The Blue View - Repairing the GPS

We have a nice Raymarine chartplotter/GPS in the cockpit that we installed in 2004. The teak panel it is attached to has a cutout that was made to fit it. Unfortunately, it died about three years ago, and when we tried to order a replacement, we found that it was no longer being produced. This was somewhat of a dilemma since we couldn't find anything else that would fit the cutout. We did manage to find a used one on eBay, but it, too, died about a year later.

Eventually, we will buy another panel mount GPS and modify or replace the teak panel, but in the meantime, it isn't all that critical. We have several other GPSs aboard -  a couple of handhelds, an antenna that feeds locational information to our Nav station chartplotter, and our tablets all have GPSs built in. We like having GPS information displayed in the cockpit, however, so I mounted one of the handhelds on the pedestal.

The problem with our handhelds is that they run on two AA batteries which need to be replaced every ten hours or so. On a long passage, that's a lot of batteries. We have Garmin handhelds which do provide the option of using a power cable, however, so I routed a cable to the cockpit. Everything was fine for about six months. Unfortunately, the power cable is not all that weather resistant, and the contacts soon corroded. No problem – I cut the connector off and soldered the wires directly to the GPS contacts, then gooped it all up with silicone. That lasted about a year and a half, but eventually, enough moisture made its way in to corrode these connections as well. It stopped working about a week ago on our first attempt to leave Cape Town.

So our options were: do without, find a GPS locally that will fit into the teak cutout, or start over with another handheld GPS. There was one more option. I could make a 12 VDC to 3.2 VDC power converter and connect it to the battery contacts inside the battery compartment of the GPS. Since the battery compartment is nearly watertight, the connections inside should last a long time.

The voltage converter is a trivial circuit, and the parts can be found at any electronic hobby outlet like Radio Shack, Jaycar, or Jayco. Being the geek I am, I actually had the parts I needed aboard. I built, tested and installed the circuit on a rainy afternoon.

The circuit is based on an LM317 adjustable voltage regulator. By selecting the correct resistors, the device will convert the vessel's 10.5-14 VDC to any voltage between 1.2 VDC to about 8 VDC. All that is needed is the LM317, two resistors and two capacitors, plus a small generic circuit board to mount everything.

The output voltage is determined by the two resistors based on the following equation: Vout = 1.25 * (1+R2/R1) + Iadj * R2 Iadj is very small, and if we keep R2 small, the last term can be ignored. This simplifies the equation to: Vout = 1.25 + 1.25(R2/R1)

For an output of 3.2VDC, if we select a value of 240 ohms for R1, the equation becomes: = 1.25 + 1.25(R2/240) Solving for R2: R2 = (3.2-1.25)*240/1.25 = 374 ohms. Since 374 ohms is not a standard size, I used the closest standard size of 360 ohms for R2.

I mounted the circuit board inside the battery compartment – it was smaller than one AA battery – using hot melt from a glue gun to hold it in place. I drilled a hole in the battery compartment just large enough to slide the power cable through, then soldered it to the input pins of the LM317. The output of the regulator was connected to the plus and minus battery connections.

If your soldering technique is a bit rusty, we have three short videos on the subject. BTW, completing this project earns you a Geek 1st Class merit badge.

Editor's note from Marcie: I would rather suck water out of the bilge than re-read this blog post, but I know you geeks out there will appreciate it. ;-)

The Blue View - Where is Daw Island?

 

It is difficult to pin down the exact location of Daw Island, our destination on the west end of the Great Australian Bight. Our Australian paper charts give its position as 33S51, 124E04. Our Raymarine chartplotter gives the position of the anchorage as 33S50.55, 124E08.35, about 3.75 nautical miles further east. We use Navionics Gold software on the chartplotter, but also have Navionics on our iPad. Even the Navionics software on the two devices differed. The satellite view provided by Google Earth gives the position of the anchorage as 33S50.55, 124E08.04, about 0.25 nm west of the chartplotter location. So where the heck is it anyway? Which position do we use? Take the average?

Modern GPS based electronics will locate you on the earth's surface to within a few feet. This accuracy is astounding, and it becomes very easy to rely on those electronic marvels almost exclusively. Therein lies a danger.

Many of the nautical charts, both electronic and paper, are based on surveys that were done before the advent of the GPS. The GPS may locate you on the earth's surface to within a few feet, but the chart may be off by as much as half a mile or more. Thus, the chartplotter may show you being well clear of an offlying danger, for example, when in actuality you may be about to hit it. Conversely, there have been any number of times our chartplotter showed Nine of Cups being high and dry in the middle of an island when we were safely anchored in 50 feet of water a fair distance off the beach.

Fortunately, Daw Island is easy to find in the daylight. It is by far the tallest and largest island in the immediate area. We used the chart plotter to get us to the general vicinity, made sure we arrived in the daytime, then used a combination of the paper charts, electronic charts and our eyes once we could see the island. The shoals and rocks lying off its west and north ends were easy to spot, so it was relatively straightforward to find our way into the anchorage without running into any hard spots along the way.

So which position was correct? This time it was Google Earth. This isn't always the case, however. You would think that a satellite photo would be right on, but we have been in a number of places that Google Earth was also off by a significant amount, along with the chartplotter and paper charts, and all by differing amounts.

The speed at which navigation technology is evolving makes me think that all these issues are short term. A decade from now, we will all undoubtedly have 3-D systems that will safely navigate our vessels for us, between any two points in any of the oceans of the world. Until then, however, I don't think we will be relying entirely on our GPS and chartplotter to find the next anchorage for us, no matter how much it thinks it knows where it is.