Autopilots

I said before that on watch we only need to check for traffic and other changing conditions. If that’s all we have to do, then who’s steering? The answer is the autopilot!

Wonderland has two autopilots on board. The first one is an electronic one. The other one is analog and operates off of the power of the wind and the boat going through the water. They are both pretty interesting, so I’ll explain them both.

The electronic one is tied into the rest of the electronic instruments via an onboard network called SeaTalk. It gets the current heading from an electronic compass (which lies a lot) and it can also find out the direction of the wind from the mast-top windvane. Finally, it has a sensor of the steering mechanism to know where the rudder is. It uses all that data, plus the course you tell it you want to either steer to the course, or keep a constant angle to the wind. If our GPS was on SeaTalk, it would be able to send the bearing to the next waypoint into the autopilot too. I said the electronic compass lies, but luckily it lies in a manner consistent enough that we can ignore the absolue reading it gives, and just let the autopilot use the changes in the compass reading to steer the boat. The thing about the electronic autopilot is that it takes a lot of electrical current to run it. It’s not the electronics that take so much, it’s the mechanical steering arm that turns electricity into the force necessary to move the rudder. When it is steering, you can go below and look at the power consumption and literally see spikes when it steers left then right over and over to counteract the waves. Power use is important because sailboats are engineered to have small engines, small diesel tanks, and small electrical systems. The windmill and the solar can provide enough power for basic needs, but if you were going to use the electric autopilot 24 hours a day, you’d need to charge the batteries with the diesel engine a lot. When you are at sea for over a week (and there’s always a chance that one week will turn to two or more if various things outside your control go wrong) conserving fuel is important. And there’s always a chance the engine will fail.

In fact in Las Palmas de Gran Canarie, we met a boat named Chant Pagan which had it’s engine fail on the passage. They had only an electric autopilot, so in order to save battery for things like sailing instruments and the radio, they had to steer by hand. There were only three crew on hand experienced enough to steer, so they had a very long passage, each steering on average 8 hours a day. Luckily for them the weather cooperated – almost too well, as they had 25 knots of wind from the NE and tall following seas. Steering in those conditions takes a lot of concentration! Poor Chant Pagan.

Safety comes from redundancy, which is part of why Evi has the other autopilot on board. It is a Rube Goldberg contraption that hangs off the back of the boat. For the first several hours I watched it I swore it couldn’t possibly work. When reality proved me wrong and I looked a little closer at it (after 2 days of puzzling) now I see how it works. The idea is pretty simple. If you stood at the back of the boat holding a piece of cardboard up in the air edge on to the wind, but then the boat turned a little with respect to the wind (due to a wave, say) you’d feel a force exerted on one side or the other of the cardboard, right? And it might be possible to use some ropes and pullies to hook that piece of cardboard up to the steering wheel, right?

So that’s all there is to a windvane steering system. Or it would be, if the piece of cardboard was the size of a billboard, which would never do. The rudder requires quite a lot of force to do its job, so in turn the windvane (cardboard) would need a lot of force to turn the rudder. So what we need is an amplifier. My buddy Cary could whip one right up with a soldering iron and a trip to Fry’s, but then we are back to using too much power (sorry, Cary). What we want is an autopilot that uses no electricity. Where else can we get power on a sailboat? Well, it is a good bet there’s some wind when you want to be using the autopilot, but it’s hard to get enough power out of the wind. And it seems kind of silly to decorate the back of your sailboat with something sail-like to harness the wind you need to steer your boat. If only we could borrow some of the power coming from the mainsail and jib in some form we can use to amplify the control input from the windvane into control output on the shaft of the helm. Well there is, and it’s really clever.

Know how you put your hand out the window of a car, and you can use the wind passing over it to make it go up and down? You can do the same thing in the water flowing behind a moving boat!

So a windvane autopilot works like this: you’ve got a rudder-looking thing hanging off the back of the boat down in the water. It is attached by a Rube Goldberg set of ropes and pulleys to the helm by two ropes, a clockwise one and a counterclockwise one. This rudder thing does NOT steer the boat. It steals power out of the water in order to turn the helm. And the helm leads to the rudder via the usual cables. The rudder steers the boat. Above the autopilot’s rudder is the windvane. It is coupled to the rudder with yet another magical set of linkages that turn the input from the windvane flopping back and forth in the wind into instructions to the rudder to swing back and forth through the water, stealing power and steering the boat.

This is by far the coolest analog computer I’ve ever seen!

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