We have been users of computerized navigation for several years now. We started back in 1995 with the program NavTrek on a laptop computer on our Catalina 36. We hooked it up to our Magellan handheld GPS and Autohelm ST4000 autopilot. It worked quite well even under fairly heavy following seas.
Naturally when we bought our new Catalina 400 we wanted the same kind of electronic navigation. This time we bought a Magellan surface mount model and hooked it up to Autohelm’s ST6000 Plus autopilot which also connected to the boat’s Raytheon radar, and Autohelm depth, wind and speed meters. And we have kept up with the latest Nobeltec software, now the Visual Navigation Suite 4.1. We have since replaced the Magellan GPS because its LCD display became unreadable. Now we have the Furuno GP30. It has a fantastic LCD display.
Two Problems and their solutions: The first problem we noticed happened on our trip home from the dealer with our brand new boat. We motored out into Long Island Sound and headed from New London towards Plum Gut about 15 miles away. We set sail and motor-sailed ESE towards the Gut. We programmed its Lat/Lon coordinates into the GPS, turned the autopilot on, put it into AUTO and then TRACK mode to track the GPS. This it did right away, but only for a couple of minutes. The boat soon started to veer quickly upwind and then come about. As we ran to the wheel to take control we noticed that the autopilot indicated that it was in STANDBY mode rather than TRACK. We checked that the GPS was reading correctly which it was. Then we reset the autopilot to try it again in TRACK mode, this time keeping a careful eye on it. A couple of minutes later the autopilot lost control again and went back into STANDBY. Very frustrated, we manually steered the remainder of the trip home. The saga of this failure and switching to STANDBY is a long one of trying several different fixes including repowering, ground strapping, etc. A number of contacts were made at boat shows and phone calls to Autohelm concerning the problem. Autohelm even contracted a service technician in our area to try to trace the problem. We won’t go through all the details of all the parts that were replaced and all the trial runs and failures that ensued.
To make this story short we jump to last summer when we had our engine replaced and also put in a new alternator regulator. The miraculous thing that happened was that the autopilot began working flawlessly. For the remainder of last summer we used the autopilot by itself as well as in conjunction with the GPS and it did all we asked of it both under motor and under sail. It handled fairly heavy seas downwind when the autopilot motor is most taxed. Not a hiccup, not a whimper. The bottom line, and this is only an educated guess, is that there was never a problem with the Autohelm instruments. The problem was when the boat was under motor, the alternator regulator was sending out dirty power and would give out momentary glitches that would cause the autopilot to go into standby. The new regulator and/or the new wiring to the regulator have cleaned up the problem and we can relax a little more on longer runs “hands free.” We extend our apologies to the Autohelm Corporation for the trouble they went through and for the less than friendly tones we gave them on the phone as the problem solving process seemed to go nowhere. And we note the expertise of Brewer Yacht Yard, Greenport in replacing our engine and setting up our alternator regulator properly.
Problem 2: In using our computer software for navigation we discovered that one item of communication was not getting through to the computer, variation. We hooked up the NMEA output of our GPS to the Course Computer 100 of the autopilot. The Course Computer NMEA output was connected to our Raytheon radar. We paralleled that signal and plugged it into the COM1 port of our laptop computer. It continually read GPS OK and was able to tell very well where we were and gave us all the particulars from the GPS about our heading, speed over ground, time to destination, etc. But what was missing was information from the GPS about the local variation (in our case 15 degrees west). In the NMEA 0183 sentence $GPRMC, the GPS sends out the GMT time, latitude and longitude readings and its calculation of the local variation followed by E or W. The importance of this information for the computer is that any magnetic bearing between two points needs the local magnetic variation to be accurate. With the GPS turned on and the computer showing the variation at the boat’s position our variation number kept changing every several seconds.
The solution was to do a test of the actual NMEA signals coming out of the Autohelm Course Computer. We found that the $GPRMC statement that had the variation information in it was missing. The instruction book for the Course Computer didn’t mention that statement either. The change we made was to move the wires that feed the radar/computer from the NMEA output of the Course Computer to the NMEA input thus paralleling the cable coming from the GPS. Thereby the $GPRMC statement from the GPS made it all the way through to the computer. The GPS became the sole sender and all other instruments became listeners. The result was that the computer locked into the local variation and all seems to be well again.
p.s. We are looking forward to the improved GPS signals now that the government has shut off its “Selective Availability” thus giving us a much greater accuracy without having to install the DGPS receiver and antenna.