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post #1 of Old 05-04-2004 Thread Starter
Bruce Caldwell
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Centerboards and Swing Keels

Raising the centerboard off the wind brings bursts of speed and access to shallow waters.
Sailboat designers know that most sailboats involve a compromise of some sort. If, for example, a very fast boat is the objective, items that provide comfort but add weight are usually sacrificed. When a sailboat is intended to be in shallow waters or needs to be easily launched from a trailer, the sacrificial victim is usually the fixed keel.

Fixed keels do two very basic things: provide stability, and prevent the boat from sliding to leeward when sailing upwind. There are several ways to achieve one or both of these benefits without a fixed keel, but they are all compromises, and compromises bring mixed blessings.

A centerboard configuration allows the sailboat to do things that a fixed keel would prohibit.  A sailboat without a fixed keel can sit on its bottom in a vertical position (when the tide is out), instead of heeling over; it can sail much faster because of reduced wetted surface and drag; and its center of lateral resistance below the waterline can be adjusted for different points of sail. Consider that on a recently designed racing sled, the daggerboard can be used to replace a broken rudder (see Vendée Globe—Entering a New Era), and you begin to understand the value of adjustable appendages.

However, the alternatives to fixed keels bring their own set of problems. While a centerboard, swing keel, or stub keel/centerboard combination are extremely simple, fixing them is not necessarily a simple matter when something goes wrong.

It is also not a simple matter to use them. There can be many more actions to take than just raising the board while going downwind and dropping it when going to windward. For instance, by experimenting with the position of the centerboard on my catboat Kirsten, I’ve found that it’s possible to balance the tiller so she can sail herself on many points of sail.

The simplest alternatives to a fixed keel are the daggerboard and the centerboard. These provide the lateral resistance needed to prevent the boat from sliding sideways to leeward, but they are not usually ballasted and do not provide the stability that a ballasted, fixed keel offers.

The daggerboard in the down position allows this Laser to sail effectively upwind.
The daggerboard is just what its name implies: a piece of wood or fiberglass that slides through a slot in the bottom of the boat in much the same way that a dagger slides in and out of its sheath. Pull it up, and the underwater drag is vastly reduced; push it down, and the boat will resist moving sideways when sailed into the wind. Daggerboards are  found in a wide varieties of dinghies, such as Sunfish and other dinghies, and in  the more performance oriented  catamaran. In their smaller versions, daggerboards weigh very little, making it possible to adjust most up or down by hand without any mechanical assistance.

Centerboards are different from daggerboards in two respects: they are housed in a casing fitted into the bilge of the boat, and they are raised and lowered by a pennant—a rope or wire—by hand or with a winch. Some boats use a more complex system involving a piston, wire pennant, turning blocks, and winch for this work. Apparently, small variations between a centerboard and a daggerboard make a world of difference when something goes wrong because most centerboards are inaccessible from inside the boat. If the rope or wire that raises and lowers it breaks, someone will have to dive in to fix it, or the boat will have to be hauled out for repair. If a piston is involved, hauling the boat out to repair the piston is nearly unavoidable.

If the line or wire that raises the swing keel breaks or fouls, it's either back to the yard or time for a swim.
The most common problems with centerboards involve the pennant and the centerboard pin. If the pennant breaks above the deck, it can be replaced by attaching a new pennant to the end of the old one and pulling it through the hole in the deck to fasten it to the hole in the centerboard. If the pennant breaks below the deck, a new pennant can be attached to the centerboard and threaded through the hole from below. A long piece of flexible wire helps with the procedure if the pennant is a rope. Both wire and rope pennants can fray and break from friction during use, and wire pennants can get kinks when the board is suddenly kicked up by running aground. Replacing the pennant at the end of each season is the best way to avoid these problems.

A broken centerboard pin, a short metal rod that allows the board to pivot up and down, can be a tough or easy job to repair, depending upon its accessibility. Bronze is the best metal for the centerboard pin, especially in salt water, because stainless steel pins can suffer crevice corrosion.

Swing keels are a function of their parts. Here the swage exhibits signs of corrosion and should be replaced.
Both daggerboards and centerboards can get jammed in their casings when sand, mud, or gravel gets into the case during a grounding. Menger Boatworks in Amityville, NY, designs the centerboard casings of its catboats so that the casing is larger at the bottom than at the top, making it easier to dislodge whatever is jamming the board. Casings with little room between the sides and the board will jam quickly. If the board bumps against the casing while sailing, it may be an annoying sound, but it means that jams are less likely to occur. Fixing rubber strips or anything else to the inside walls of the casing to stop the board from bumping is a solution in search of a problem. If the board has more than a quarter-inch of lateral play, however, The Handook of Trailer Sailing by Robert F. Burgess suggests that it may be necessary to correct this by installing a sleeve or shim on the mounting post or pin, or by inserting a larger pin.

The centerboard pin leads a rough life. Here crevice corrosion signals approaching failure.
Casings can be flushed out from above or below with a high pressure hose, but that’s a dockside operation. If the board is jammed while sailing, it might be freed by moving the board up and down, or by heeling over sharply to move the board from side to side in the casing. Otherwise, the best course of action is to sail to the nearest beach or dock on whatever point of sail is possible with the jammed board.

When a small boat with a daggerboard capsizes, it is usually easy to right it by standing on the daggerboard and gripping the gunwale or mast. Because centerboards can pivot into a casing, care must be taken to ensure the boat doesn't turtle completely.

Because centerboards can occasionally scrape along the bottom, wearing off epoxy or fiberglass coatings, centerboards should not have steel, iron, or plywood cores that will rust or rot if exposed to water. Most centerboards can be easily repaired with fiberglass, but if a centerboard is broken beyond repair, replacement can be difficult and expensive. If the boat’s original manufacturer is no longer in business, or did not keep the original mold for the centerboard, replacing the centerboard will become a custom effort. Someone skilled in do-it-yourself work might be able to undertake it, but it may be best to seek out a boatyard that can handle the project.

Swing keels may or may not have a casing, but they are ballasted, providing additional stability that a daggerboard or centerboard cannot. This makes a broken pennant a much more serious problem, since the swing keel can be severely damaged if it runs aground, on rocks, or is deeply embedded in sand or mud, while a centerboard is usually light enough to bump harmlessly up into its casing. Diving overboard to fix or replace the pennant may be necessary to avoid more trouble.

Because stability is compromised by the use of an alternative to a fixed keel, most sailboats without fixed keels will be much wider, or beamier, to make up for the loss of the ballasted keel. This increases wetted area and drag, slowing the boat unless the bottom is flat enough to get up on a plane, in which case it can often go much faster than a boat with a fixed keel.

Swing keels may or may not be housed in a casing.
A beamy boat won’t heel over as fast or as far as a boat with a narrower beam, but without a ballasted keel, it also won’t right so easily itself if it capsizes. That can limit the weather conditions and seas that a boat without a fixed keel can safely sail in if it is too large to right by hand easily while you’re treading water. And, if treading water sounds like something you never want to picture as part of your sailing, then you’ve forgotten what fun it can be to get dunked while sailing a Sunfish or dinghy.

Boards and swing keels can take sailors up creeks and into shallow bays and, after a trailer trip, onto remote mountain lakes that are less accessible to sailors with fixed-keel boats. They may be a compromise that bars some bluewater sailing, but they open up other worlds—no small feat for something as simple as a board.

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