I have a Raymarine 4000+ autopilot and a Standard Horizon GPS/Chartplotter. While on an autopilot directed course, I noticed that the pilot course and the GPS/Chartplotter COG (i.e., Course over ground) differ. Can I assume that I need to adjust the autopilot to the GPS/Chartplotter? If not how do I syncronize the two so that they are both reading the same heading? Thanks
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Course and heading
The autopilot reads a magnetic heading which is the direction the boat is pointed. It doesn't necessarily do this with great accuracy. Its job is to hold a heading when you press auto. The GPS looks at the angle of your most recent positions relative to Mag or true North and displays that as your course. This is the direction you are moving, not the direction you are pointing. If you are tracking a desired course on the GPS, the autopilot steers to try to keep you on the line from where you set the GO-TO to your GO-TO waypoint. Steering corrections are made by the autopilot using cross track error (XTE) received from the GPS. Hope this helps, Jack
The autopilot reads a magnetic heading which is the direction the boat is pointed. It doesn't necessarily do this with great accuracy. Its job is to hold a heading when you press auto. The GPS looks at the angle of your most recent positions relative to Mag or true North and displays that as your course. This is the direction you are moving, not the direction you are pointing. If you are tracking a desired course on the GPS, the autopilot steers to try to keep you on the line from where you set the GO-TO to your GO-TO waypoint. Steering corrections are made by the autopilot using cross track error (XTE) received from the GPS. Hope this helps, Jack
Course and heading
Course and heading may be different as stated before. Currents and leeway may require that a different heading be maintained in order to track to your desired waypoint. Just watch that the cross-track error is kept to a minimum to verify you are staying on course. If you have a chart plotter that can display the "highway" display, you can easily see how far off your desired course you may be wandering while under autopilot. Les Murray s/v Ceilidh '86 C-36 #560
Course and heading may be different as stated before. Currents and leeway may require that a different heading be maintained in order to track to your desired waypoint. Just watch that the cross-track error is kept to a minimum to verify you are staying on course. If you have a chart plotter that can display the "highway" display, you can easily see how far off your desired course you may be wandering while under autopilot. Les Murray s/v Ceilidh '86 C-36 #560
Cross Track Error
John, I would say no to the assumption. When you are tracking the autopilot is working off of cross track error. As the CTE grows the autopilot should react to it and alter the course. I really do not think that the heading or bearing have anything to do with it. I think ours has a course computer that will correct for set and drift. I think, in a perfect world, the both of them should agree. I would first check the setup menu on the autopilot and the gps to be sure that they are both magnetic or both true. If you are on a magnetic heading with the autopilot and you are out of sync, this MAY indicate an adjustment to the fluxgate compass, is necessary.
John, I would say no to the assumption. When you are tracking the autopilot is working off of cross track error. As the CTE grows the autopilot should react to it and alter the course. I really do not think that the heading or bearing have anything to do with it. I think ours has a course computer that will correct for set and drift. I think, in a perfect world, the both of them should agree. I would first check the setup menu on the autopilot and the gps to be sure that they are both magnetic or both true. If you are on a magnetic heading with the autopilot and you are out of sync, this MAY indicate an adjustment to the fluxgate compass, is necessary.
Basic Navigation - Course - Set - Drift
Mike I tend to disagree with your assessment. A study of basic chart plotting (not electronic) in Dutton's and Chapman's indicates that set, course and drift will HAVE TO make the readings different with current in the mix. Course over ground and boat heading (i.e., compass heading or autopilot heading) can be very different. To best understand this, assume you're going through Deception Pass with a wicked current that slews your boat around while you're in the Pass. The basic course is, say, west, 270 degrees. You made the mistake of NOT going through at slack, and the current is flowing west. The wicked boils and overflows in the rapids within the pass slews your boat around facing south, but the boat is still moving west! because of the current going through the Pass. You could be going sideways, facing south (heading reads
180, while course over ground is still 270. Am I missing something? Another example: you're heading north (000). Current is going west to east (current's going 090. To go straight north to a destination, you MUST head your boat further WEST (i.e., say 300) [heading] to make a course made good [course over ground] of 000 to get to your 000 destination to overcome the east set of the current. Simple vectors. In order to properly understand and utilize all these electronics (improvements or dumbing-down, I haven't figured that choice out based on these types of discussions), all sailors STILL have to learn BASIC NAVIGATION concepts. Current inherently and mathematically causes a difference of (boat's) heading (simply to develop the offset to reach the desired destination) and, hence, course over ground. Stu PS Please note that I have not included leeway in this discussion for simplicity. Leeway when sailing just gets added into the vectors.
Mike I tend to disagree with your assessment. A study of basic chart plotting (not electronic) in Dutton's and Chapman's indicates that set, course and drift will HAVE TO make the readings different with current in the mix. Course over ground and boat heading (i.e., compass heading or autopilot heading) can be very different. To best understand this, assume you're going through Deception Pass with a wicked current that slews your boat around while you're in the Pass. The basic course is, say, west, 270 degrees. You made the mistake of NOT going through at slack, and the current is flowing west. The wicked boils and overflows in the rapids within the pass slews your boat around facing south, but the boat is still moving west! because of the current going through the Pass. You could be going sideways, facing south (heading reads
180, while course over ground is still 270. Am I missing something? Another example: you're heading north (000). Current is going west to east (current's going 090. To go straight north to a destination, you MUST head your boat further WEST (i.e., say 300) [heading] to make a course made good [course over ground] of 000 to get to your 000 destination to overcome the east set of the current. Simple vectors. In order to properly understand and utilize all these electronics (improvements or dumbing-down, I haven't figured that choice out based on these types of discussions), all sailors STILL have to learn BASIC NAVIGATION concepts. Current inherently and mathematically causes a difference of (boat's) heading (simply to develop the offset to reach the desired destination) and, hence, course over ground. Stu PS Please note that I have not included leeway in this discussion for simplicity. Leeway when sailing just gets added into the vectors.- Status