The Osprey 14 is a high performance planning catamaran, that is basically a pair of co-joined windsurfers. The hulls have flat bottoms for quickly getting up on a plane, and are powered by a pair of freestanding (bi-plane rig) windsurfer sails. At first glance it might seem like the leeward sail would be operating in foul air, but this is not the case. Both sails operate in clean air because at planning speeds the apparent wind shifts forward, allowing the air to flow smoothly between the sails, which are 6 feet apart.
Stability is also better with the bi-plane rig. Compared to a single sail of equal area and aspect ratio, the bi-plane rig lowers the center of effort by a factor of two, which reduces the probability of capsize. This, plus the 7-foot beam, and high volume Hulls, makes the Osprey 14 extremely stable.
OSPREY 14
14’ LOA
7’ BEAM
60 - 120 SQ.FT. SAIL AREA
200 POUNDS
The hulls are tied together structurally with two six inch square, 7 foot long, box beams. The box beams can be quickly removed for easy storage or car topping, and a wide variety of gear can be stored inside them.
Below the water, the boat is controlled with three foils, two centerboards and a rudder. They are only 7/8" thick, for reduced drag, and are built out of laminated plywood and fiberglass. The rudder is designed to tip up when beaching. The area between the Hulls and cross beams can be fitted with a nylon trampoline, or simple foot braces.
Used windsurfer sails come in a variety of sizes, and are a low cost way to evaluate various sizes. It’s a good idea to have several pairs of smaller area sails. They can be used under high wind conditions or when the helmsman is inexperienced and you want to minimize the possibility of capsize. Any small boat will capsize, especially a catamaran. Practice recovery techniques before going out in heavy winds, and always wear a PFD
Construction Overview
The boat is built out of plywood panels using the stitch and glue technique. The panels are formed out of 1/8" mahogany door skin plywood that comes in 3’ x 7’ sheets. Door skin is an ideal material for lightweight construction, and its relatively inexpensive, compared with marine grade plywood. The strength, and stiffness of each panel is determined by adding fiberglass skins and Airex core stiffeners. All outside surfaces are covered with fiberglass. The inside surfaces are coated with resin, for durability, and reinforced with six bulkheads. The bottom panel is stiffened with a 6" wide Airex core, covered with fiberglass.
To save time, most of the fiber glassing is done first, on full sheets of door skin. A full sheet is laid down on a flat table, and the best side covered with one layer of 6 oz. Fiberglass. When all the air bubbles have been worked out, the door skin is covered with a sheet of ¾" particleboard that has been waxed and sprayed with a release film (PVA). Sand bags are placed on top of the particleboard, squeezing out any extra resin. After curing, the particleboard is removed using several soft wooden wedges, revealing a smooth surface that requires almost no sanding.
Ten sheets of door skin are covered with glass, and eight of them are spliced into four 14-foot long panels. A simple butt joint splice is used, consisting of a 4" wide door skin block glued across the inside of the joint, and two layers of 4" wide fiberglass tape across the outside of the joint. The five sets of panel dimensions are transferred to the 14’ panels, and cut out using a skill saw.
The hull is built right side up, using three female building forms. The forms are aligned and secured to the floor, ground, or 2x6 strong back. The bottom panel is secured to the forms first, then the sides, and finally the shear panel. The panels are drawn in and secured to the nosepiece and transom, and any open joints are drawn together with nylon wire ties. The outside portion of the joints is covered with duct tape, which prevents resin from leaking out. The insides of the joints are then covered with an epoxy fillet and two layers of 4-inch wide fiberglass tape. Bulkheads and stiffeners are added, and the hull is notched for the cross beams. The deck is cut to size, and glassed in place after giving all exposed wood a generous coating of resin. The hull is now removed from the building forms, turned upside down, and the joints covered with two layers of 4" tape. Only the joints need sanding, and the hull is ready for painting.
BUILDING FORMS ASSEMBLED ON STRONGBACK
(nose piece included)
Building Forms
The five hull panels are supported by three female building forms. The forms are built out of 1x6 stock, to the dimensions shown on the Building Forms drawing. They can be bolted to a level floor, fastened to posts set in concrete, or attached to a rigid strongback, at stations 4, 8, and 12. The bow of the boat is formed by attaching the panels to a nose piece. The nose piece is simply a tapered board (4" at the top, 1" at the bottom), 20 inches long.
The stern of the boat is defined by the station 12 form, and the transom added later. Two transom options are available:
1. No transom. Leave the stern open up to the bulkhead at station 12. This is the lightest way to go, but some floatation is lost.
2. Put a vertical transom at station 153, taper the sides back to form a "sugar scoop". This picks up some of the lost floatation and the sugar scoop makes it easy to get on the boat from the water. The panel dimensions are for this option.
The three building forms should be set vertical and closely aligned with other. Check with a line and level, then anchor them in place.
Hull Details:
When your ready to splice the panels together, the outside edges of the scarf joint should be hit with a belt sander so that the glass tape used to join the panels does not form a bulge. After the scarf joint cures, the door skin blocks can be knocked off and replaced with a strip of glass.
Referring to the Plywood Layout drawing, rough out the hull panels and stack them, according to location, together. Make sure that half the panels have the glassed side up and the other half have the glassed side down. Lock the stacked panels together with a few drywall screws, then layout a reference line with verticals every 24 inches. Loft the shape of the panel, using dimensions from the Patterns drawing, and fair the points with a batten. Then cut out the finished panel using a saber saw or circular saw. Use a long board sander to fair the panels, then round over the edges and arrange them in the correct sequence for each Hull. Keep all scraps; they will be used for foils, bulkheads, and longitudinal stiffeners.
Check the fit of all the panels in the building forms. The hulls are symmetrical, so they can both be built on the same fixture. Put the bottom panel in place first, attaching it to the forms with dry wall screws. The fiberglassed side of the panel should be facing outside. Position the side and shear panels, and check the fit. Use more screws and wire ties to close the joints. Lock the panels to the nose piece with dry wall screws.
Check the alignment of each panel by standing off ten feet or so, and looking along the edges for bumps or hollows. Open or close the joints as necessary. Trust your eyes. When everything "looks true", cover the outside joints with duct tape, mix up some thickened epoxy (about the consistency of peanut butter) and form a liberal fillet along each joint. Push the epoxy down into the joint and over the plywood an inch or two. Clean up the fillets with a putty knife, and sand them smooth when cured. Cover each joint with two layers of 4-inch wide fiberglass tape.
Bond the mast step to the hull at station 60, and the centerboard trunk between station 74 and82. Install the four cross beam support bulkheads at station 90, 102, 132, and 144. Note that each of the cross beam support bulkheads has a 4" wide "shelve" glassed to its upper surface. The shelves face away from the cross beam. The hull is notched for the cross beams, and the open surface covered with door skin and glassed over the shelves.
Longitudinal strips of Airex,1/2 inch thick by 6 inches wide, are glued to the inside surface of the bottom and deck panels. Scrap door skin is then glued to the Airex, with the fiber-glassed side "up", and covered with fiberglass. Three 6-inch diameter opening ports are located in the deck to allow access to the cross beam mounting screws. All bulkheads and stiffeners are glassed to the hull and deck with two layers of fiberglass. Add two layers of glass tape to the nose, both outside and inside. Finish sand the nose, and attach a ¼" stainless "U" bolt to it, 2 inches down from the deck.
Deck Details:
The fore deck is cut from a 7-foot panel and placed at the edge of the first cross beam cutout. Cut out for the centerboard trunk and mast step, then fasten one-inch square blocks, 6" long, every 18" along, and flush with, the shear. Hold in place with dry wall screws and adhesive. Mark the center of each block on the hull, lay the deck down in position, and drill a screw hole at each mark. Three access ports are called out, one just ahead of the forward cross beam, one just aft of the aft cross beam, and one between them. They can be home made "lids" or purchased plastic ports. There main function is to allow for reaching the assembly bolts, but they also let you check the bilge for leakage. Make sure each bulkhead has a limber hole, clean up the insides, check one last time for any missing parts, and glue the deck sections in place. Back up the adhesive with a screw into the center of each block. Set the screws slightly below the surface. Sand the edges smooth, and glass the edges with two layers of 4" wide fiberglass. Cover the cross beam cutouts with another layer of glass, running at least 6" onto the deck.
Cross Beam Details:
The cross beams are simple 6" square sections, 7’ long. They are made in halves, glued together and fiber glassed. The cross beams fit into matching notches in each hull, where they are held securely with bolts. This allows the boat to be broken down for storage or car topping.
Two bulkheads are taped in place 18 inches from each end and the exposed inside surfaces glassed. Two more bulkheads are glassed inside the cross beams, two inches from the end. These bulkheads add support for the straps, provide floatation, and can be fitted with access ports for dry storage.
The cross beams are set in 90 degree notches that are glassed to two bulkheads and the sides of the Hull. Two molded fiberglass straps, 2" wide, cover the cross beam and bolt to the Hull with two 3/8" bolts per strap. The straps are molded over a simple male tool, 18" wide. Build the thickness up to ¼" using scrap glass and two yards of woven roving. To insure strength in all directions, arrange half of the layers at 45 degrees, and the other half at 90 degrees. When cured, cut off 8, 2" wide straps, using a saber saw and lots of blades.
The cross beams should be clamped in position and the entire assembly drilled for the locking bolts. Bolt the cross beams in place using oversize washers to spread out the load. When all the 16 bolts are tight, the cross beams should be locked to the hull. If any motion is noticed, shim the joint by adding another layer of glass to the "vee" cut out in the deck. This will raise the cross beam slightly, allowing the straps to make better contact. Conversely, if the straps are too tight, add a layer of glass to the bolt surface.
Rig Details:
Two freestanding wind surfer rigs are used to power the Osprey 14. The masts are beefed up to handle the bending loads by bonding aluminum tubing inside and outside the mast, then wrapping the mast with glass tape from the wishbone to the foot. EMT tubing can be used if you can’t locate aluminum, but its heavy and will rust if not painted. The mast step should be drilled to fit the tubing, and the upper step slotted to allow for fine tuning the rig. The downhaul holds the masts in place. Standard windsurfer sails and wishbones are used. The wishbones can be linked together, or sheeted individually. Look on the WEB for good buys on used windsurfer gear.
Rudder and Centerboard Details
All foils are built up from scrap. Six layers are used. The inside pieces do not have to be full length, and can be built up from shorter pieces. The assemble will be sanded into an airfoil shape. You can save a lot of time if you cut the center two layers 8" wide, the next two layers 7" wide, and the outside layers 4 ¾" wide. Arrange the short pieces so that the joints are at least 6" apart form each other. Glue the layers together, with the forward edge of the outside layers ¼ inch back from the nose. Sand the foils to the dimensions called out in the Details drawing, then cover them with one layer of fiber glass. The centerboard trunks are formed by using a finished foil as a male mold. Wax the foil, then wrap it with several layers of plastic sheet, held in place with masking tape. Wrap two layers of 1 ½ oz. Matt around the foil, wet it out with resin, then add a layer of cloth. When cured, knock out the foil and trim the molding to length.
