Requirements: Safely carry four adults and full provisions for extended offshore cruising. Seaworthiness, crew comfort, cost, and building time are primary design parameters.

Concept: The Osprey 41 is a large, seaworthy (relatively heavy), open wing Trimaran , designed for rapid construction using plywood and low cost composite materials. The center hull is fitted with a simple galley, "booth" style dining table, navigation area, and head. Four berths are built into the inboard portion of the cross beams. Additional storage is available in the Amas.

Both hulls and decks are built using "stitch and glue" technique

Center Hull: 41’ LOA, 38.5’ LWL, 6’ beam 10044 # Disp.

AMA: 34.7’ LOA, 32.6’ LWL, 3’ beam, 1894 # Disp. (each)

The displacement is high for this class of boat because seaworthiness has been given more weight then performance. Since the boat is heavy by design, no high tech fibers and core materials are used, just off the shelf exterior grade plywood, stitched roving, and vinyl ester resin. Marine grade plywood is not necessary, since all exposed surfaces are covered with fiber glass. This adds weight, but a heavy boat, all things equal, is always more seaworthy and comfortable than a light one. Comfort is an important factor in seaworthiness, since an exhausted crew will be slow to respond, physically and mentally, to emergencies. Many human factors studies have shown the linkage between fatigue and rapidly changing acceleration. Light weight boats are tossed around more than there heavier counterparts. My general rule is that an offshore capable boat should not weigh less than 15000 pounds, fully provisioned. The Osprey 41, fully provisioned, displaces 16000 pounds.

The Osprey 41 is a modular design, consisting of the main hull, two amas, four cross beams, the main cabin, and the cockpit. They are all plywood structures, with no compound curves, and several of the components are small enough to build in a single car garage. Construction time is reduced by laying out and cutting the panels on a long, flat, table. The inside butt joints are tied together with butt blocks and dry wall screws, and then the outside surface is fiber glassed. No time consuming scarf joints are needed. Additional time is saved by finishing the outside surface while the panel is flat and easy to fill and sand.

The center hull and amas are shaped using four female forms. The panels are laid in the forms and bent or twisted into position. The joints are then "stitched" together using nylon wire ties, copper wire, and sheet rock screws. The assembly is then checked for distortion and shimmed around until all surfaces are fair, then locked in place by filling the joint areas with thickened epoxy fillets. After curing, the insides are cleaned up and fiberglassed completely, with extra material over the joints. After the primary bulkheads are glassed in place, the deck is built, and the boat turned on its side to finish the outside joints.  The components can be trucked to a final assembly area.  The final assembly starts by inserting the cross beams in the hull and ama cutouts, lining everything up, and bonding them in to the four carry through bulkheads in both the center hull and both amas. The connections are heavily glassed, and then backed up with bolts. Bias ply tape is then wrapped around the outside of eight joints, providing redundant load paths and completely sealing the assembly.

By moving all the berths into the cross beams, the interior arrangement becomes quite simple. The head and shower is located forward, and fitted with a large deck hatch. Each berth is fitted with a small overhead hatch which provides light and ventilation, a privacy curtain, and storage for personal gear. In the main cabin, a settee faces a fold up table and smaller booth seat. A simple galley is to port, across from the ice box / chart table. The stairs to the cockpit are broken up by the top of the engine box, which is just the right height for viewing the horizon, while still sheltered from wind and spray. Wet gear is stowed aft of the stairs.

Performance under sail will be good, but its not this boats strong point. The sail area / displacement ratio is 18, which is low for performance oriented multihulls, but about right for a couple or short handed crew. A ketch rig with a self tacking staysail, 120 Genoa, and cruising spinnaker is an option for a husband and wife team to handle and many of the components can be purchased used from the mono hull fleets. Two, 12 feet long keels are fitted to the AMA’s to help upwind performance. They are 18" deep, just 3" less than the center hull. If grounded, the vessel will likely settle slightly, coming to rest on all three hulls.

Propulsion is a single 40 hp Yanmar diesel sail drive. These install quickly and do not require accurate alignment. If more maneuverability is desired, a bow thruster can be fitted at about 25% of the cost of a second engine.