Why your settings matter on a catamaran

A catamaran is a forgiving boat in shallow water — until it isn't. With a draft of perhaps 1.2m on a cruising cat and a beam of six metres or more, you can tuck into anchorages that would make a deep-keeled monohull skipper nervous. But that same beam, and the bridgedeck slap in a chop, mean you cannot heel your way out of trouble. Good chartplotter settings are not about trusting the screen more; they are about knowing precisely what the screen is telling you.

Set the datum before anything else

Most UK leisure charts are now drawn on a GPS-compatible datum, and most plotters default to WGS84. Older charts, and a handful of small-scale harbour plans, were drawn on OSGB36, where the offset from WGS84 can be 100 metres or more. That is the difference between clearing a headland and putting yourself on the rocks.

Check two things: the datum printed on the chart or chart pack you are using, and the datum set in the plotter menu. If you are navigating from a paper chart on the old datum while your plotter reads WGS84, either change the plotter to match the chart or apply the correction when transferring positions. Never mix them without thinking it through. For everyday coastal cruising on modern vector charts, WGS84 is the right default — but verify it, because a previous owner may have changed it.

Depth units and keel offset

UK charts are metric. Your sounder may not be. Make sure the plotter and the sounder are both displaying metres, then set the transducer offset properly.

• Depth below transducer is the raw reading, and it is almost never the number you want on screen.
• Depth below keel is what matters when you are creeping into a drying harbour or a shoaling channel. Enter your actual loaded draft — not the brochure figure — as the offset.
• On a catamaran, the sounder usually sits in one hull. The other hull, the rudders and any saildrive leg may sit differently. Measure it with the boat loaded, don't assume.
• Check the units on your handheld echo sounder too, if you carry one. Feet and fathoms still lurk in older kit.

If your draft is 1.2m with full tanks, water and cruising stores, say so in the menu. A one-metre error here is the most common cause of "the chart said it was fine".

Safety contours and shallow water shading

Vector charts let you set a safety contour — the depth at which the display highlights water as dangerous. Set it to your draft plus a sensible margin for swell, tide and the way a loaded catamaran settles. For a 1.2m draft boat, 3m is a reasonable coastal starting point; many skippers use 5m for passage planning and relax it only when pilotage demands.

Then use the shading. On vector charts, shallow water is coloured differently from safe water, and drying areas — those above chart datum — are shown separately. On the East Coast and in the Bristol Channel, where spring ranges can exceed ten metres, "drying" is a genuinely useful category rather than a technicality. Learn to read the contour lines, and remember that all depths are measured from chart datum, close to the lowest astronomical tide. Add the height of tide to get the real depth, and be honest about what the tide is actually doing, not what you hoped it would do.

Adjusting the display for local waters

Presentation choices make a real difference in UK pilotage, where buoyage is dense and channels shift.

• North up for passage planning and for comparing the screen with the paper chart on the table.
• Course up for close-quarters work in a buoyed channel, where you want the next mark straight ahead.
• Declutter at small scales. Too many labels and light sectors turn a chart into noise.
• Range should match the job: wide for offshore legs, tight when following a winding channel.
• Brightness and contrast need adjusting for bright sunlight and for night watches. A dim screen at dusk is worth the two minutes it takes.

Raster charts look like the paper you know and are excellent for pilotage. Vector charts re-scale cleanly and let you use contours, but occasionally simplify a feature. Know which you are looking at, and do not let either replace a proper look out of the window.

Keep paper aboard and keep it current

Electronics fail. Batteries flatten, connectors corrode, screens fog, and GPS jamming is no longer a theoretical concern in some areas. Carry paper charts covering your intended passage and a sensible alternative, plus tide tables, a pilot book or harbour guide, and a hand bearing compass. Keep them in a dry place, and keep them corrected.

Before each trip, run a short checklist: datum, units, keel offset, safety contour, and the paper set for the day. It takes five minutes at the chart table and it is the cheapest insurance you will ever buy. Do it once, do it properly, and your plotter becomes a genuine partner rather than a confident stranger.