I have mentioned that one of the things that has really worked well for us is our scanning sonar. Of all the electronics, I initially thought I was being extravagant when I chose to put it on the boat. Not any more!
Scanning sonar is like an underwater radar. Radar shows the location of objects above the water – rain showers, other boats, and land masses. It is the electronic equivalent of a map displaying everything around you with simple methods to determine the direction and distance things are from the boat.
With the newer systems like those on Iron lady, you can actually lay the radar image over the electronic image of a nautical chart. Why would you do that? Because many of the charts (even the electronic versions) were created from surveys made as early as the seventeen and eighteen hundreds and they have not been significantly updated since. Thus, land masses on the electronic charts do not necessarily coincide with the actual land mass location that that the radar sees. On this trip, there have been a number of occasions where if we had believed the chart, we would have been on the reef. There have been other occasions where we have been safely at anchor but the GPS location on the chart puts us on dry land.
The radar overlay gives us a good idea of just how accurate the charts are since the radar “sees” precisely where the land is – and not where the chart says it should be.
The obvious corrolary to this is that underwater obstacles are charted with even less accuracy then the land masses shown on the charts. Hence the value of a scanning sonar that looks out around the boat and “sees” what the underwater terrain is.
The passage in to Nava’U, while not dangerous, does a pretty good job of showing what I am talking about.
In this first image, you can see the boat overlaid on the electronic chart as we approach a section of narrows entering the harbor. The red on the image is the radar overlay, so you can see that the chart and radar are in very good agreement about the location of things. That adds a significant comfort level to the transit.
In the second image, the boat is approaching the first set of narrows.
This is the image that the scanning sonar is seeing under the water at the same time. The red indicates hard bottom and the numbers at the top right of the screen show that the water directly ahead 160 meters from the boat (500 feet) is 10 meters deep – just about what the chart shows. Another confidence builder.
In the next shot, you can see the boat approaching the second set of narrows where will have to make a turn to starboard as we pass.
Taken at the same time, the scanning sonar is showing precisely the same thing. Our route will keep us in the blue color (deeper) water if we turn to starboard as we approach the narrows. To continue straight at this point would put us on the rocks.
We use the sonar all the time. For entering anchorages scanning for coral heads, reefs and shallow water. Once at anchor, we do a full 360 degree scan to make sure nothing dangerous is in range as we swing about the anchor.
Makes you sleep a whole lot better.
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