A Different Tradition of Innovation
Mon Tiki is a novel design, by which I mean she’s a departure from the glass reinforced plastic (GRP) sloops that typify modern sailing craft. Though these boats are often derided as “clorox bottles” by folks who fancy themselves traditionalist, there’s nothing inherently wrong with these boats.
Back in the Sixties the CAL 40 was one of the first boats to fully embrace the design/performance possibilities of GRP. Designer Bill Lapworth gleaned that GRP was a new material and rather than imitate plank on frame lines, he designed a hull to take full advantage of this new material’s properties. Almost 50 years after their introduction, CAL 40s still deliver performance that leaves most sailboats in their wake.
Southern California after the war was a hotbed of this sort innovation. Aerospace, surfing and sailing cross-pollenating with new materials and new design ideas.
When I was a kid, my dad had friends who shaped surfboards. To this day, the smell of polyester resin carries me back to a workshop in the Sorrento Valley where a green pintail gun was taking shape. My dad had other boards, but that’s the one I remember the most.
A couple of these guys also designed an award-winning aircraft: foam and glass, like a surfboard. I remember seeing it in their garage, before it won the award. That was amazing to me, that they were building an airplane in their garage. One of the big innovations in their plane was the use of carbon-fiber, a material that’s flowed upward from small workshops and home-built planes to military and commercial jets.
The difference between the way that glass and resin is used in the CAL 40 or other “clorox bottle boats” and how glass and resin is used in surfboards (or my dad’s friends’ airplane) is important.
Boats like the CAL 40 are formed in or around molds (male or female); the idea being that you get the shape just right, make a mold and then churn out a bunch of them. There’s a big investment in the mold and that discourages experimentation and risk-taking. Getting it just right, where performance, looks, cost, market demand all meet (like the CAL 40) is the exception.
In a surfboard, the glass and resin is cast around a light-weight foam core.
Until very recently, each of these foam cores were shaped one at a time, by hand, often to the exact specifications of the surfer. Bespoke surfboards (like my father’s green gun) were the rule rather the exception. Upfront costs for surfboard making were low; the materials workable with ordinary tools; design experimentation and innovation were the norm. Even with the recent advent of machine-shaped boards, The Shaper holds a venerated position in surf culture.
The design for Mon Tiki’s lashed hulls is inspired by Polynesian voyaging canoes. Her gaff-topped schooner rig is borrowed from cod schooners and other properly Yankee craft. Aside from the aesthetic appeal, there are well-founded practical reasons for this.
In combination they yield a vessel with an easily driven hull form and a lower center of effort rig that generates lower stress.
Mon Tiki’s design inspirations may be traditional (although at this point, what constitutes “traditional” maybe up for grabs) but her construction method is not. She’s made of wood, aye, but this wood is a long way from Polynesia’s dug-out tree-trunk, or a pilot schooner’s carvel planking.
Both interior transverse members and hull planking are from plywood. Working in plywood is less construction time, lighter weight, more water tight, and less expensive than plank on frame construction.
That means Mon Tiki will be faster than most monohulls; but less highly strung that even detuned production cruising catamarans; with deck space for carrying passengers vastly larger than any other sort of 38 footer, and (with a little luck) will she’ll be launched at a cost less than “traditionally” designed USCG Inspected Sailing Vessels of similar passenger capacity ratings.
Plywood is a miraculous, if under-appreciated material; a composite material from before the word had cache.
Plywood is made by taking thin sheets of wood and then gluing them with the grain of each layer set at 90 degrees opposed to the layer on each side. The result is a material that can be milled, machined and worked with hand-tools, but has a strength to weight ratio on par with steel or aluminum; but better because (in smaller scantlings) a similar strength plywood plate is substantially thicker than steel or aluminum, so it’s stiffer and more puncture resistant than either.
For a boat-builder, working in plywood offers many of the same advantages that a foam blank offers a surfboard shaper.
The “core” of our boat is light and strong, but workable with simple tools.
This means that unlike the mold/GRP method, set up costs are low, low-enough to justify even one-off construction, and designers have responded by dreaming up plywood designer for everything from children’s punts to ocean-crossing liveaboards. A builder/operator can select a boat that best suits his own needs, rather that buying a design that triangulates on a diverse market. And like a surfboard, the finishing touch for our “core” is to skin it with protective skin of fiberglass and resin, as you can see us doing in the time-lapse video above.
Somewhere between mold-based GRP construction and plywood construction lies cold-molded construction.
Cold-molding gets its name from the process of molding veneers of wood around frames and stringers in much the same way that we mold our plywood skin around Mon Tiki’s frames and stringers. It’s called “cold” molding because unlike earlier glues used in high-strength wood construction which required heat to set properly, cold molding uses epoxy, which cures at room temperature.
This makes cold-molding a good, if somewhat expensive choice for one-off construction. It’s like molding your own custom-shaped curved plywood around a skeleton of frames and stringers.
The reason I bring up cold-molding is because when you build a boat for yourself, you can build whatever you want. But when you build a boat to USCG inspected passenger vessel you have to build to one of the construction standards the Code of Federal Regulations recognizes; either ABS standards, or Lloyds of London standards.
Both ABS and Lloyds standards have been developed primarily for insurance purposes. Insurance companies want to know what they’re insuring; Lloyds and/or ABS provide engineering data and best practices for everything from ferry boats to offshore oil-rigs to offshore racing yachts.
Well actually, that last one, offshore racing yachts, is a sort of.
There is an ABS Guide for Building and Classing Offshore Racing Yachts. It was last updated in 1994. But it is still an official ABS designation, recognized by the Coast Guard.
Within the ABS guide there’s a section on cold molded hull plating. There is not a section on plywood hull plating. The reason there’s a section on cold molding used to be popular for producing high-end, one-of-a-kind yachts. High-end one-of-a-kind yachts are a risk. Not to their owners especially, but to the companies that insure them. And so you have ABS standards for cold-molding:
“(W)ith cold molded shell and deck plating, the cold molded wood plies in which the grain runs parallel to the ply, are generally laid at plus and minus 45 degrees to the longitudinal axis of the boat. Tests conducted by ABS Americas have determined that the ultimate strength in the principal axis of a panel laid up in such a manner is about 22% of the ultimate strength in the principal axis of a panel in which the plies are laid parallel to the principal axis. Thus, without the builder conducting material tests on the cold molded wood laminate, the modulus of rupture value to be used in the design calculations must be 22% of the particular wood species’ established modulus of rupture.”
Following an inverse train of logic, you can see why there is no ABS standard for plywood planking. Plywood planking is an inexpensive material/construction technique, popular with one-off builders, builders who exists at the margins of regulatory and insurance concerns. In other words, no one stands to loose enough money to make the rationalization of plywood hull-plating a high priority.
In fact, at this point in history, the entire sailboat category is a bit of an orphan.
The ABS Guide for Building and Classing Offshore Racing Yacht was published in 1986, updated in 1994, and has not been updated since. (If you follow the link, you’ll see the .pdf of the publication isn’t even searchable text; it’s simply a scan of a paper document.)
Whatever material and technical developments have taken place in the last 17 years – vacuum bagging, carbon fiber, computer-aided design and modeling, etc – none of them have found their way into the ABS standard. To incorporate any new ideas into a USCG-inspected vessel means costly and time-consuming empirical testing.
That’s where we ran into our snag with our choice of meranti plywood for our hull plating.
The ABS standard says that plywood must be “of marine quality and manufactured in accordance with a national standard”. Ours our plywood is British Standard (BS) 1088, the highest standard for marine plywood in the world, and recognized by Lloyds of London.
The ABS standard also says cold-molded plating shall be rated at 22% of the materials strength of the wood (see quoted section above.)
Our engineer made the logical and conservative step of applying the cold-molded formula, a formula for a hand-made laminate, to our factory-produced sheets; the rationale being that whatever strength a cold-molded plate would achieve, a factory-made sheet – laminated under precise condition of heat and pressure – should meet or exceed those specs. In his 20 years of designing boat for USCG this is an argument he’s used successfully many times.
But not this time.
This time the reviewing naval architect at MSC rejected the leap of using the cold-molded formula for a hand-made meranti laminate and applying it to a factory-made laminate from the same wood. Now before you get your libertarian panties in a bunch, just settle down a minute and listen to his reasons (I had a chance to talk to him on the phone.)
He agreed that our argument was not unreasonable, and believed it would be supported by testing data, but he felt he could not approve the argument under the cold-molded rule for two reasons:
1) In cold-molding the wood veneers are saturated in epoxy resin; in plywood the veneers are laminated under pressure using a heat-activated glue that does not penetrate as deeply into the wood. Both processes yield a bond that is strong than the wood itself, but he felt the processes were sufficiently different to disallow the formula for one to be applied to the other.
2) In cold-molding the resulting hull-plating has the strong axis of the wood (the grain) running in courses at 45 degrees to the fore and aft axis of the vessel; it’s this 45 degree off-set combined with the thinness of the veneers that allows complex shapes to be molded. With a plywood hull, the courses run at 90 degrees (idea) and 0 degrees (theoretically less strength; think about splitting a log with an axe). Again, the MSC naval architect felt this was simply too dissimilar to allow our usage by applying the cold-molded rule.
But remember what I said above? Plywood is a composite material, but rather than being a composite of different materials (fiber and resin, steel and concrete, etc) it’s a composite of the axis of strength (with help from the glue). The veneers are laminated in layers set at 90 degrees to each other, and the result is that the plywood is strong in every direction. The modulus of rupture for meranti lumber is around 10,000 to 11,000 psi (that’s testing along the axis of strength). When we got our test results back for our hull plating (testing across multiple axes) the figure was 10,500, more than 4 times higher than the figure arrived at by applying the cold-molded rule. (As I said, in using this approach our engineer was being conservative!) Cold-molded sounds sexier, or at least it does in the yachtsmen’s set, but plywood is stronger!
Anyway, we were all relieved when the tests came back. Our engineer was relieved because it’s his reputation on the line; the MSC NA was relieved because he’s not looking to scotch our project, but he’s got his job to do, and his job is to apply the rules prudently; and I was relieved because our entire concept is predicated on the miraculous but under-appreciated qualities of plywood!
One more thing about plywood. We are no where near the first to test the possibilities of plywood in critical applications.
On the eve of war with Germany, the British Air Ministry began developing specifications for a fast, high altitude bomber that was to constructed of non-strategic materials.
The result was the de Havilland Mosquito, a high performance aircraft built almost entirely from wood, including plywood. Nicknamed “the wooden wonder” these planes saw service throughout the war, much to Hermann Goering’s distress:
“In 1940 I could at least fly as far as Glasgow in most of my aircraft, but not now! It makes me furious when I see the Mosquito. I turn green and yellow with envy. The British, who can afford aluminium better than we can, knock together a beautiful wooden aircraft that every piano factory over there is building, and they give it a speed which they have now increased yet again. What do you make of that? There is nothing the British do not have. They have the geniuses and we have the nincompoops. After the war is over I’m going to buy a British radio set – then at least I’ll own something that has always worked.”
And those friends of my father, the surfboard shapers?
Carl Ekstrom continues to shape surfboards, but he also became a leader in the design of water park wave machines. Like my boat design hero Phil Bolger, Carl has a long-standing embrace of asymmetrical design.
Al Nelson lives in Baja California. According to a piece in the most recent issue of The Surfer’s Journal, he’s still building airplanes in his garage.
And boats too. Yes, Al and his friends built little boats they’d launch through the surf to fish the kelp-beds, but that’s a post for another day.