Drying out your boat
- Thu, 24 Sep 2009
There are compelling arguments in favour of a boat which can take the ground. The ability to dry out opens up a wealth of cruising possibilities, to say nothing of the financial advantages and convenience of being able to scrub and antifoul between tides or fix that rattling anode. The upper reaches of some of the most beautiful rivers and estuaries are only accessible to boats able to dry out on the falling tide. Where the bump of keel on sand strikes dread into the heart of the deep-fin sailor, it is welcomed at the end of a day's sailing by those who own boats with drying ability. Creeks and rivers surrender their secrets to the intrepid crew of a small centreboarder; the multihuller can creep across the waving fronds atop a coral reef, secure in the knowledge that he can swing to anchor in waist-deep water. The luxury of not having to fret about tidal ranges cannot be overestimated.
There are many boats on the market that can dry out, from small Beneteaus and British Hunters, to large Moodys and Westerlys to name but a few.
We took seven examples of shoal-draught vessels of varying vintage, owned by readers based around Chichester, and put them on the beach at Pilsey Island, a popular drying-out spot in Chichester Harbour. They were a Red Fox 200S with dagger bilge boards; the Edel Cat Adventure multihull; a Southerly 115 swing keel; a Westerly Griffon bilge-keeler; two swing-keel Dufours a 39 and a 30 - and a little Seamew with bilge plates.
The British love affair with bilge keels has always had our American and European counterparts, who favour centreboards, scratching their heads. Considering the huge number of indifferent bilge-keelers produced over the last half-century, you can see their point. Many builders merely bolted a pair of iron fins to the hull with little regard to the hydrodynamic penalties, or, worse, moulded them in at right angles to the waterline. The result was a generation of boats that pounded nastily and sagged to leeward when on the wind. Most needed at least a metre of water to stay afloat so lacked even the advantages of true shoal draught. Some older boats combined short bilge plates with centreline ballast keels, another compromise that did nothing for performance.
It is only recently that bilge keel design has been refined to the point where some of them offer windward performance comparable to that of a single keel. The David Thomas designs for Hunter Boats are probably the best example, and later Westerly and Moody bilge keels are a vast improvement on early versions. Many sailors happily trade a little sailing efficiency for the bilge-keeler's versatility.
For bilge keels:
• Simple and rugged
• Keeps hull clear of the bottom
• Can dampen rolling downwind
• Modern designs perform well
Against bilge keels:
• Generally not as good to windward
• Draw more than centreboarders
• Can put stress on hulls
Design and construction has evolved into two distinct types of centreboard. The simplest, and oldest, is the daggerboard, as seen on dinghies, some cruising yachts and some catamarans. The swing keel was a later step and has become the preferred option for most centreboard cruising boats. Both variants have in common the ability to retract fully, so they can stay afloat in shallower water than bilge-keelers.
Within these two categories lurk a multitude of refinements. Some fin-keel or long-keeled boats with several feet of draught are equipped with boards to give a bit more bite to windward. Others have token stub keels, more to protect the hull than anything else, through which the boards swing or drop.
Many blue water cruising yachts are equipped with centreboards to let them sneak into anchorages that are too shallow for most fixed-keelers. Traditionally, such boats are very beamy to make up for a lower ballast ratio than their fixed-keel counterparts, and tend to have a higher centre of gravity because ballast is carried closer to the hull. Most - but not all - centreboarders are more tender than fixed-keelers.
Daggerboards all work on the same principle - a rectangular board, made of wood, metal or GRP, slides down a casing or trunk. The bigger the boat, the stronger the trunk must be, because a board can generate considerable stresses. When combined with ballast at the tip - Parker Yachts' daggerboards have wings on which the boat can settle to keep the hull off the bottom - they are very efficient.
• Keel shape can be sophisticated for good windward performance
• Few moving parts
• Easier maintenance than swing keels
• Can be ballasted
• Lifting tackle is simpler
• Keelbox intrudes into accommodation - typically the height of the saloon
• Hitting an obstacle with keel down can damage keel, keelbox or hull
• Can be difficult to raise when under way
Swing keels pivot on a pin inside the keel casing. The most common way of raising/lowering them is by means of a wire or rope pendant, which can come back to a cockpit winch or to a geared handle on the keelbox. On bigger swing-keelers like Southerlys, hydraulic rams are used to make operation effortless.
For swing keels
• Keel will automatically pivot up if it hits the bottom
• Keelbox has less effect on accommodation
• Keel often has greater lateral area
Against swing keels
• Pivots can be difficult to replace
• Keel shape often not as efficient as daggerboard
• A swing keel generally adds to the price of a new boat
These are nothing more than twin lifting keels, one on either side of the hull. There are plenty of variations, either ballasted or not; some swing up into stub keels or into slots in the hull, others lift daggerboard-style through trunks. The David Thomas-designed Red Fox is a successful example of the latter type. Their use is mainly on smaller boats, but you do see the occasional larger cruiser, mostly one-off designs, appearing with these functional keels.
For bilge boards
• Keels are set outboard so do not affect accommodation
• Draught increases as boat heels
• Properly designed, give good windward performance
Against bilge boards
• More complex than fixed bilge keels
• Can be fiddly to operate
By their very nature multihulls are associated with the ability to sail in shallow water and to take the ground. Even large cruising multis rarely draw more than one metre and many moderately-sized designs can float in just a couple of feet. Most cruising multihulls are catamarans rather than trimarans as cats offer more accommodation. There are many keel variations. Some, like the Wharram Polynesian designs, rely on deeply veed hulls for lateral resistance. Others have stub keels, to improve windward ability and to protect the hulls when drying out. High-performance multis often have a daggerboard in each hull; others have pivoting centreboards.
• Length for length, cats have more accommodation than monohulls
• Good average speeds and light air performance, plus exciting acceleration
• Manoeuvrability good with twin engines
• Extra beam incurs higher marina charges
• Poor performance of some when beating