Reuel B. ParkerReuel Parker designed and built the 43′ ketch PEREGRINE for his own use. The box-section main and mizzen masts are both made of easily sourced Douglas-fir plywood. They have proven themselves at sea on several successful cruises between Parker’s home ports of Brooklin, Maine, and the Bahamas.
My preferred way to build hollow masts has been, for two decades, the bird’s-mouth method (see WoodenBoat Nos. 268 and 149). I have also made traditional box-sectioned hollow spars using solid wood. The common box-spar method involves making the forward and after components from thick material, the edges of which are rabbeted to receive thinner material for the sides. Many thousands of spars have been made with this method, which dates to the mid-19th century.
I have designed several rotating wing masts of plywood, but a few years ago it occurred to me that box-sectioned spars of lighter weight than the usual solid-wood construction could be designed and built of plywood and solid-wood corners. Because the loads on masts are primarily vertical and circumferential, plywood intuitively seems a good choice. Solid wood has only moderate tensile strength across the grain, while marine plywood has excellent tensile strength in all directions. Hence, I developed a “new” mast design based on this concept. (I do not doubt that other spar-makers have thought of this and utilized it).
1. I chose Douglas-fir for my solid corners and Douglas-fir AB marine plywood for the front, back, sides, and also for the internal blocking. All the joints were glued with epoxy, backed up with pneumatically driven stainless-steel brads. My scarf ratio was 8:1, which resulted in a 4″-long joint for the 1⁄2″-thick plywood and 3″ long for the 3⁄8″ plywood. I used a power planer and a sander-polisher with 8″ stick-on sanding disks to shape the scarfs.
2. I cut scarf slopes in both ends of the sheets leaving the ends factory square. Then I ripped the full sheets to the maximum widths of the front, back, and side panels and glued the scarfs to yield panels of the full lengths of the masts. After the glue cured, I used a long, stiff batten to scribe the mast taper onto the plywood, then ripped the pieces, including the tapers, with a handheld circular saw.
I tapered my front and back panels along both edges for athwartships symmetry, but I tapered only the leading edges of the side panels so that the back faces of the masts would be perfectly straight. I placed sawhorses every 8′ with their top surfaces exactly aligned so that during the construction the back face would lie directly on them.
3. I used nominal 2x4s of straight-grained Douglas-fir for the corners of my mainmast. The 3⁄8″ plywood mast panels were glued up full-length, then cut out, including their tapers, with a handheld circular saw. The design calls for fixed radii—one continuous shape for the corners—for the full length of each mast, which simplifies milling. Only the plywood panels are tapered; the corners are not.
I ripped the corners on a tablesaw, cut square rabbets to receive the plywood panels, and made faceted cuts for the approximate diameters required during final shaping of the outer surface. I made 8:1 scarfs on the ends of the corner segments and glued them up to match the full lengths of the masts, as with the plywood panels.
4a and 4b. Most modern spars have one edge straight for the length of the spar. In other words, the back side of a mast forms a straight line, while the front typically tapers above the spreaders. Because one edge of any given mast or boom may be designated straight, I first glued the front panels to their corner posts and the back panels to theirs. I made a number of forms to align the corners to the faces, and epoxy-glued the mast components together while also using 1″ stainless-steel brads to fasten the plywood to the solid corners.
5. I next glued and fastened the sides to the back, with the mast back lying flat and straight on the aligned sawhorses. Because of the inherent stiffness of the materials I was using, and because I used fastenings instead of clamps, my supports did not have to be as closely spaced as they would for conventional sparmaking. I used solid fir blocking where hardware would be through-bolted to the mast, and plywood backing blocks where screws would be located. Note that epoxy glue, rather than plastic resin glue, must be used on the mast sections in this method to encapsulate the end-grain of the plywood.
6. After gluing up the front and back mast assemblies, I epoxy-sealed their inside surfaces and ran copper ground wire for lightning protection in both masts. In addition, I ran coaxial cable for the VHF antenna in the mizzenmast and 14-guage marine wire for the masthead steaming light in the mainmast.
7. Seen from the head, this is the mainmast after the glue-up was completed. The masthead has been shaped and sanded using an 8″ angle grinder with a soft pad, and the “fan” masthead’s construction has begun.
8. After sanding the spars to shape (8a), I epoxy-sealed, primed, sanded, and painted them. While I considered adding a sheathing of Xynole set in epoxy, I decided against it due to its added weight and expense. I used Industrial Imron linear polyurethane, which from a distance almost looks like varnished Douglas-fir or Sitka spruce (8b). I always paint my spar tips white so I can see them at night.
9. My boatbuilding partner, Bill Smith, and I make all of our stainless-steel hardware from scratch. Bill is a welder, and his work is meticulous.
10. I design for ultimate strength, because rigging failures are unacceptable, but with conscious effort to minimize weight aloft. These tenets convey to my spreader design and construction. I use thin-wall stainless-steel tubing welded to flatbar mast fittings (10a). The finished spreaders (10b) have a flag halyard block and split tubing welded to their tips for shroud attachment.
Reuel Parker is a yacht designer, boatbuilder, and author who regularly contributes to WoodenBoat and Professional Boatbuilder. A lifelong cruis-ing sailor, he currently lives in the Bahamas aboard PEREGRINE and sails seasonally between Maine and Florida. He ventures farther as time and tide permit.