Why a catamaran behaves differently at anchor

A catamaran is not simply a monohull with a spare hull. The wide beam, the twin hulls and the enormous windage of that high bridgedeck mean she lies to the wind and tide in her own particular way. Where a fin-keeled yacht settles down and stays put, a cat will often sheer gently from side to side, hunting for equilibrium, especially in a gusty breeze or when the tidal stream turns.

The consequence for your anchor watch is simple: the boat does not always return to the same spot. A monohull swinging through a lazy arc might be predictable to within a metre or two. A cat on a bridle can sweep through a much wider envelope, and the stern quarters — the two far corners of the hulls — travel further than anything else on board. Set your alarms for the corners, not the bow.

Add to that the fact that many cats sit to a bridle rather than a single bow roller. A properly rigged bridle reduces sheering and takes the snatch load off the windlass, but it also changes how the boat points. Get to know that behaviour in daylight before you trust it overnight.

Before you settle: swing room and neighbours

Do the arithmetic on deck, not in your head at 2am. Work out the radius your boat will cover from the anchor: the length of chain or warp you intend to veer, plus your overall length, plus a margin. Thirty metres of chain on a twelve-metre cat gives a radius of roughly forty-two metres from the anchor. Now look at the boats around you and ask what they are likely to have out. A monohull in eight metres of water with a five-to-one scope has a comparable circle. Two circles that just touch in settled weather will overlap when the wind backs thirty degrees.

Things worth checking before you drop:

• Depth and seabed. Sand and firm mud hold well; thin weed over rock does not. A cat can take the ground comfortably, but a dragging anchor at low water is still an unpleasant business.
• Tidal range and stream. In UK waters the stream will reverse twice a day. Your boat will swing through 180 degrees at each turn, and the anchor will be asked to reset each time.
• Other people's rodes. Mooring buoys, fore-and-aft moorings and boats on long scopes all lie differently to you. Assume the worst case.
• Late arrivals. Leave extra room on the side where a boat is most likely to anchor after dark — usually the deeper or more sheltered corner.

Setting the anchor and the bridle

Approach slowly into wind or tide, whichever is stronger, and bring the boat to a standstill before you lower the anchor. Pay out chain as she drifts back rather than dumping it in a heap. Once you have your chosen scope — five to one as a minimum in settled conditions, seven or eight to one if there is any swell or a blow forecast — take a gentle strain astern, then increase the revs steadily until you feel the chain come up hard and the boat stop.

Now is the moment to take your bearings, before you fit the bridle and while the anchor is freshly set. Snub the chain, attach the hook and nylon bridle to both bow cleats, and veer enough so the hook sits comfortably below the waterline but clear of the seabed. A quiet boat is a useful thing: with the chain snubbed, you can hear a dragging anchor as a low rumble through the hull, and you can hear the change in wave noise when your lie alters.

Bearings: the simplest reliable check

Bearings remain the most trustworthy anchor watch going, because they need no power, no signal and no screen. Pick three fixed objects around you — a headland, a lit mark, the corner of a harbour wall — spread widely apart. Take a compass bearing to each and write them in the log with the time.

The interpretation is straightforward:

• If all three bearings change by roughly the same amount, you are swinging. Normal.
• If one bearing changes noticeably more than the others, you are dragging or the anchor has tripped and reset.
• If a bearing to a distant object has not changed at all but your depth has dropped, check the tide tables before you panic.

Repeat the readings every hour for the first few hours, then at longer intervals, and always after a significant wind shift.

Depth alarms and GPS drift alerts

Set the depth alarm to sound on a decreasing depth, about half a metre below the shallowest you expect at the bottom of the tide. Remember that a cat draws little, so the alarm is really telling you that the anchor is dragging uphill or that you have swung over a shoal patch. Check the tide curve so you know whether a change in reading is the sea or the boat.

GPS anchor alarms are excellent, provided you set them honestly. Drop the alarm point at the anchor position rather than the boat, then set the radius to cover your full swinging circle plus a margin of ten metres or so for GPS error. Test it before dark, keep the device on charge, and turn off any power-saving mode that might put the receiver to sleep. Remember that a GPS alarm only speaks once you have already moved — it is a backstop, not a substitute for bearings.

The overnight routine

Before you turn in, log the position, the bearings, the scope veered and the forecast. Set two alarms — a phone timer and your depth alarm — so that a single failure does not leave you unwatched. Sleep in a bunk where you can hear the chain and feel the motion change.

If the wind is forecast to veer strongly, or the stream is due to turn against a fresh breeze, consider shortening your sleep and checking on deck at the change. If anything at all seems wrong, start the engine, take the strain off the anchor and re-lay it while you still have light and room. On a catamaran, with two hulls to keep clear of your neighbours, that decision is almost always the cheaper one.