FLAT CAT: Design # 991

I arrived at the design of this 15 footer after observing many different catamarans. Its size is such that it fits in with a large group of small boats that can be home built, trailered, and launched and retrieved from a ramp. The idea was to create a versatile craft using straightforward “egg crate” construction with its outer surfaces fiberglass covered. The resulting design has hulls that are slender for low resistance and spaced wide apart for stiffness. The centerline tunnel generates lift. There’s a small cuddy forward under the top of the box, a center console with a seat half way to the stern, and plenty of room for guests and the engine.

Principal Characteristics:

LOA: 15'-6"; LWL : 13'-9"; Beam: 6'-11"; Draft: 10".

Weight of structure: 450 Lbs. Materials: Marine plywood, epoxy.

Maximum capacity: 8 persons.

The Flat Cat hull is a new and efficient concept and is within the capabilities of many amateurs to build. The hull is a “false” catamaran with a tunnel that’s partially immersed. A built-in “venturi” minimizes the wetted surface once the Flat Cat gets up to speed. The combined effect is of a center plane that contributes to planing and a deep-Vee hull to give stability, good wave handling, and maneuverability. The main advantage of the boat’s geometry is its great form stability when at rest, which will be appreciated by fishermen and divers. It also reduces pitching and helps the boat track well.

Six Steps to get the result.

The first step is to buy the plans. The dream is palpable. You can already envision future outings.

The second step is to find a place to build the boat and to buy the construction material.

The third step is to study the drawings and to cut the various components according to the construction sequence.

Next comes the assembly of the parts. As the pieces go together, the project quickly takes shape.

Then . . . Oh well. About 500 screws to be located, pre-drilled, and screwed in. Even with modern tools, this is the longest part of the project.

Sixth. When the actual construction is completed, to fill the screw heads and coat the entire hull with epoxy. There is no alternative to the sanding needed between coats of resin or paint.

Construction and materials

About 500 ft of 1"x1" material for cleats.

2 longitudinals of 3/8" plywood, 14" high, provide the fore and aft structure over the length of the boat. Six bulkheads tied into these give great rigidity. The space below the sole is filled with polyurethane foam. The bottom is made of 3/8" plywood; the topsides, the sole, and the deck are of 1/4" plywood. The transom is a 3/8" panel, heavily reinforced by cleats. Also, a massive 1"x6" piece of solid wood is bolted and glued over the entire width of the transom to support the engine, to absorb vibration, and for strength. The Flat Cat 15' has a vast cockpit which provides open space for touring, fishing, and diving. In the bow is a shelter for stowing gear. The steering console has space for instrumentation and the seat doubles as an icebox. Built into the structure is the air conduit with openings under water to feed the venturi. Further aft, the motor well is surrounded by lockers and drawers. The side decks are 6" wide, allowing for comfortable seating all around the boat. 

The self-bailing cockpit is 1'-10" deep and the straight sheerline gives the boat some character. The ideal power plant, to give the most favorable ratio of consumption to speed, is around 60 to 70 horsepower. The maximum should not exceed 100 hp.