Autopilot & GPS, Finding a Common Bearing

[John J Frank]

1)I have a Raytheon ST4000+ autopilot with compass sensor mounted on the centerline, in the cabin, but only a couple of feet above 3800 pounds of cast iron keel. Is this a problem? It feels like it should be. 2) Can I use the GPS course bearing as the 'Gold Standard'for setting my autopilot so the two are in agreement? Thanks

 

[Paul McGhee]

1) Maybe. 2) No

Two feet from all that iron does sound a little dubious. But if your autopilot is taking you where you want to go, don't sweat it.Bearing and heading are two different things. Your autopilot works with heading, since the function of your autopilot is to move the rudder to keep the nose pointed in a certain direction, which is your heading.Your GPS works with bearing, since it has no idea which way the boat is pointed. It can, however, estimate where the boat is actually going.Imagine this: You have the autopilot set at Zero degrees. You're sailing across a three kt cross current and the log says you're making 5 kts over the water. In order to get to a destination having a bearing of zero degrees, you'll have to change the autopilot to point the boat at a heading equal to arctangent(3/5). Or is it arctangent(5/3)? I forget.Trigonometry aside, what you do in practice is adjust the autopilot heading until the boat's track is taking it to your destination. The heading of the boat and the bearing of that destination will rarely agree.Paulsv Escape Artisth336

 

[Augie Byllott]

Common Bearing?

With respect to your #2, maybe not "Never the twain shall meet", but it will be rare when they do. It's called cross track error. I like to think of it this way. If one were in a car rolling down an airport runway, headed for its end, the heading and bearing would be the same. The friction of the tires prevents a crosswind from pushing the car off the runway.On the water, the GPS "draws a line" from point A to point B and memorizes it. Whenever the GPS indicates that wind or current has pushed the boat off the memorized line, all one can do is frequently point the boat in the direction that will get it back on the line. Else the pilot will maintain a compass course that is parallel to the GPS "line". If the run is sufficiently long, the waypoint may be passed at a great distance.Coupling the two so that the GPS feeds course corrections will keep the boat moving along the line although it may not be pointed directly at the target waypoint. Current, wind, and wave action will introduce an element of "crabbing".Bearing is where the boats should be going; heading is where the boat is actually going. No doubt the manuals have a chapter or two on the subject with far more detail than can be included here.