Mark Chisnell recalls how a tiny calibration error with a boat’s new instrument system turned a tactical move into an awkward groundingIt\u2019s 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.
\nWe\u2019re racing a 40-footer, and on handicap it\u2019s the quickest boat in this fleet. So, we\u2019re leading everyone towards the Lee-on-Solent beach, going to a mark that\u2019s a mile away upwind. We know the left-hand side will pay on this leg, we\u2019ve done it many times and, so far, the strategy \u2013 to win the pin and get as far in towards the beach as we can \u2013 is working.
\nThe big question for the navigator at this moment (and that\u2019s me) is how far we can go before we\u2019re forced to tack by a lack of water. I\u2019m working on it \u2013 or at least I would be if I had a bit more faith in the accuracy of the wind direction. It\u2019s early in the season and the calibration of the wind angles is some distance from being sorted. So, I don\u2019t have an answer to the question that the tactician really needs to know the answer to. At least, not an answer that I\u2019m confident in.
\nI keep rechecking it because something doesn\u2019t feel right, and I can\u2019t quite put my finger on it. Still, even allowing for that feeling, I\u2019m 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.
\nThis, I note \u2013 slamming the door after the horse has bolted \u2013 was probably what didn\u2019t 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\u2019t going to be finishing the race. The second step was to put the engine on and try to motor off. No dice there either.
\nA classic run down the Green for Cowes Week competitors this year required confidence in their yacht\u2019s depth calibration. Photo: Paul Wyeth\/CWL
\nThe 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\u2019t working\u2026 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.
\nThat and the horsepower of the rescue boat. At which point, we cut our losses and went home. It didn\u2019t 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 \u2013 and therein lay the problem.
\nCalibrating Depth
\nNow, depth is one of the more straightforward calibrations that must be done when setting up a yacht\u2019s instrument system, largely because there is a fixed measurement that it can be calibrated against \u2013 the depth of the water. However, a warning; depth finders sometimes have trouble getting clear readings in crowded marinas.
\nSo, if it\u2019s reading erratically in any way, wait till you get a calm day and do the calibration while you\u2019re 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\u2019t been on a boat with an actual leadline since I was a Sea Scout, but it\u2019s 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\u2019m adjacent to the depth sensor, and let it sink so the winch handle is just touching the bottom.
\nIf it\u2019s 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\u2019s a lot of current). Then, just pinch the halyard where it exits the water and, if you\u2019re worried about losing the spot, wrap a turn of tape or thread tightly around it.
\nHaul 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\u2019s a choice. The depth sensor will have a calibration number, which is an offset.
\nThis 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\u2019s 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.
\nThe depth sensor (this from A+T) is a deceptively simple looking device, but immensely useful\u2026 if calibrated correctly
\nNow, we\u2019re 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\u2019t 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!
\nHowever, I\u2019ve always set the depth sensor to read depth of water under the keel. This is the number that matters to me, and it\u2019s much more relevant to how I use the sensor. The tides range in the Solent means there\u2019s 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\u2019t always have on a race boat.
\nDepth Under the Keel
\nThere are a few situations where it\u2019s worth working out the water depth in advance of being somewhere \u2013 when a route option includes sailing over a sandbank or reef, for instance. Otherwise, and that\u2019s for 99% of the time I spend racing in the Solent, I sail to the measured depth \u2013 after all, if you\u2019re trying to get out of a foul tide, then you\u2019re going to go as close in as you dare, managing the risk depending on the circumstances.
\nTo set up the instruments to display depth under the keel, we\u2019ll need to subtract the distance between the sensor and the bottom of the keel from the sensor\u2019s measurement of depth. So we\u2019re going to enter a calibration number\/offset with a negative (\u2013) sign. The distance can be obtained from a yacht design drawing, or measured when the boat is out of the water.
\nIn the case of this particular yacht it was 2.5m between the sensor and the bottom of the keel \u2013 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.
\nAwkward
\nOn that Sunday I\u2019d been preoccupied with the calibrations that control the wind direction data. I\u2019d been taking the depth sensor for granted, and on this occasion got bitten hard by the assumption. My knowledge was \u2013 quite literally \u2013 not keeping me afloat.
\nIt 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 \u2013 but these are the lessons we can learn from. So, this one\u2019s 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\u2026 but especially when you have done any work on the instrument processor or computer.<\/p>\n
If you enjoyed this\u2026.<\/p>\n
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The post How to check your depth sensor calibration (and why you need to) appeared first on Yachting World.<\/p>\n","protected":false},"excerpt":{"rendered":"
Mark Chisnell recalls how a tiny calibration error with a boat’s new instrument system turned a tactical move into an awkward groundingIt\u2019s the second day of a race weekend, an early season shakedown for boat and crew. A pleasant 12-14 … <\/p>\n