Autopilot - if you don't know...just ask

[jcollins]

I installed a Raymarine autopilot a couple of years ago since my orginal autopilot was promoted to crab-pot-scout. At that time I did not interface the ap with my gps. My thought was that although the Raymarine unit, compared to others, is less expensive, it tends to be off 4-5 degrees (sometimes more) from the gps heading. I have been waiting for someone else to ask but have not seen a posting about my question.

When you connect the ap to your gps which device controls the course? The ap gyro compass or the gps when following a route?
If it's the gps will it correct course deviation caused by wind, current, etc. ?

Probably a "newbie" question but I'm hoping those more experienced than I will answer.

Thanks!

[loubennett]

When you interface the autopilot to the GPS, the GPS will control the course when you set it to navigate to a waypoint. It sets a course to the waypoint and keeps you on that course line. Inititiating this mode is probably controlled from the autopilot. You will set the navigation to the waypoint on the chartplotter then set the autopilot to navigate. Its usually one of the buttons along with the Auto and Standby buttons. In the presence of crosswind or cross current, it will steer the boat to a heading that keeps you on course. In these conditions the heading will not be directly toward the waypoint, but left or right of that heading to keep your course over ground directed toward the waypoint.

I think you have a Lowrance chart plotter and a Raymarine autopilot. I'm not sure of the interface, but it's probably a NMEA 0183 connection. The course and position information will be passed over that interface to the autopilot. Your autopilot display will probably still show the heading indicated by the autopilot's compass, but it will steer the boat to maintain the course over ground from the GPS. One of the display pages on your chartplotter should give you cross track error. That indicates how far off the course line you are.

I have a Simrad autopilot and Simrad chartplotter so the integration is fancier. The autopilot can even be controlled from the chartplotter. Crossing brands will work fine, but it probably won't have all the bells and whistles. They're not necessary, just gadgets.

[jleonard]

GPS directed autopilot is the best. I have mixed brands now, but the Garmin plotter guides the Sitex just fine.
My "ex boat" had a Raymarine autopilot guided by a Micrologic gps. The autopilot was always very slow to take the gps data. I think that is a Raymarine design thing. They do that so you will match their electronics up and use their "seatalk" system. At least that is my opinion.

[mhanna]

If there was no cross wind or current then the ap running off its own compass would probably do a fine job.
John I think you have the ray st5000 ap? I have the same one and when I had a ray plotter they were connected via seatalk. I used the ap almost every time out but not often from the plotter, most of the time I hit auto and then just + - 10.
When I replaced my plotter with a simrad I found that the st5000 does not do nmea183 and I would need to buy an additional conversion box to interface them. I decided not to do this and continue to use the ap on its own, it works well for me

Matt

[jcollins]

Thanks for the replies. The AP is a ST6000. This is more of a minor annoyance than anything else. Unless I'm planning a trip to Bermuda being off a few degrees is not a big deal. I've learned to just ignore the heading on the AP and follow the gps. But, it would be nice and I'll probably go ahead and connect the two. I guess I'm just looking for something to fix since nothing has broken lately. Of course, now that I said that, I did have problems with the windless over the weekend but that's another post.

Thanks!

[Captn_Dwt]

Hello!
I've been running Ray-marine ST4000 wheel pilot for years on the sailboat, and now the 6000 system on the Albin. These autopilots have a flux gate compass...should have been "swung" as a part of setup, and that process will remove the bulk of the locally caused deviation (influence of the engine and other IRON on the boat)...but it will not remove it all. So, as a stand-alone system, the autopilot will mostly not show a perfect "magnetic" heading. But, once you connect to the nav system with GPS...that will control the course when the auto pilot "tracks". I've just gone through the learning phase with tracking...it has its issues...but once your vessel is "on track"...it is like being on rails. wind, tide, and any other effect is compensated for automatically. Once you get used to it, it's very very nice. I think it could also allow one to overlook dangerous cross-currents, which are a real issue in Maine. I keep a close eye on the cross-tracking, which shows on the plotter data and the difference between the predictor and the heading indicators.
enjoy!!

[jbutzer]

Late reply here, but here goes. All of the postings have great info. I would like to add the following, which was prompted by the 'going to Bermuda' reference. Somewhat bored on runs over glassy water, 120nm, Fort Myers Beach-Key West, my experience between 'going-to' to the Northwest Channel marker; and finding the (corrected) bearing to it, then letting the A/P track the radial generated by it's electric compass: YACHTA YACHTA's wake is straighter when the A/P is NOT coupled to the GPS. A guess on the reason is that the A/P goes to the GPS on pre-set time intervals for course corrections, and no matter how good the GPS, nor how smooth the water, there will always be some subtle GPS corrections that are fed back to the A/P. Bottom-line is that the A/P seems to love it's own radial bearing more than continuous updates from the GPS. Of course when you get nearer to your destination, there may be a 'manual correction' required. But if you are using a go-to from the GPS, you need to take it off of that so that you don't hit it!

Again, this is definitely splitting hairs on the subject. Hope I haven't wasted anyone's time.

BUT WAIT THERE'S MORE!!! 'Splitting hairs' that is. Hey, digital ink is cheap:

'Splitting hairs' on a GPS just got a whole new meaning for me. Traveling in North Dakota last week, stopped at a farmer friend and got an extended ride on one of those monster John Deere $500,000 Combines. They steer with a coupled auto-pilot. The driver/operator/farmer was as shocked that we drive boats the same way -- as I was to see no hands on the steering-wheel of this huge machine. For these farmers, signals off of the satellite aren't good enough, as they only hold +/- 8". So, they run the satellite signal to a tower on the ground, which resends a better signal to the Combine. That refined GPS signal has this monster holding +/- 1" (!!!). Sounds impossible until you see the laser-beam-straight lines that are + 1/2 mile in length. All this technology paid is for by the savings in diesel fuel. Amazing stuff. Hope it wasn't out-of-line to share on the Albin site.

[DougSea]

Jbutzer - good comments, but a thought on that "wake is straighter" observation. A boat traveling on a compass bearing will be drifting off of it's course line as wind, waves, and currents move it around. The longer the run, and the stronger the offset forces, the more this would come into play. A GPS waypoint WILL be constantly correcting.

And yes, it's not a good plan to have your GOTO point be something you don't want to hit! Setting your point on an offset is always a good idea.

[Chuck Waygood]

Jbutzer - Amazing note about how they maintain those straight lines with the combines. Who would think something like a marine autopilot would be used to plow, plant or harvest crops?? But how does the repeater improve performance?