We are often asked, “Is the depth instrument on this boat set to the depth of water or depth below keel?”
This is a question we will not likely answer. The answer is probably, “Neither!” and I’ll explain why:
There is a setting in most depth instruments called ‘offset’. Anyone or their guests can easily change this setting with a few button presses. This is the part that makes us reluctant to answer the above question. What if we answered it and we were wrong – and that caused you to run aground? Unless you know all of the details about the specific boat you are on (Depth of the sounder, depth of the keel) AND how to check or change the offset, there is only one way to be sure about the depth reading that you are seeing on your instruments. That method involves using a lead-line or another means of measuring the water depth and comparing it to what your sounder reads.
We are going to help with this by doing one of two things at first opportunity: Either having lead-lines available for sale in the office or painting actual depths on some of the pilings around where our boats enter and exit D-dock. I prefer the second option for business reasons, but the first would help you wherever you happened to be on a boat, so both are under consideration. If we were to paint depth markings on a piling, you could compare water depth on the markings as you pass the piling to that of your sounder and would then know your offset.
Now on to the good stuff: How does it work?
Check out the diagram:
As you can see, the sounder is about 2′ below the water-line on this boat, so:
If the ‘offset’ in the depth instrument settings is set to ‘0’ (this is most often how you will find them and why I said earlier the answer to the question is most likely, “Neither!”), the instrument reads 8′. 8′ is not the depth of the water, nor is it the depth below the keel!
If the ‘offset’ is -4, (8′ minus 4′), the instrument reads 4′, or depth below keel.
If the offset is +2, (8′ plus 2′), the instrument reads 10′, or depth of water.
Now on to the next question we are asked during this conversation, “What setting do you prefer?”
Most people I have this conversation with answer with something along the lines of, “I want it set to depth below keel so that I don’t have to do any math – I just know how much water is between the bottom of my keel and the earth!”
At first glance, this would seem the simplest method. If that’s what works for you, great – as long as it keeps you out of the mud!
My personal preference is depth of water, and this comes from personal experience navigating in waters that I am unfamiliar with. As an example, in the Grenadines or around the islands of Tahiti, there are plenty of reefs which you would like to navigate around. These pop up very quickly from a safe depth to almost no depth at all, making them very dangerous. They are well marked on the charts and on the chart-plotters. Imagine I am picking my way through a passage where there are visible reefs on one side, scattered invisible reefs on the other, and the water is so clear that it is almost impossible to tell if the rocks you are looking at over the bow are 15′ down or 3′ down under the water. My main concern when picking my way through is to know at all times exactly where I am in comparison to my chart. In order to know this I use the visual marks around me to determine where believe I am on the paper chart (always folded to show my current area and held in one hand) while repeatedly scanning my chart plotter and depth meter to make sure that all three match. If all three sources match, I am comfortable that I am navigating safely and am not going to run aground on a bunch of jagged rocks and coral!
What does this have to do with the offset? Easy: If I am set to depth of water, the reading on my depth meter matches exactly with the reading on my chart plotter and the reading on my paper chart without having the do any conversions except for current tide. If I were set to depth below keel, I’d have to check the chart, add in the tide, and then add in the difference between the bottom of my keel and where my sounder is mounted to know if my three reference points all match! It’s not a lot of math, but enough that it will tire the brain when you are navigating in unknown waters and constantly comparing your three sources of information. Most places we travel, I already have to do the conversion from meters to feet in order to form a good mental picture of what’s actually under the boat, since the charts, plotters, and depth meters all use the metric system!
I hope this helps keep you out of the mud and off of the rocks!