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Basic Hull, Keel, and Rudder Shapes

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Explore the different boat structures, including multihulls, monohulls, keels, rudders, centerboards, and daggerboards. Learn how to choose the right design for your needs.

The hull, keel, and rudder configuration is another major decision area. Your expectations for underbody performance, safety, practicality, and aesthetics should all be matched against the type of sailing you intend to do.

Multihulls

Multihulls include outriggers, catamarans, and trimarans. You probably will not be concerned with the outrigger unless you’re looking for a fast, exotic day sailer. Catamarans have two hulls and are built as production daysailers in many sizes. There are a few production builders of cruising catamarans, as well as many home-built cats using purchased plans.

Trimarans have three hulls – a large center hull with two smaller outside hulls or sponsons. They are usually designed for cruising and are typically larger than catamarans. Most trimarans are home-built to plans.

The advantages of multihulls are several. Light weight makes them very fast and enables them to use smaller sail plans without an appreciable performance loss in light or moderate wind conditions. Shoal draft permits them to enter shallow harbors and to be easily beached on a low tide for hull maintenance. Multihulls heel very little and provide a stable sailing and living platform. They offer a huge amount of interior and deck space at a relatively low cost per square foot. Finally, because they lack a heavy lead keel, they can be fitted with enough flotation to give them positive buoyancy.

Photo of a trimaran
This immaculate trimaran clearly shows the tremendous amount of deck and living space available in a large multihull

Unfortunately, the disadvantages of multihulls are many. They are only fast if they are kept light and provided with adequate sail area. Since many cruisers load their boats down with tons of gear and then use a small, conservative sail plan, much of their speed advantage is lost. The multihull has very high initial stability but no ultimate stability. This means that in severe conditions the boat could be flipped upside down from carrying too much sail, by catching a leeward hull, or by a rogue wave.

Once flipped, the large multihulls are very stable in the inverted position. Because of their high degree of windage they point poorly, tend to yaw and sail around their anchors, and can be difficult to bring into a tight anchorage or dock if there is any wind blowing. Their size also makes fitting into the average marina berth rather unlikely.

Monohulls

Monohulls, a broad category, includes all boats with a single hull – from light, shallow planing hulls to deep, heavy displacement hulls. Like multihulls, monohulls come in a variety of sizes, from six-foot prams to eighty-foot luxury yachts.

Monohulls with light, shallow planing hulls are usually referred to as having a dinghy hull. Most tenders to larger boats, day sailers, and small one-design racing sailboats have dinghy hull shapes. This shape is also appearing with increasing frequency in much larger racing boats and in racer-cruisers. The dinghy hull has a shallow draft, sometimes measured in inches, which enables it to sail shallow lakes and rivers and to be easily beached, trailered, and launched from docks.

The lay-up of the Characteristics of Different Types of Construction Materialshull involves less material, thus reducing costs. The dinghy hull is a lighter hull, making it generally easier to handle. Light, shallow, and flat, the dinghy hull moves easily in light airs and gets up on a plane quickly, producing high downwind speeds. The relatively flat bottom causes pounding when going to windward. In boats intended for cruising, it produces less displacement (for interior accommodations, stores, fuel, and water).

The larger racing and day-sailing boats with dinghy hull shapes rely on a weighted keel to produce stability, to give the boat a self-righting capability, and to increase its ability to carry sail. The smaller day sailers and one-design racers utilize a centerboard or daggerboard to provide lateral stability, enabling the boat to track and sail to windward. While centerboards and daggerboards work well for that purpose, they don’t provide ultimate stability, and if the boat is heeled over too far it can lay on its side or even turn turtle without righting itself. Then it is up to the crew to right the boat, which hopefully has enough positive flotation to prevent it from sinking.

Sailboats with deeper, heavier hull shapes are said to have displacement hulls. With more weight and wetted surface, and a larger bow wave, this hull is slower in light air and more difficult to get up on a plane. For most purposes the speed of this hull shape is limited to its displacement or hull speed. More room is available for accommodations, stores, and equipment, which are important on larger boats used for overnight trips and cruising. While the initial stability of this hull shape is not as high as that of the true dinghy hull, it increases as the boat heels.

To provide even more stability and a self-righting capability, weight in the form of ballast is attached to the bottom of the keel. If everything has been designed correctly the boat shouldn’t turn turtle, and if it does, it should right itself. Because of the enormous weight of the ballast (typically 30 to 50 percent of the boat’s total displacement), it is not usually possible to have positive flotation. Consequently, if the displacement keel boat is holed, it will quickly sink unless the water can be removed or the leak stopped.

Centerboards and Daggerboards

Centerboards and daggerboards are usually used on sailboats up to around twenty feet LOA to provide stability and lateral resistance. After twenty feet, weighted keels become more prevalent.

Centerboards are raised or lowered on a hinge with a winch or block and tackle. When not in use, they are stored in a centerboard trunk, an enclosed slit in the keel or bottom of the hull. With the centerboard down, the boat has increased lateral resistance (resistance to being pushed sideways) and substantially improved windward performance. With the board almost all the way up, the boat has decreased wetted surface for better downwind performance.

Centerboard trunk
A dinghy hull with centerboard

In the various positions in between full up and full down, the board can be used to help balance the boat and obtain the ultimate possible performance. Changing the position of the board moves the center of the lateral plane fore and aft to match the movement of the center of effort caused by different sail combinations and sail trim. With the centerboard up, the boat also has a greatly reduced draft, enabling it to be beached or to enter very shoal harbors.

The centerboard does have some disadvantages, however. The trunk can clutter the cockpit or cabin; the centerboard may bang; and the trunk may gurgle, spit, or even leak. The centerboard can also jam up or down. On larger boats, where it is used to create shoal draft, the centerboard will increase the cost, maintenance, and complexity of the boat.

Read also: Comprehensive Collection of Common Sailboat Rig Types and Designs

Daggerboards are raised or lowered straight up and down through a narrow trunk or slot in the hull. They serve the same purpose as the centerboard, but with less mechanics. Because they aren’t hinged, however, they are much more apt to be damaged in grounding, and they present more of a storage problem. Daggerboards, although common on small dinghies, are rarely seen on larger boats.

There are many variations and combinations of the center-board, daggerboard, and keel. Common combinations include the weighted centerboard or daggerboard and the fixed weighted keel with a centerboard trunk inside it.

Keels

Keels come in many shapes. While they all provide lateral resistance to enable the sailboat to track and sail to windward, they represent significant differences in performance. Traditional full keels run the entire length of the bottom of the hull. In some cases they blend into the hull so completely that they are indistinguishable from the deep, heavy displacement hull to which they are usually matched.

Modern hull and keel design has borrowed heavily from aerodynamics. Close examination of a modern keel reveals its similarity to the foil section of an airplane wing.

Fibreglass boat
A full-keel fiberglass boat fresh from Taiwan. An outboard rudder (tiller steered) is hinged to the keel, and the propeller sits in an aperture in the keel

Modern keels perform much better than their historical counterparts, which were drawn by eye and to traditional lines. There is a high probability that the full keel will not be a modern foil section since most full keels are older designs or newer copies of older designs.

Boat with full keel
A typical shape for a full keel. As the forefoot is cut away, the boat becomes more maneuverable

The design restrictions of the full keel on a very deep hull also make production of a good foil section difficult, since the keel often has a low aspect ratio and is broadly faired into the hull at the root (top of the keel).

Fin keel
A moderately high-aspect fin keel. The moderately high-aspect rudder is mounted on a partial skeg

Two improvements were made to the full keel that greatly enhanced its performance. One was to cut away part of the forefoot (hull underbody at the bow), and the other was to remove part of the keel at the stern section. Both of these changes reduced wetted surface and weight. Cutting away the forefoot improved the steering responsiveness and ease of tacking.

A major advantage of the full keel boat is that it usually tracks well, holding a course with little rudder action. It also hauls out or careens for maintenance easily. Matched with a deep hull and heavy displacement, as it so often is, the keel provides room for water and fuel tanks deep in the hull and in the upper part of the keel, where their weight is best placed.

Typical fin keel
A moderately high-aspect fin keel with medium sweepback angle to the leading edge

The disadvantages are primarily in performance. The full keeled boat will have a high wetted-surface area, will be heavier, and consequently will be slower. Although it will track well, it will not be as responsive to the helm, will be more difficult to tack, and may not steer well in close quarters maneuvering. Finally, it will generally not point as high because it lacks the lift that comes from a modern foil-section keel.

Fin keels are exactly what the name implies — fins or vertical wings attached as an appendage to the bottom of the hull. Fin keels are usually matched with hulls that have less displacement and depth (closer to a dinghy hull shape), though this varies. Like a wing, the fin keel has a leading edge that is thicker than its trailing edge (aft end). There are many variations on the shape of the fin, including bulbs (like wing-tip tanks on a fighter plane) where the ballast is held. Fin keels are primarily distinguished by their aspect ratios, the ratio of depth (span) to length (chord).

Boat with high-aspect fin keel
A high-aspect fin keel and spade rudder on a large lightweight ocean racer

Keels with higher aspect ratios have more lift and hence point better than those with lower aspect ratios. If matched with a shallow dinghy hull shape, they also often have less wetted surface area and hence are generally faster. As the aspect ratio increases to the extreme range, however, they track less well and create tremendous strain on the structural attachment of the keel to the hull.

Fin keel boats are also more difficult to haul out, and in careening and accidental grounding the boat may want to roll on its side or bow. The very deep fin keels are more difficult to navigate safely through shoal waters.

Rudders

Like keels, rudders come in many shapes and are attached in several locations.

Keel hung rudders offer superior protection in a grounding or collision. They also dampen the steering, which results in better tracking under conditions that make steering difficult. Unfortunately, the dampening effect makes them less effective when turning in close quarters. Further, a keel hung rudder doesn’t provide a separate foil.

Construction of boat with fin keel
A popular compromise between the full and extreme fin keels is the low to moderate fin keel with a skeg of varying size. This shape combines much of the tracking performance of the full keel with maneuverability that is close to that of the spade rudder

Since the leading edge of the keel or rudder is what gives it most of its windward lift, a separate foil-shaped rudder improves the windward performance of a boat.

Spade rudders are freestanding foil sections (again, like an airplane wing) attached to the hull only by the rudder shaft. Like fin keels, they have aspect ratios with similar performance characteristics. While spade rudders provide excellent responsiveness and turning characteristics, the boat can develop “squirrelly” steering downwind and require constant helm attention.

Spade rudder
A high-aspect spade rudder with a very small skeg. Note the gel coat bubbling, a common problem with fiberglass hulls

This problem is magnified as rudder and keel aspect ratios increase. The rudder is also under more stress and is more at risk in a grounding or from logs and debris.

Skeg hung rudders are a compromise between spade rudders and keel hung rudders. A skeg is a small isolated section of keel with a large cutaway in front of it, aft of a fin keel.

High-aspect rudder
This high-aspect rudder is hung from a small but full-length skeg. Note the bearing at the bottom of the skeg, which supports the rudder

Skegs come in various sizes and shapes, but they all dampen the steering, improve tracking ability, provide protection for the rudder, and strengthen the attachment of the rudder by linking it to the skeg.

Outboard rudder
This outboard rudder with tiller is both transom and keel hung. The large open slot between the rudder and keel and the propeller aperture will create turbulence and reduce the effectiveness of the rudder

The skeg configuration has less wetted surface than the full keel and provides another foil section for improved lift to windward. Combining a skeg hung rudder with a moderate aspect ratio fin keel makes a nice compromise between the high aspect spade rudder and fin keel and the traditional full keel. This configuration is now seen quite often on what are being called “performance cruisers”.

Stern profiles
Three different stern profiles matched
to their typical hull and keel shapes

Outboard hung rudders may be fastened to a full keel or a skeg or be freestanding like a spade rudder. Attached to the transom, they bring the rudder back to the farthest point aft, increasing its relative power.

Tillers and self-steering vanes are easier to install on the outboard rudder, and inspections and repairs are facilitated by its accessibility. They are more susceptible to damage, however, because of their location.

Sterns

Many of my students ask about the characteristics of different types of sterns. There are four basic types, with many variations and combinations.

One type is the counter stern, which can overhang the waterline either moderately or extremely. This stern provides reserve buoyancy, which reduces the probability of being pooped by a large following sea, and additional waterline when the boat is sailing beyond its displacement speed.

It also provides a higher proportion of deck space for a given waterline, perhaps providing an aft deck for sunning or an attachment point for backstays without resorting to a boomkin (which is basically a stern bowsprit). A locker aft of the cockpit (lazaret) is also usually available for additional storage.

One disadvantage of this stern is that if heavily loaded, particularly if the overhang is extreme, it will increase “hobby-horsing” (the fore-and-aft, up-and-down movement of the boat) in a seaway. The overhang also increases the cost of construction slightly for a specific waterline length.

The transom stern is cut off nearly flat or straight from the deck to the waterline. It is very common on dinghies, small day sailers, and pocket cruisers. If cut off before the beam is severely pinched, this stern creates large buoyant aft sections that contribute to the boat’s ability to get up on a plane downwind. It also creates a larger cockpit and more storage in the cockpit lockers.

The reverse transom brings the hull out beyond the deck and cockpit at the waterline so that it points aft, inversely to the angle of the bow. This is commonly done on racing boats and in moderation on some cruiser-racers. The transom is reversed to gain additional waterline length with a minimal gain in weight and cost. In its extreme forms, it may make anchoring, docking, or tying up a dinghy from the stern difficult. Installation of equipment such as swim ladders and self-steering vanes may also be a problem.

The last group of sterns that are of interest include the double-ender (also known as the Colin Archer or Scandinavian) and the canoe. The former is usually found on full keel boats, while the latter may be used with a variety of hull and keel shapes. Despite almost incredible claims of seaworthiness, these sterns are probably no better than other types.

Many of the myths about canoe and double-ender sterns developed because they were used on pilot boats and How to Choose the Perfect Sailboat: Tips on Selection, Ownership, and Alternativessmall sailboats that were successfully sailed in severe conditions and were later written about by their survivors.

These boats were close to “state of the art” when they made their reputations, but design and construction have further evolved and many seaworthy designs are now available. The only clear claim that I can substantiate is that these sterns are very appealing aesthetically.

On the other side is a string of disadvantages. If carried to an extreme and matched to a full keel, as on a Colin Archer type, these sterns may be seriously lacking in buoyancy in the aft sections. Narrower aft sections also make the cockpit more cramped and reduce cockpit locker space. The earlier and the more severely the stern is pinched, the greater will be the reduction in the volume of the entire boat. The extremely pinched stern will also be less apt to get out of the trough behind the bow wave and surf when running or reaching.

Author
Author photo - Olga Nesvetailova
Freelancer
Literature
  1. Cruising World, Subscription Service Dept., P. O. Box 953, Farmingdale, NY 11737.
  2. Motor Boating & Sailing, P. O. Box 10075, Des Moines, IA 50350.
  3. Multi-hulls, 421 Hancock St., N. Quincy, MA 02171-9981.
  4. Nautical Quarterly, 373 Park Avenue South, New York, NY 10016.
  5. Sail Magazine, P. O. Box 10210, Des Moines, IA 50336.
  6. Sailing, P. O. Box 248, Port Washington, WI 53704.
  7. Small Boat Journal, P. O. Box 400, Bennington, VT 05201.
  8. Soundings, Soundings Publications, Inc., Pratt Street, Essex, CT 06426.
  9. The Practical Sailor, Subscription Dept., P. O. Box 971, Farmingdale, NY 11737.
  10. Wooden Boat, Subscription Dept., P. O. Box 956, Farming-dale, NY 11737.
  11. Yacht Racing/Cruising, North American Building, 401 North Broad Street, Philadelphia, PA 19108.
  12. Yachting, P. O. Box 2704, Boulder, CO 80321.
  13. Beiser, Arthur. The Proper Yacht, 2nd ed. Camden, Maine: International Publishing Co., 1978.
  14. Chapman, Charles F. Piloting, Seamanship and Small Boat Handling, 56th ed. New York: Hearst Marine Books, 1983.
  15. Coles, Adlard. Heavy Weather Sailing, 3rd rev. ed. Clinton Corners, N.Y.: John De Graff, Inc., 1981.
  16. Pardey, Lin and Larry. Cruising in Seraffyn and Seraffyn’s Mediterranean Adventure (W. W. Norton, 1981).
  17. Roth, Hal. After 50 000 Miles (W. W. Norton, 1977) and Two Against Cape Horn (W. W. Norton, 1968).
  18. Royce, Patrick M. Royce’s Sailing Illustrated, 8th ed. Ventura, Calif.: Western Marine Enterprises, Inc., 1979.
  19. Kinney, Francis S. Skene’s Elements of Yacht Design, 8th ed. New York: Dodd, Mead, 1981.
  20. Street, Donald M., Jr. The Ocean Sailing Yacht, Vols. I and II. New York: W. W. Norton, 1973, 1978.

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