Sunday, August 13, 2017

Dawho River - Beware the Magenta Line

We all know that the magenta line does not always lead you to the deepest water, in fact, it often does not. It's a handy guide to finding the general direction but you often need more data. I compiled a series of charts by various navigation programs to compare the routes through one of the more challenging areas of the ICW, namely, the east entrance to the Dawho River south of Charleston.

Below is the chart as shown by Navimatics Charts and Tides with a route plotted that I followed for a minimum of 5.5 MLW on 4/25/2017.

The chart correctly shows deeper water 400 to 450 ft north of G115. The yellow line is the path I took on 4/25/2017. If you have the latest charts, you should see a similar path highlighted corresponding to the lighter shade of blue above.

Next up is the same area as shown by the Charleston ACOE survey of 11/17/2016.

Unfortunately, the original channel has shoaled down to 3 ft but it's deeper farther north. Also unfortunately, the Charleston ACOE does not survey outside the old channel so we are left to our own devices - trial and error. I hand-drew the blue line to show the same route shown in the Navimatics example at the top.

Let's take a look at the Garmin chart of the same area.

In this case, the brown line is the exact path I took on 4/25/2017 for 5.5 MLW. I converted it into a GPX route which can be download at BDawho. Notice where the magenta line goes, it's much too far south. Compare that excursion south against the ACOE survey chart, it's shallow!

Next up is the Navionics chart for Dawho.

It shows a better depth using the more northerly route but the magenta line is still too far south.

Finally, let's take a look at the Navionics Sonar chart of the same area.

This is not bad although it shows less depth than what I experienced following the blue route. The Sonar chart by Navionics looks good for this passage but you never know the time stamp on the soundings and which areas are based on actual soundings and which areas are interpolations (or whatever). It would be nice if the Sonar chart showed where real data was used and given a time stamp.

So what to do? Active Captain is a source of current data along with alerts from Cruisernet and similar services. To get everything in one place, the 2017 ICW Cruising Guide is another option, which also comes in ebook format (free, with the hardcopy version). As I head south, I'll be updating Active Captain posts under the handle Bob423 and adjusting the GPX routes as needed. After all, the whole point of cruising down the ICW is to enjoy the ride. There are lots of sunsets to see, friends to meet, and quiet times just enjoying the trip.