Depth Sounder / Depth Meter Explanation

, Depth Sounder / Depth Meter Explanation

When sailing school students step on our boats, one of the first questions they ask is how to read and understand the depth meter. And what a good place to start thinking about boating!

, Depth Sounder / Depth Meter Explanation

How the depth sounder works

The depth meter sends the signal from the transducer at the through-hull fitting to the bottom of the ocean floor. The transducer counts how long the signal takes to go there and bounce back, does some fancy math, and spits out the answer.

, Depth Sounder / Depth Meter Explanation

Sorting out the numbers

On most of our 30 to 40-foot sailboats, our magic number is three. By that, I mean when the gauge reads three feet, the boat will be aground.

That is because there is three feet clearance between the transducer and the bottom of the keel. That’s not three feet from the waterline, or the top of the freeboard, or even the cabin top. It’s from the transducer to the ocean floor.

, Depth Sounder / Depth Meter Explanation

Adjusting the readout

The transducer reporting digital readout can be adjusted to read zero when you are aground. I think for years and years, maybe it was not possible to calibrate the device, so maybe everyone got used to the number three meaning aground. Or, if it was possible, it was like setting the blinking time readout on a VCR; just too much trouble.

, Depth Sounder / Depth Meter Explanation

So, when the transducer reads anything less than three, we are in a panic. If it was set to read that zero means we are aground, we would probably be pulling in and out of our slips actually with anywhere from one to two feet of clearance, but with that readout, we would think we are plowing through mud.

That must be why people are reluctant to calibrate them to read the true depth. Possibly they are reluctant to consult the manual.

, Depth Sounder / Depth Meter Explanation

Three different ways to measure depth

1. Depth from the top of the water to the sea floor. 

To get that depth, you would have to add from the transducer to the waterline.

2. You can have a reading from the transducer to the bottom of the seafloor.  

This requires no adjustment. The device comes out the box reading from it to wherever its signal bounces.

3. You could have the instrument read what is actually between the very bottom of your keel and the sea floor. 

This would require you subtracted the distance between the bottom of the keel to the transducer in order to get a “clearance only“ read out.

, Depth Sounder / Depth Meter Explanation

Depth and Anchoring

When you’re anchoring, you are using the depth meter to decide how much scope to lay out. 

It’s actually rather misleading to use your depth meter readout to determine scope unless you have it set to read from the bottom to the waterline. The depth of the water is not really 12 feet; it is more likely closer to 15 feet, and then you still need to add another three feet to your depth calculation to add your freeboard!

, Depth Sounder / Depth Meter Explanation
The line to the mooring ball covers the distance known as “freeboard” – the length between the waterline and the top of the rail.

So, if your depth meter is reading 12 feet, and the transducer is three feet below your waterline; you need to figure for 15 feet. Then you need to add another three feet from your waterline to your anchor roller! That gives you 18 feet times your preferred ratio of line to depth. I guess this is why I always just go ahead and multiply it by 10 instead of seven! 

Which do you prefer?

Your depth meter to read actual depth, depth from the transducer, or clearance below the keel? Please leave your comments below.

What to read next: How to Pick up a Mooring Ball

Coming Soon – Anchoring!

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  • David Gale February 23, 2019 at 5:25 pm

    Nice piece in today’s ASA newsletter!

    • Kimberly February 25, 2019 at 3:27 pm

      Thank you so much – I really appreciate your visit!

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