Engineering Insights: Have Surfboard, Will Travel
The surfer’s dream is to travel the world in search of the Perfect Wave. For those who can afford such dreams, taking one’s prized surfboard along has been a perennial challenge. Transport by car requires an external rack, and airlines charge $50 to $100 – one way.
The concept of a “travelboard” was first realized in 1965 by surfboard designers Karl Pope (Ojai, Calif.) and Tom Morey, who later invented the Morey Boogie Board. Their Trisect Travelboard, folded in thirds and packed in a suitcase-like carrier, could be put into the trunk of a car or checked as luggage at an airport at no extra charge. In 1999, Pope updated the concept with the Pope Bisect, an otherwise conventional polyurethane foam-cored fiberglass board that separated into two pieces using a unique telescoping joint. Although the new concept was highly successful, Pope believed the time was right to optimize the Bisect design. While today’s high-performance, glassed-foam boards can weigh less than 10 lb/4.5 kg, their ultrathin skins dent easily as the rider’s feet or knees concentrate weight in localized areas, and they are at risk of catastrophic failure in heavy surf. In search of a solution, Pope benefited from lessons learned in the 1970s when he, Morey and naval architect Bob Johnson (Island Packet Yachts, Largo, Fla.) built the WAVE (Water Apparatus and Vehicular Engineering), the world’s first waterproof coreless surfboard, which put the weight once consumed by the foam blank into the laminate to increase strength. Pope credits Johnson with a “tremendous contribution” to the design of the WAVE’s shell, a 0.25-inch/6.35-mm thick sandwich of glass/epoxy prepreg and aluminum honeycomb, and reasoned that a hollow Bisect made from lighter, stronger and stiffer carbon/fiber reinforcement would require its owner to sacrifice nothing in terms of portability or performance. The result was the Pope Bisect Hollowcarbon Stealth.
A Telescoping Joint
Introduced in 2001, the Stealth updates the Bisect’s joint system, which was first conceived, Pope says, while he was shopping one day for pool supplies. “I found skimmer poles that could extend from 5 ft to 20 ft or longer by means of two aluminum telescoping tubes,” Pope recalls. From there, Pope evolved a design comparable to that used to attach removable wings to another lightweight piece of sporting equipment, the glider. In the case of the glider, cylindrical booms are inserted into sleeves on each side of the fuselage; a sleeve in each wing slides over the boom and then the wings are pinned in place. Pope added two clamps, one on the deck and the other on the bottom (see drawing, p. 54).
In the first iterations, Pope used aluminum tubes, but these were available only in standard sizes, and the clearance between the outside diameter (OD) of one size and the inside diameter (ID) of the next larger tube was too great, allowing excessive flex in the joint. He found that tubes of unidirectional carbon/epoxy prepreg could be manufactured to tighter tolerances at lighter weight. The carbon fiber tubes are filament wound by Miramar Strategic Ventures (MSV, formerly HST Inc., San Diego, Calif.). Each board requires two sleeves that begin as a single 610-mm/24-inch long tube, helically wound at a 45° angle over a machined steel mandrel to 51-mm/2.0-inch OD, with a wall thickness of 1.6-mm/0.063-inch using Hexcel (Dublin, Calif.) 200g/m2 (0.65 oz/ft2) unidirectional carbon fiber/epoxy prepreg tape. The board’s 603.25-mm/23.75-inch long tube (boom) is similarly wound to a 48-mm/1.875-inch OD, with a 2.0-mm/0.080-inch thick wall. The removable tube OD is made slightly larger than the sleeve ID to allow for precision grinding, after cure, for a very close fit. After winding, tubes and sleeves are sealed on both ends to prevent water ingress and are oven cured at 121°C/250°F. MSV then centerless-grinds the removable tube’s exterior to a final OD leaving a scant 0.127-mm/0.005-inch tolerance between sleeve and tube. “This is key to the whole concept,” says Pope. “The tolerance is so close that, when you bend the board, there is virtually no flex in the joint.” Centerless grinding is a precision OD grinding process wherein the cylindrical workpiece is supported on its own outer diameter by a work blade located between the grinding wheel and a slower, smaller-diameter regulating wheel; this method allows grinding without drilling the piece for external support fixtures.
At only 6.8 kg/15 lb, the Stealth offers serious surfers greater strength than state-of-the-art one-piece boards and comparable maneuverability as well. “The carbon composite tube-and-sleeve joint combined with a carbon shell construction gives the Bisect the highest strength and stiffness at the best weight,” says Pope.
The Stealth design consists of two elements: the configuration (board shape) and the composite structure. Pope’s Stealth configurations are purchased from top shapers in the surfboard industry. “We don’t take a board off the rack and make a Bisect mold from it,” Pope says. “The hollow carbon structure dictates new patterns and new molds to accommodate the different buoyancy, center of gravity and moment of inertia of this different construction.”
Despite its high-tech construction, the just-right combination of length, complex curvature (rocker) and variable thickness (foil) that make a “sweet” surfboard – one that that feels like part of the surfer on the water – is a product primarily of the shaper’s experience, intuition and what competition surfer/shaper Wayne Rich (Wayne Rich Custom Surfboards, Santa Barbara, Calif.) calls “real-time R&D.”
Rich, a key Pope design source, cuts master models by hand and, because “you can’t match board shape perfectly from one side to the other,” numerically scans what Rich considers the best side of the model in incremental reference points, using a Kahuna Kalai Ltd. (KKL, Oceanside, Calif.) scanner. KKL subsequently cuts the part on its CNC machine, reversing the best-side data to cut the other side, thus producing a symmetrical master that Rich then refines by hand. The master becomes the male plug from which Pope pulls high-temperature (204°C/400°F) carbon/epoxy molds that have a useful life of about 1,000 parts.
Pope designs the composite structure by making test panels according to projected ply schedules. Dead loads are applied by an in-house, custom fixture because Pope seeks the right balance to minimize weight and maximize stiffness. Based on the tests, full-size prototypes are molded from promising ply schedules and given to the shaper for wave testing. “Because the loads are so complicated,” says Pope, “the only way to really prove a design is to build a prototype board and have a capable surfer test it in different kinds of surf.”
Testing validated a 6-mm/0.24-inch thick sandwich construction for the shell, built up using 200 g/m2 (0.65 oz/ft2) 0°/90° woven carbon/epoxy prepreg for the exterior skin (from Newport Adhesives & Composites, Irvine, Calif.), a 6.35-mm/0.250-inch thick, 2.27-kg/5-lb PVC foam core (Alcan Airex, Sins, Switzerland), and the same prepreg for the interior skin. Total board thickness is approximately 76 mm to 82.5 mm (3 inches to 3.25 inches).
The shell/joint system incorporates safeguards against catastrophic board failure when the rider and board drop into a wave and the rider’s weight imposes its maximum load in the turn at the bottom of the wave or a massive 10 ft/3m wave crashes down on it, compressing the board’s top and pulling its bottom into tension. Subject to a bending moment beyond the laminate’s fracture strength, the Stealth’s clamps release and the connecting tube, rather than the board shell, will break or crimp. Failure of the Stealth sandwich occurs at about 327 kg/720 lb of dead load, but ultimate clamp strength is 227 kg/500 lb, and its release load is 159 kg/350 lb.
Mold First, then Cut
In production, board components are molded and assembled as if for a traditional one-piece surfboard. Pope ships molds to a manufacturing plant recently established in mainland China, where top and bottom structures are hand layed separately in two composite molds prepped with Chemlease semipermanent mold release (from Chem-Trend, Howell, Mich.). Although other hollow board makers use a seam at the side rail, the Stealth’s bottom continues up and around the curve of the rail, and its top is molded to fit like a lid, with the seam in the deck area. “The rail takes a lot of impact in surfing,” Pope explains. “A rail seam is liable to crack eventually, even when it’s glassed over.” The shell layups are vacuum bagged and autoclaved for two hours at 121°C/250°F at 1 bar/14.7 psi pressure. A removable fin is similarly hand-layed using the same materials.
The joint components are then assembled over the demolded bottom structure. The sleeve is encased in a block of 2.27-kg/5-lb polyurethane foam and supported by four prefabricated bulkheads (see drawing, p. 54). Each bulkhead is a three-layer sandwich consisting of three plies of 1-inch/25.4-mm thick, 2.27-kg/5-lb density PVC foam with two plies of Newport’s 0.17-kg/6-oz, 0°/90° woven glass/epoxy prepreg between each foam layer, with two more prepreg plies applied on each outside face. Two additional plies are layed up inside a central opening through which the sleeve will pass. Other openings are cut into the two centermost bulkheads for integrally molded clamp boxes and alignment pins (see photo, this page). The bulkheads are bonded to the interior surface using a polyurethane adhesive from Gorilla Glue (Cincinnati, Ohio).
When the top and bottom shells are bonded together, Pope notes that the deck-seam arrangement resists torsional twist or rocker distortion during bonding. “We could actually hand-tape the top on,” he says. “We don’t need any pressure.”
The assembled board is finished with a clear polyurethane gloss coat, secured in a padded fixture and then precision-cut through its width into halves between the centermost bulkheads. “This way,” Pope explains, “the sleeves are perfectly aligned for insertion of the carbon tube.”
Alignment pins are installed just inboard the side rails to locate the halves rotationally; aircraft-type over-center latches are screwed into the recessed clamp boxes. Vacuum-formed, slightly textured ABS faceplates are bonded over the exposed bulkheads and finished edges of each board half, and the clamps are locked down until the polyurethane adhesive cures. Finally, the faceplates are trimmed flush with the board surface and an injection-molded Gore-Tex (Elkton, Md.) vent-and-drain plug is screwed into a threaded insert in each faceplate. The plugs resist water intrusion but permit air exchange to relieve internal pressure due to thermal expansion, and remove easily if water needs to be drained.
The Future for Globe-Trotting Surfers?
Pope currently offers eight shapes, from 6-ft, 10-inches to 11-ft, 6-inches (2m to 3.5m) in length. New shapes are added each fall. While the Bisect comes at a premium – $1,400 to $1,600 – standard glassed-foam surfboard prices have climbed an average of $120, some say, since the industry’s major foam blanks supplier, Clark Foam, closed its doors (see CT June 2006 p. 36 or www.compositesworld.com/ct/issues/2006/April/1342). Today, a 9-ft 6-inch (2.9m) standard board retails for $900 to $950. Pope targets a more competitive Bisect retail price of $995 in the near future.
Surfers give the Stealth high marks. Rich says the board assembles and disassembles in about one minute and he’s comfortable riding the Bisect in up to 10-ft/3.0m surf. While the Stealth’s life cycle is yet to be determined, Pope points out that glassed-foam boards from the 1950s are still around. “These stronger boards will probably last 100 years, if you take care of them.”