Pilotage is perhaps the single most important skill for any competent skipper. The wind will blow you along fairly happily even if your sails are set like sacks, but get the tide wrong, or miss the south cardinal mark, or forget to read the depth, and you could be stuck, or worse.
After transits, my next most favourite pilotage indicator is the good old depth gauge. Set it up right, and it will tell you, hour after hour, month after month, exactly how much water you have under you. Know your draft, know the depth at low tide, and you can usually stay afloat. But first of all, you have to set it up right.
No matter where it's fixed on the hull, the modern depth gauge can be set to read the depth of water from the surface, or from the bottom of the keel, or from any useful point on or below the hull. Read the friendly manual (it's probably on the web if you don't have it to hand) to learn how to calibrate it.
Some people like to know the absolute depth from the surface, and subtract the draft in their heads as they go. Others, and I am one of them, like to set the depth gauge to read from the bottom of the keel.
A few like to set it to read from a metre or two below the keel 'for safety', but I don't like that practice. It prevents you from squeezing into harbours or marinas that are 'marginal', and if you do decide to give it a go, you will never remember exactly how much leeway you allowed yourself. The facts are always friendly, even if they're against you, so knowing exactly how much you have under the keel will always help.
Before you trust a new depth gauge, and especially when skippering a new or unknown yacht, always test its reading against an accurate lead line. Take a reading from a point approximately in line with the stays, from both the port and starboard side of the yacht, somewhere where there's unlikely to be a rock or a shopping trolley under the transponder.
If there's a difference, go for the median point. You now know the absolute depth. Check your carefully obtained lead line readings against the depth gauge, and calibrate the depth gauge accordingly. You now know within a few centimetres how much water you have under the keel.
A depth contour line is essentially an accurate Line of Position, or LOP. It may twist and turn a bit, but an LOP it is, and you can use it as such. It is particularly useful in fog or poor visibility, and used in conjunction with a lookout on the bow, it will probably get you into harbour safely.
Suppose you're stuck in thick fog a mile or so off shore. The chart shows that the ten metre line runs along the coast about half a mile off shore, and then turns inshore and up the river Muddy to pass Bimbo marina.
Using dead reckoning, backed up by the GPS or chart plotter (if you have one), aim for a point about a mile to one side of river Muddy, where you have a clear run along the ten metre line. Don't aim for the river mouth, because if you miss it, you won't know which way to turn. If you can aim uptide of it, then you have a bonus, and might want to give yourself more than a mile.
Read off your tidal curve to work out the height of tide, and add it on to the ten metre line. Suppose there's two metres of tide at that point, all you need to do is head inshore until the depth gauge reads 12 metres, and you're on the ten metre contour line. You have a line of position, and, what's more, one that will lead you safely home: you don't really need much more than that.
Next, give clear instructions to the lookout on the bow, and give him the hand-held VHF if necessary (but set it to an inter-ship channel, not 16). You don't want to hit anything, or drive over a lobster pot. Next, turn the yacht towards the entrance of River Muddy and going slowly, probably no more than two or three knots, you can creep along the contour line towards home.
At some point you will find that you need a fairly sharp change of course to keep on the line. You will, of course, know already that when you turn to head 375 magnetic in order to keep on the contour line, you have turned up into the river mouth, because you've looked on the chart.
Just keep clear of moored yachts, lobster pots, and the fishermen in tiny rowing boats who seem to love fishing in the fog, measure the distance from the turn as accurately as you can by using the log, and you're home!
Use the depth reading in conjunction with a transit, or as one part of a fix, and you have a solid, reliable reading. It's not for nothing that the Admiralty spent a good few millions of our tax money getting accurate contours of the sea bed, so you might as well use them.
Richard Thomas holds a Commercially Endorsed RYA Yachtmaster and Cruising Instructor (Sail) certificate. He runs his own delivery business, www.yachtmovers.co.uk
and is available for deliveries, assisted passages, own-yacht tuition, and yacht management.
