Sounder — noun — from the French sonde: a probe.
Does your depth sounder tell you the depth of the water? It might. But is that the number you need?
Unless calibrated with an offset, your depth sounder merely tells you the distance between the transducer and the seabed. I guess that’s an interesting bit of trivia, but we can do better.
What is the purpose of a depth sounder? I like to use mine to:
help me avoid running aground and
calculate the scope of my anchor rode.
Not running aground is my most common use of the depth sounder, so what I want to know is:
How much water is between my boat and the seabed?
Let’s say I turn on my depth sounder and it reads 7 feet. This could mean:
There is 7’ of water from the transducer to the seabed.
There is 7’ of water from the surface to the seabed.
There is 7’ of water from the bottom of my keel to the seabed.
Number one is the most likely, as depth sounders are often installed with no offset programmed. They will simply display the distance from the transducer to the hard stuff below. However, transducers are not usually installed on the deepest part of your boat, especially if on a sailboat. There could be several more feet of boat below the transducer, meaning your boat will be aground well before the depth sounder reads 0’.
Number two requires an offset to be programmed into the depth sounder unless the transducer is at the waterline, not very common on sailboats or larger powerboats. This is also not a particularly useful number unless you’re calculating your anchor rode scope. Even then, you still have to add in the height of your deck above the waterline. Otherwise, the total depth of the water is not particularly useful to know aboard a boat.
Number three is the magic number I want to know to avoid running aground. When navigating shallow waters, I prefer to avoid doing math several times per minute to figure out how much water is below my keel. When my depth sounder says 1’, I should have 1 foot of water below my keel. When it says 0, I should be aground. Easy! If I want to calculate my anchor rode scope, I know my draft plus deck height at the bow is about 8’; I need to add 8 to my scope formula once.
How To Determine Your Offset
So how to calibrate your depth sounder to give you the most useful number? Every depth sounder is different, but most of them have a way to program an offset. Refer to your manual on how to do that. What I can help you with is how to figure out what that offset needs to be.
For this, we need two bits of information:
The water’s depth
Your boat’s draft
To determine the water’s depth, use a leadline (or a weight on a line) to measure the distance from the surface to the seabed. This job is easiest done from a dock or your swim ladder as you will need to reach the line with your hand right at the water’s surface.
Lower the weight until you feel the line go slack.
Keeping the line slightly taught, but not lifting the weight from the seabed, get down and tie an overhand knot on a bight, or otherwise mark the line, at the surface.
Bring up the weight, lay the line out on the dock or deck, and measure with a tape measure from the bottom of the weight to the knot.
Take this opportunity to measure your wingspan from pinched fingers to pinched fingers. Know this distance, and you won’t need a tape measure next time.
Ready for some math?
Let’s say you measured the water’s depth at 10’, with the depth sounder reading 7’. This should mean the sounder is mounted 10'–7' = 3’ below the waterline. If true, your depth sounder should currently have an offset of 0’. Confirm by checking your sounder’s programmed offset. If offset = 0, you now know that the transducer’s location is 3’ below the surface.
Let’s say you know your vessel’s draft to be 6’.
10–6 = 4, so there should be 4’ of water below your keel. Therefore, we want the depth sounder to read 4’ instead of 7’. 7 – 4 = 3.
If your offset is currently 0, set it to -3, so it reads 4.
The most helpful thing your depth sounder can do is let you know how far you are from running aground (water depth — draft). Plugin your offset and enjoy more sailing with less math.