're motoring it's all a bit simpler.