How to check your depth sensor calibration (and why you need to)

Mark Chisnell recalls how a tiny calibration error with a boat's new instrument system turned a tactical move into an awkward grounding

It’s the second day of a race weekend, an early season shakedown for boat and crew. A pleasant 12-14 knots is blowing from the eastern entrance of the Solent. The first upwind leg takes us eastwards from Hill Head, along the mainland shore towards Portsmouth and the shining white Spinnaker Tower.

We’re racing a 40-footer, and on handicap it’s the quickest boat in this fleet. So, we’re leading everyone towards the Lee-on-Solent beach, going to a mark that’s a mile away upwind. We know the left-hand side will pay on this leg, we’ve done it many times and, so far, the strategy – to win the pin and get as far in towards the beach as we can – is working.

The big question for the navigator at this moment (and that’s me) is how far we can go before we’re forced to tack by a lack of water. I’m working on it – or at least I would be if I had a bit more faith in the accuracy of the wind direction. It’s early in the season and the calibration of the wind angles is some distance from being sorted. So, I don’t have an answer to the question that the tactician really needs to know the answer to. At least, not an answer that I’m confident in.

I keep rechecking it because something doesn’t feel right, and I can’t quite put my finger on it. Still, even allowing for that feeling, I’m really not expecting what happens next. The boat hits the bottom with a horrible lurch, tipping everyone forward. I glance at the depth a moment after the impact, and it reads 5m of water under the keel.

This, I note – slamming the door after the horse has bolted – was probably what didn’t quite feel right. The first step was to try and sail the boat off. This got us nowhere, and it quickly became apparent we weren’t going to be finishing the race. The second step was to put the engine on and try to motor off. No dice there either.

The third and final step was to call in the rescue boat and get them to try and tow us off, and when even that wasn’t working… ouch. It was going to be messy if we were still there at low tide with the racing paint wallowing in the mud and gravel. In the end, it was plain luck that we got the yacht safely back afloat.

That and the horsepower of the rescue boat. At which point, we cut our losses and went home. It didn’t take long on the way back to the dock to work out what had gone wrong; the boat had just had a brand-new instrument system processor/computer installed. Some of the calibration values had been (very helpfully) transferred across from the old system – and therein lay the problem.

Calibrating Depth

Now, depth is one of the more straightforward calibrations that must be done when setting up a yacht’s instrument system, largely because there is a fixed measurement that it can be calibrated against – the depth of the water. However, a warning; depth finders sometimes have trouble getting clear readings in crowded marinas.

So, if it’s reading erratically in any way, wait till you get a calm day and do the calibration while you’re stopped somewhere outside the marina. A second point to watch out for is that the depth can be a rather fuzzy measurement if it is a muddy bottom, so try to find a solid seabed.

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I haven’t been on a boat with an actual leadline since I was a Sea Scout, but it’s easy enough to improvise. I usually use a winch handle tied (securely!) and taped to the end of a line, but any heavy weight will do. I drop it over the gunwale, making sure I’m adjacent to the depth sensor, and let it sink so the winch handle is just touching the bottom.

If it’s a nice solid seabed I can feel when it lands, making sure that the halyard is tight, creating a straight line between the winch handle on the seabed and my hand at the surface (this can be difficult if there’s a lot of current). Then, just pinch the halyard where it exits the water and, if you’re worried about losing the spot, wrap a turn of tape or thread tightly around it.

Haul it back up, pull the halyard tight between the bottom of the winch handle and the taped mark, and measure the distance. Once we know the depth of water from the surface to the seabed there’s a choice. The depth sensor will have a calibration number, which is an offset.

This can be set to measure and read the depth from the surface, or to measure and read the depth below the keel types or rudders (depending on which is deeper). It’s straightforward to set it up to read depth from the surface; the calibration number/offset should be set to the distance between the sensor and the waterline.

Now, we’re adding this amount to the depth the sensor is measuring, so we need to enter the number with a positive (+) sign. Even if we don’t know this distance, we do know the depth of the water, and the calibration number/offset should be entered (using trial and error if necessary) as the value that makes the number on the instrument display match the measured depth. Or maybe a little bit less than the measured depth, so that any error is in your favour!

However, I’ve always set the depth sensor to read depth of water under the keel. This is the number that matters to me, and it’s much more relevant to how I use the sensor. The tides range in the Solent means there’s around 3-4m of difference in depth from high water to low water, so it takes a bit of time to work out what the water depth should be at any given spot. And time is something I don’t always have on a race boat.

Depth Under the Keel

There are a few situations where it’s worth working out the water depth in advance of being somewhere – when a route option includes sailing over a sandbank or reef, for instance. Otherwise, and that’s for 99% of the time I spend racing in the Solent, I sail to the measured depth – after all, if you’re trying to get out of a foul tide, then you’re going to go as close in as you dare, managing the risk depending on the circumstances.

To set up the instruments to display depth under the keel, we’ll need to subtract the distance between the sensor and the bottom of the keel from the sensor’s measurement of depth. So we’re going to enter a calibration number/offset with a negative (–) sign. The distance can be obtained from a yacht design drawing, or measured when the boat is out of the water.

In the case of this particular yacht it was 2.5m between the sensor and the bottom of the keel – a number I had in my calibration notes. So, it should have been easy, on that bright and breezy Sunday morning, to check the number entered into the new instruments was minus 2.5m. Unfortunately, while the right number had been entered into in the new processor, it had been entered with the wrong sign. Instead of subtracting 2.5m of water from the reading, it was adding it. So, zero depth was therefore reading as 5m of water under the keel.

Awkward

On that Sunday I’d been preoccupied with the calibrations that control the wind direction data. I’d been taking the depth sensor for granted, and on this occasion got bitten hard by the assumption. My knowledge was – quite literally – not keeping me afloat.

It was a very particular kind of embarrassment to end up on the Lee-on-Solent beach in front of the whole Hamble River race crowd thanks to a measurement error – but these are the lessons we can learn from. So, this one’s on me: always check the calibrations or the readings on all the main instrument sensors before you leave the dock; apparent wind speed and angle, compass, boatspeed, depth and GPS… but especially when you have done any work on the instrument processor or computer.

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