Shallow draft catamaran
United States Patent 3910214

In a shallow draft sailing catamaran, resistance to leeward sliding so as to permit sailing to windward in very shallow water is provided by substantially vertical surfaces on the outboard sides of the hulls. The resulting asymmetric shape of the hulls also permits a centerboard to be positioned so as to impose less interference on the usable space within the hull.

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Primary Class:
International Classes:
B63B1/12; B63B41/00; (IPC1-7): B63B1/12
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US Patent References:
3259093Sailboat hull1966-07-05Taylor

Primary Examiner:
Blix, Trygve M.
Assistant Examiner:
O'connor, Gregory W.
Attorney, Agent or Firm:
Smith, Harding, Earley & Follmer
I claim

1. A shallow draft sailing catamaran capable of making headway without the need for having an extended centerboard, said catamaran having port and starboard hulls secured in fixed relationship to each other, the exterior of each hull being convex and rounded, in transverse section, on its inboard side and having a substantially flat, vertical surface on its outboard side extending throughout a substantial area below the water line whereby the hull offers substantial resistance to leeward motion.

2. A shallow draft catamaran according to claim 1 in which each hull is provided with a centerboard substantially offset from the center of the hull toward the side having the substantially flat vertical surface, and means mounting said centerboard for extending and retracting movement in a substantially vertical plane.


This invention relates to sailing catamarans, and particularly to a shallow draft catamaran. Normally a shallow draft catamaran has a length to draft ratio approximately in the range between 25:1 and 35:1 when fully loaded and with the centerboard fully raised.

In order to sail with the wind direction either forward or abeam, a catamaran must be either of a deep draft design, or, if shallow draft, it must be sailed with the centerboard extended so that the effect is that of a deep draft catamaran.

There are two categories of shallow draft catamarans. The first is characterized by a vertically movable centerboard located more or less centrally with respect to its hull. In the other category, the centerboard is arranged obliquely as viewed from bow or stern in order to provide more usable room within the hull. However, the latter design has the disadvantage that the centerboard or hulls may be damaged when the tide recedes with the catamaran in relatively shallow water.

In either case the shallow draft catamaran has a rounded hull primarily in order to keep the wetted surface area at a practical minimum, and to avoid the generation of conflicting waves between hulls. However, with a semi-cylindrical hull, the shallow draft catamaran will not sail to windward unless the centerboard is extended. Thus, the catamaran is difficult to maneuver in very shallow water. In accordance with this invention, a sailing catamaran is provided with asymmetrically shaped hulls, each hull being rounded on its inboard side and having a substantially flat, vertical surface on its outboard side, extending throughout a substantial area below the water line in order to offer substantial resistance to leeward motion even when the centerboard is fully retracted. The centerboard is preferably positioned along the substantially flat vertical surface, and may be made vertically movable to avoid the aforementioned possibility of damage under conditions of receding tide. This positioning of the centerboard leaves considerably more room within the hull than is available in the conventional shallow draft catamaran.

The principal object of this invention, therefore, is to provide a shallow draft sailing catamaran which is adequately maneuverable in very shallow water. Another object is to provide a shallow draft catamaran in which more available room is provided within the hulls without the risk of damage under conditions of receding tide.

Still further objects of the invention will appear from the following detailed description when read in conjunction with the drawings.


FIG. 1 is a perspective view of a sailing catamaran in accordance with the invention;

FIG. 2 is an elevation of the catamaran of FIG. 1 as viewed from the starboard side;

FIG. 3 is a diagrammatic view looking toward the bow of the catamaran showing the hull contours, the contours being numbered and corresponding to the stations indicated in FIG. 2; and

FIG. 4 is a transverse section of the starboard hull of the catamaran, showing the centerboard in its extended condition.


The catamaran in accordance with this invention, as illustrated in FIG. 1, has the usual twin hulls 12 and 14, which are rigidly connected together. Cabin space includes not only the interior of the hulls, but also the interior of the interconnecting thwart 16 (FIG. 3). Deck house 18 provides an enclosure for the cabin. Thwart 16 supports mast 20 which is suitably guyed to the respective hulls. The sailing catamaran in accordance with the invention may be of any desired size. The one illustrated in the drawings is approximately 36 feet long, and has an interior space capable of accommodating several crew members.

The hull of a typical shallow draft sailing catamaran has a substantially symmetrical, rounded shape, and either a vertically movable centerboard located centrally within the hull or, if more interior space is desired, a slanted centerboard. The catamaran in accordance with this invention, however, differs from the typical shallow draft catamaran both in hull shape and in the position of the centerboard. Each of the hulls is rounded on the inboard side, as best shown in FIG. 3. However, unlike the hulls in a typical shallow draft catamaran, the hulls in accordance with this invention are shaped differently on the outboard side than on the inboard side; that is, each hull is asymmetric. A substantially flat vertical surface is provided on the outboard side of each hull, the flat vertical surface of hull 14 being indicated at 22 in all of the figures of the drawing. This surface 22 preferably extends from above the waterline 24 (FIG. 4) throughout a substantial area below the waterline (FIGS. 2 and 4). The function of this vertical area below the waterline is to resist leeward motion, thereby making the catamaran capable of sailing to windward with the centerboard fully withdrawn. A catamaran with its hulls constructed as shown in FIGS. 2 and 3 is therefore capable of sailing to windward even in very shallow water.

The substantially flat vertical surfaces permit the centerboard to be substantially offset from the centers of the hulls so as to provide increased room for the crew inside the hulls. As shown in FIG. 4, centerboard 26 is positioned alongside surface 22, leaving a large unobstructed space 28 above deck 30. The centerboard extends downwardly from the lowermost portion of the hull, and is extended and withdrawn in a vertical motion.

The principal advantage of the catamaran shown in FIGS. 1-4 is its maneuverability in relatively shallow water. Although its sailing performance with the centerboard withdrawn in not as efficient as it is with the centerboard extended, the catamaran in accordance with the invention is nevertheless capable of sailing to windward with the centerboard fully withdrawn, and that is not the case with the ordinary semi-cylindrical symmetrical-hull catamaran.

The second important advantage afforded by the invention is the increased usable space within the hulls. This space is made available by reason of the fact that the vertical surface permits the centerboard to be offset from the usual central position and positioned alongside the vertical surface. This is an improvement over earlier attempts to provide more usable space by the use of a slanted centerboard, since, in accordance with the invention, the centerboard is vertically movable and therefore much less likely to be damaged in a receding tide. The vertical centerboard is also more efficient than the slanted centerboard.

The vertical surface 22 may be located on the inboard sides of the hulls with a similar effect in resisting leeward motion. However, with the vertical surface on the outboard side of the hull, the center of buoyancy (considering a single hull by itself) is farther outboard than in a symmetrical hull configuration. This positioning of the center of buoyancy gives rise to improved stability in heeling, and, in the preferred form of the invention, the substantially flat vertical surfaces are therefore on the outboard side of the hull. The positioning of the weight of the centerboards and the centerboard well structures next to the outboard sides of the hulls is also instrumental in improving the stability of the catamaran.