Sunday, May 31, 2009

Navigating by Color

We're all used to navigating using a map (or a chart if afloat). Most of us even combine the map with a GPS to get an up-to-the-second birds-eye view of our location. Since we arrived in the Bahamas however, I've had to learn a new technique - color. That is, the color of the water.

The water here is exceptionally clear. Its not unusual to be able to look down into 20' of water and see the bottom as clear as a bell. It's also very shallow. Huge areas are less than 20' deep and all the interesting parts less than 10'. There's a limit to the accuracy of both charts and GPS and that limit is far greater than you'd like to trust when your props are concerned. Hitting bottom is not good and if the bottom is rocky, can make for a really bad day, so its something to be avoided.

So when we nose into an anchorage, I use the color of the water to tell me where to go. It's called VPR - visual piloting rules. Our boat goes 3' 8" below the water (aka draught), the deepest part being our props at the back.

Deep blue water is over 30' deep - in the case of the ocean thousands of feet - and is no problem as far as depth goes. Between 30' and 20' the water turns a lighter shade of blue. The Bahamian Bank is mostly this color. 20' - 10' and there's another color change, this time green. Between 10' and around 6', the water is a light green-blue. Less than 6' and its white. While 6' is still plenty deep enough, there's no way to tell how shallow it goes, so its to be avoided.

These colors assume a white, sandy bottom. Grass makes everything look darker, as do cloud shadows, so you need to keep an eye on the chart to have a rough idea of the depth. Also complicating things are rocks and coral heads. These show up as black blobs. Unless the blob is moving, in which case its a stingray or a shark. Reefs usually show up as brown patches.

So the rule is, stay out of the white and avoid black and brown. I also keep an eye out for the tide when anchoring as a 3' drop when at 6' will ground us out.

When we're heading in or out through a shallow channel, Steph is performing her "Titanic" impression, standing at the bow, keeping a look out for rocks and coral heads.


Louise said...

We learned about this in one of our trawler schools, so it is interesting to know you really use the colors.

Martin said...

It really takes clear water to work. As I'm sure you saw during your school, the water in the US isn't really clear enough. Even in the keys, it's not as obvious as it is here.