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

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

We’re racing a 40-footer, and on handicap it’s the quickest boat on this fleet. So, we’re main everybody in the direction of the Lee-on-Solent seaside, going to a mark that’s a mile away upwind. We all know the left-hand facet can pay on this leg, we’ve executed it many instances and, thus far, the technique – to win the pin and get as far in in the direction of the seaside as we are able to – is working.

The large query for the navigator at this second (and that’s me) is how far we are able to go earlier than we’re pressured to tack by a scarcity of water. I’m engaged on it – or a minimum of I might be if I had a bit extra religion within the accuracy of the wind path. It’s early within the season and the calibration of the wind angles is a long way from being sorted. So, I don’t have a solution to the query that the tactician actually must know the reply to. At the least, not a solution that I’m assured in.

I preserve rechecking it as a result of one thing doesn’t really feel proper, and I can’t fairly put my finger on it. Nonetheless, even permitting for that feeling, I’m actually not anticipating what occurs subsequent. The boat hits the underside with a horrible lurch, tipping everybody ahead. I look on the depth a second after the affect, and it reads 5m of water below the keel.

This, I be aware – slamming the door after the horse has bolted – was in all probability what didn’t fairly really feel proper. Step one was to attempt to sail the boat off. This received us nowhere, and it shortly turned obvious we weren’t going to be ending the race. The second step was to place the engine on and attempt to motor off. No cube there both.

The third and remaining step was to name within the rescue boat and get them to attempt to tow us off, and when even that wasn’t working… ouch. It was going to be messy if we have been nonetheless there at low tide with the racing paint wallowing within the mud and gravel. Ultimately, it was plain luck that we received the yacht safely again afloat.

That and the horsepower of the rescue boat. At which level, we lower our losses and went dwelling. It didn’t take lengthy on the best way again to the dock to work out what had gone improper; the boat had simply had a brand-new instrument system processor/pc put in. A few of the calibration values had been (very helpfully) transferred throughout from the previous system – and therein lay the issue.

Calibrating Depth

Now, depth is without doubt one of the extra simple calibrations that should be executed when organising a yacht’s instrument system, largely as a result of there’s a mounted measurement that it may be calibrated in opposition to – the depth of the water. Nevertheless, a warning; depth finders generally have bother getting clear readings in crowded marinas.

So, if it’s studying erratically in any means, wait until you get a peaceful day and do the calibration when you’re stopped someplace outdoors the marina. A second level to be careful for is that the depth is usually a quite fuzzy measurement if it’s a muddy backside, so attempt to discover a strong seabed.

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I haven’t been on a ship with an precise leadline since I used to be a Sea Scout, but it surely’s straightforward sufficient to improvise. I often use a winch deal with tied (securely!) and taped to the top of a line, however any heavy weight will do. I drop it over the gunwale, ensuring I’m adjoining to the depth sensor, and let it sink so the winch deal with is simply touching the underside.

If it’s a pleasant strong seabed I can really feel when it lands, ensuring that the halyard is tight, making a straight line between the winch deal with on the seabed and my hand on the floor (this may be troublesome if there’s a number of present). Then, simply pinch the halyard the place it exits the water and, when you’re anxious about dropping the spot, wrap a flip of tape or thread tightly round it.

Haul it again up, pull the halyard tight between the underside of the winch deal with and the taped mark, and measure the gap. As soon as we all know the depth of water from the floor to the seabed there’s a selection. The depth sensor may have a calibration quantity, which is an offset.

This may be set to measure and browse the depth from the floor, or to measure and browse the depth beneath the keel sorts or rudders (relying on which is deeper). It’s simple to set it as much as learn depth from the floor; the calibration quantity/offset must be set to the gap between the sensor and the waterline.

The depth sensor (this from A+T) is a deceptively easy wanting gadget, however immensely helpful… if calibrated accurately

Now, we’re including this quantity to the depth the sensor is measuring, so we have to enter the quantity with a constructive (+) signal. Even when we don’t know this distance, we do know the depth of the water, and the calibration quantity/offset must be entered (utilizing trial and error if mandatory) as the worth that makes the quantity on the instrument show match the measured depth. Or perhaps a bit bit lower than the measured depth, in order that any error is in your favour!

Nevertheless, I’ve at all times set the depth sensor to learn depth of water below the keel. That is the quantity that issues to me, and it’s rather more related to how I exploit the sensor. The tides vary within the Solent means there’s round 3-4m of distinction in depth from excessive water to low water, so it takes a little bit of time to work out what the water depth must be at any given spot. And time is one thing I don’t at all times have on a race boat.

Depth Underneath the Keel

There are a couple of conditions the place it’s value understanding the water depth upfront of being someplace – when a route choice contains crusing over a sandbank or reef, as an example. In any other case, and that’s for 99% of the time I spend racing within the Solent, I sail to the measured depth – in any case, when you’re making an attempt to get out of a foul tide, you then’re going to go as shut in as you dare, managing the chance relying on the circumstances.

To arrange the devices to show depth below the keel, we’ll must subtract the gap between the sensor and the underside of the keel from the sensor’s measurement of depth. So we’re going to enter a calibration quantity/offset with a detrimental (–) signal. The space could be obtained from a yacht design drawing, or measured when the boat is out of the water.

Within the case of this explicit yacht it was 2.5m between the sensor and the underside of the keel – a quantity I had in my calibration notes. So, it ought to have been straightforward, on that vibrant and breezy Sunday morning, to examine the quantity entered into the brand new devices was minus 2.5m. Sadly, whereas the best quantity had been entered into within the new processor, it had been entered with the improper signal. As an alternative of subtracting 2.5m of water from the studying, it was including it. So, zero depth was subsequently studying as 5m of water below the keel.

Awkward

On that Sunday I’d been preoccupied with the calibrations that management the wind path information. I’d been taking the depth sensor with no consideration, and on this event received bitten arduous by the belief. My data was – fairly actually – not conserving me afloat.

It was a really explicit form of embarrassment to finish up on the Lee-on-Solent seaside in entrance of the entire Hamble River race crowd because of a measurement error – however these are the teachings we are able to study from. So, this one’s on me: at all times examine the calibrations or the readings on all the principle instrument sensors earlier than you allow the dock; obvious wind velocity and angle, compass, boatspeed, depth and GPS… however particularly when you will have executed any work on the instrument processor or pc.

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