The chief function of the keel and centerboard is to keep the boat from skidding sideways when sailing across or on the wind. Thus the underfin provides lateral resistance.
A second function of each is to help keep the boat steady on course. In this sense. the fin provides directional stability.
A helpful feature of the keel is that it supports the boat when on the ground (with the addition of buttresses or a cradle if we're speaking of the single keel). Obviously the centerboarder with its board retracted is able to sit lower and upright alone.
Still another advantage of the keel over the centerboard is that it provides stability by housing ballast internally and externally. thus lowering the center of gravity. Although it centerboard may be of cast iron, stability on a centerboarder is provided in other ways.
Basically, stability may be achieved by ballast, a wide beam or a combination of both. Ballast is obtained by the attachment of fixed weight on or in the keel or within the bilges, and by the strategic positioning of crew members. On boats lacking a ballasted keel, a proportionately wider beam (or foundation) is required. and a shifting crew plays a more dominant role. Usually the mast height is lower also.
Deep-draft boats can either be proportionately of heavy or light displacement. Heavier-displacement types generally have well-ballasted, long keels, where the light-displacement craft have less ballasted fin keels. A light-displacement design does forsake some stability but it is offset in the same way that shoal-draft boats, most often of light displacement, achieve stability, again, by relying on a wider beam and strategic crew positioning.
The advantages of stability as achieved by the lower center of gravity induced by it ballasted keel, versus that achieved by a wider beam, are worth mentioning. The former will right herself if knocked down by it strong gust. Here the ballasted keel acts as a righting moment by swinging the boat back to its natural vertical position. A boat without such a keel is not likely to return as spritely, although a boat with a wide beam is initially less apt to heel. To preclude excessive tipping on a centerboarder, the helmsman must be quick to release the main sheet (thus spilling the wind) the instant an excessive gust strikes.
A well-ballasted keel boat, because of her weight, also carries her way better. This means she is more likely to coast when steered upwind or when coming about. Such maneuvering is therefore easier.
A boat of heavier displacement, achieved by a weighty keel, tends to cut steadily through choppy waters, although in doing so, she'll ship solid water. The proportionately lighter, beamier and more shallow boats are prone to slap and splash in choppy waters. These characteristics make a difference in a continual chop. Because of the light boat's quick motion, her crew will tend to tire faster than those aboard it heavier boat.
However, what the shoal-draft centerboarder lacks in stability, she makes up in versatility. Since the board retracts, sailing range is considerably increased by ease of trailering and the ability to sail in waters too shallow for a keel boat. Retractability also allows you to employ some helpful moves.
Trailering has notable advantages. If you are restricted to weekend sailing, and you wish to sail in other than local waters, you can trailer your boat elsewhere. Also, if the boat has bunks, it can serve as a motor home en route. Trailering to a ramp each time you use the boat also cuts out summer mooring fees, but does require more time and effort. Most every sail will involve hooking and unhooking the trailer, rigging and unrigging your boat, as well as launching and hauling it.
The ability to sail in shoal waters is definitely a plus. You'll increase your access to sheltered coves where you can anchor more safely and comfortably in a blow. Gunkholing will provide an opportunity to see, with more detail, the prettiest scenes. and to smell the heady fragrance of coastal flora. And if you wish, you can synchronize with the tides to explore a beach, laying up on the beach at low tide, leaving at high.
Use of the retractable board is also a convenience for getting underway after going aground, which the most experienced sailors may do. If upwind, the boat will be blown toward deeper water when the board is hoisted. If downwind, you can motor away after lifting the board. When you wish to stop for sandwiches. you may even elect to use the board instead of an anchor. In shallow, calm water, while running free with the board nearly extended, turn upwind soon after it hits the bottom, then drop sail. Release the pennant. Enjoy your lunch, and keep in mind whether the tide is rising or failing.
The board may also be used as a when-to-tack sensor. When beating up a channel, hold the pennant in your hand (perhaps with a turn on the pennant cleat) just before you expect to come about. When the board nudges bottom, immediately lift the board slightly, tack, and release the pennant. Needless to say, avoid these tactics where bottoms are rocky.
Because the board can be retracted to different positions, or angles, you can sail more efficiently by reducing the wetted surface. thus drag. at different points of sail. When beating and the possibility of skidding is the greatest, the board should be completely extended. thus fully resisting the lateral tendency. When reaching and the tendency to skid is lessened, the board should be positioned half way, or thereabouts. When running and there is no sidewards tendency, the board should be fully tucked in, eliminating all unnecessary resistance. A board can be partially extended, also, to avoid catching and dragging kelp or seaweed if sailing in such waters.
These are a nice assortment of advantages. However, as a result of the centerboard's design, or make-up, certain characteristics are prone to be annoying. In many boats, the trunk is a hindrance—it's in the way, right in the center of the boat. But, as in most good marine designs, this structure serves a second or third use. It is sometimes employed is a mount for mainsheet hardware and hiking straps. In larger boats, those with the trunk in the cabin, it serves as a pedestal for a dropleaf table.
The gear for pivoting and hoisting the board is occasionally the source of trouble. The pivot pin may work out, or. if the pin and board are two different metals, electrolysis may weaken the pin until it breaks under stress.
Due to vibration underway, I have heard boards chattering at the pin and humming at the pennant. Surely one can get used to these extraneous noises, and even use the pennant hum as a gauge for speed—the faster you sail. the higher the pitch. But on the boats I've sailed with this affliction, I found it annoying, mainly because I identify sailing as being silent with the pleasant exception of nature's sounds. Centerboard chatter and pennant hum are intrusions. Besides that, this noise can be a disadvantage when sailing in a fog. Here silence is necessary so you can detect the fog horns or bells of navigational aids or other boats. Any extra noise diminishes your receptivity.
Although sailing in shoal waters is an asset, it may pose problems if navigational care is not taken. If you hit a rock, the board is more susceptible to damage than the keel. It may bend or the pivot pin may break from the jolt. If you run aground on a "dirty" bottom with the board up. mud or shells may lodge in the centerboard slot. jamming the board closed.
Although less versatile, the keel boat is not subject to any of these harassments. There is no trunk, it runs silently, it's stronger. and it won't jam—all good features.
Clearly, each basic design has distinct advantages and disadvantages. In an attempt to combine the best traits of each design, or to reduce certain disadvantages, several other shoal-draft configurations have evolved. They are hulls with the keel-centerboard; leeboards; and twin, or bilge, keels.
The keel-centerboard, as the name implies, is a long keel into which a centerboard fits (Figure 2). The keel essentially doubles as the trunk. Usually this type has more stability than a centerboarder, but not as much as the full-keel boat.
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