These grotty little graphics are just a couple of many I sent to the nice people at Voyager Windvanes , whom I hope will make as functional a product for us as it looks to be robust. They need the measurements to essentially custom-build a windvane for Alchemy, one upon which I will be relying to steer the boat when on passage or under sail for more than a couple of hours.
Why a windvane? Because one of the dirty little secrets of cruising any sort of distance is that actually standing or even lying down at the helm, steering 15 tons of moving vessel is tiring, exacting and, well...boring. My time would be better spent looking around for other boats, sleeping whales, rogue shipping containers and things that could be sucked into the seawater intake.
A windvane both obviates and compliments the more popular electric or electric-hydraulic, super-duper, GPS-guided, amp-eating autopilot. Yes, I will install one of those as well, but the general idea is that the autopilot will steer while we are motoring (sails down) or motor-sailing (sails up but engine on in order to maintain a certain speed. With the engine on, particularly in calm seas, there is no problem in making the amps necessary to power the autopilot, and the same is true of the array of electronics gadgets that help us navigate and help us communicate with the outside world. Generally, we would have AIS, GPS/plotter and probably would actively monitor certain VHF/SSB frequencies. The kid might be watching an education DVD, also. The combination of alternator, solar panels, and towed generator would more than take care of the energy drain during motoring or motor-sailing.
Sailing (to be preferred), it's a different story. Depending on proximity to shipping lanes, land, known fishing grounds, the time of day and the weather, we might switch to just AIS, handheld GPS, radar on low-power "guard" mode (more on this later), and only switching on the SSB at certain times for "cruiser nets". I would likely keep a VHF on low if close to land; otherwise no.
The fridge would thus be the biggest draw. Again, I would hope that the solar panels, the wind or towing would more than compensate the reduced draw-down, plus the fact that I intend to have more than the usual battery capacity for calm, cloudy days when the engine's not working...
The windvane works on servo-pendulum principles, a sort of mechanical feedback effect akin to a see-saw or a gyroscope. The course made good tends to resemble a sort of very elongated letter S, whereas the autopilot will attempt to steer directly at the desired waypoint or compass course. Frequently, an odd wave or wind shift will push the boat off-course; this will cause the autopilot to use a lot of power to compensate. The windvane does this using the wind itself, and ends up in many situations doing not only a better job of steering than a human helmsman, but a better job than the autopilot on which so many cruisers rely exclusively.
Here's an interesting head-to-head by Tony Gooch, a long-time solo sailor from Canada with more fastidious observational skills than myself.