September 23, 2009 15:32
I've just returned from driving back and forth over a 1000 mile stretch of Alaska, virtually all of it using the nav setting of my autopilot with a data feed from MacENC. I've learned several caveats to its use, and one caveat should be easily addressed by a software upgrade.
So far I've identified three sources of nav error, and the error typically manifests itself while turning from one waypoint to the next when following a route. One error is a NMEA feedback loop, which I solved last year. Another error derives from the constantly changing magnetic environment around the steering compass. The third error is the difference between the local magnetic variation and that shown in the preferences window of MacENC.
When traveling north-south from Kodiak to the Aleutians, I go from approx 18°E variation to approx 5°E variation. It doesn't matter whether I turn the computer on and off, or pull the BU353 GPS unit that I normally use—the value set in preferences will remain the same until I check and then uncheck the box for manual variation. When I uncheck the box the value will instantly change to the correct local variation and the initial course error at the next waypoint shift will be somewhat reduced. Each time I do this the bearing displays also change from true to magnetic.
The error is not a caveat by itself, but it leads toward a rule: never trust the nav function to turn you within 0.1NM of any hazard. It may seem absurd that I would need to say something so obvious (prudent mariner, proper watch, blah, blah) but it gets to the heart of why you should be using nav in the first place. The nav function is incredibly accurate. You can have all sorts of steering errors and equipment issues, but if it is reasonably possible to steer a straight line, eventually all those errors get sorted out and the computer/autopilot will keep you generally within about 100 feet of where you want to be. Thus, when faced with a particularly tight passage, like a narrow channel between submerged rocks, I always try to make the computer steer through it.
On the other hand, I never know when there will be a very large error. These can be generated by sudden volcanic activity ( a big issue in the Aleutians) or by somebody setting a flashlight by the steering compass. I've seen the boat swing 90° when faced with a 5° course change. The computer will begin sending a strong correction at about 0.1NM of XTE. That's 51 seconds at 7kn of charging toward a big buoy before the system corrects the error. You can believe that you would never get distracted for 51 seconds on watch, or you can make sure that every waypoint on a route is more than 0.1NM away from any hard object.
So far I've identified three sources of nav error, and the error typically manifests itself while turning from one waypoint to the next when following a route. One error is a NMEA feedback loop, which I solved last year. Another error derives from the constantly changing magnetic environment around the steering compass. The third error is the difference between the local magnetic variation and that shown in the preferences window of MacENC.
When traveling north-south from Kodiak to the Aleutians, I go from approx 18°E variation to approx 5°E variation. It doesn't matter whether I turn the computer on and off, or pull the BU353 GPS unit that I normally use—the value set in preferences will remain the same until I check and then uncheck the box for manual variation. When I uncheck the box the value will instantly change to the correct local variation and the initial course error at the next waypoint shift will be somewhat reduced. Each time I do this the bearing displays also change from true to magnetic.
The error is not a caveat by itself, but it leads toward a rule: never trust the nav function to turn you within 0.1NM of any hazard. It may seem absurd that I would need to say something so obvious (prudent mariner, proper watch, blah, blah) but it gets to the heart of why you should be using nav in the first place. The nav function is incredibly accurate. You can have all sorts of steering errors and equipment issues, but if it is reasonably possible to steer a straight line, eventually all those errors get sorted out and the computer/autopilot will keep you generally within about 100 feet of where you want to be. Thus, when faced with a particularly tight passage, like a narrow channel between submerged rocks, I always try to make the computer steer through it.
On the other hand, I never know when there will be a very large error. These can be generated by sudden volcanic activity ( a big issue in the Aleutians) or by somebody setting a flashlight by the steering compass. I've seen the boat swing 90° when faced with a 5° course change. The computer will begin sending a strong correction at about 0.1NM of XTE. That's 51 seconds at 7kn of charging toward a big buoy before the system corrects the error. You can believe that you would never get distracted for 51 seconds on watch, or you can make sure that every waypoint on a route is more than 0.1NM away from any hard object.