Lighter and Stronger
by Paul Butler
The pleasures of a lightweight boat are an acquired taste, sort of like basswood Piantedosi sculls, turbocharged Mini Coopers and McManus cabernet. There are multiple advantages both obvious and eventually realized, but the use and ownership requirements of a ply/epoxy lightweight gives a fresh spin to the term pleasure boating, and it’s hard to go back to the ordinary. And although we’re talking mostly about ply/epoxy cartoppers, many weight saving techniques can be appropriate for larger boats.
The real advantages begin to show up not only in performance and payload, but in getting the boat to and from launch and retrieve. Lightweights under 100 pounds can easily be cartopped, which eliminates the complexities, legal and otherwise, of pulling a trailer. If you can lift 50 pounds by yourself you can handle a 100 pound hull because you only have to lift half the boat at a time for sliding it onto a roof rack. And even a 100 pound hull can be carried on the smallest of compact-car roof racks, further increasing the options for mobility. The ability to load and unload your boat without assistance is a real convenience. For those unable to carry, a lightweight with a slick, graphite-coated bottom can be dragged without harm across parking lots, down launch ramps, and over gravel beaches. Run a tether through a bow eyelet and just start walking.
Lightweights can be faster when speed is important, or when stability is critical they can be ballasted specifically to punch through small chop, or enhance stability in wind. Ballasting weight can be distributed fore and aft toward the ends of the hull, and as naval architects like to say, this will “dampen the moment” and reduce the hobbyhorse action of a lightweight hull. Alternatively the ballast can be centered low and midships to aid stability. Removable ballast is also a considerable safety factor if you get in trouble as it’s easy to flip removable ballast overboard, and with watertight compartments the hull will support itself even if flooded. Whether the ballasting is lead shot in bags, sandbags, water filled compartments, or just camp-cruising gear and a passenger is a choice with a lightweight boat. Twenty to 30 pound bags of sand will nestle into odd corners low in the hull where ballasting is most effective and can be moved with one hand, leaving a free hand to hold oars or motor controls.
These boats do not lend themselves to production techniques but are ideal for the amateur builder. You can build a ply/epoxy boat and use it for years then give it to the grandkids in about as good a shape as when you built it. They don’t watersoak! Maintenance is simplified and longevity is enhanced. Epoxy is a stable undercoat which adds longevity to an overcoat of paint or varnish and boats often last over 15 years with the original two coats of varnish over the epoxy finish. All this on a boat that can be taken into open water, make landings in surf and get dragged over beaches to a campsite.
The design process and preparation for building should be a period of contemplation where you ask yourself how you plan to use the boat. These choices require common sense to keep the boat as simple as possible, and to start with the idea that perfection is attained when there is no longer anything superfluous to take away, then add-on wisely as you see the need. The construction of a lightweight is as much art as science and requires the use of minimal and strength-consistent scantlings throughout the boat in order not to add unnecessary weight by over building. Here's where you can push the envelope a bit, but common sense and safety are still the prime requirements.
Two opposing options are to build the hull with a thicker skin and minimal internal framing, or to go with a very thin skin and rely more on internal framing, and it really depends on how you plan to use the boat. Camp-cruisers, river boats, and drifters dealing with rocky bottoms and beach landings obviously need more abrasion protection than a boat launched from a pier and used only in deep water. Building and installing all the fussy little parts that make for internal bracing also makes for a more labor intensive hull, but it can be the lightest way to go. All interior components should also be designed monocoque with everything fastened to everything else, which is also easy to accomplish with ply/epoxy.
Compartments are the structural furniture of these boats and can easily be made into seats, live bait wells, foam lined iceboxes, dry gear lockers, and are also handy to isolate fuel tanks, batteries, food, etc. Installing truly watertight bulkheads and decks is a piece of cake with ply/epoxy and compartments can easily be added throughout the hull and accessed with plastic screw-out ports or hinged hatches. Self-bailing capability can be arranged by diverting all water aboard to drain into a single footwell. Emergency flotation capability is perhaps the coolest byproduct of compartmentalization and makes an appealing safety feature, and if the hull is sensibly compartmentalized both fore and aft to maintain trim, the hull can fill from a sloppy wave top and you can keep right on rowing, motoring, or sailing.
With ply/epoxy there is seldom need for the timber stem pieces, chine cleats, and transom framing used in traditional construction. To build a lightweight stem, stitch the port and starboard panels together at the bow and apply a large interior fillet. Remove the ties, and round and tape the stem exterior. Large radius rounds are more durable than small rounds. For extra support apply a second strip of narrower glass tape to stagger selvage edges. To add support a number of four mil plywood cant frames can be installed extending 90 degrees off the stem angle, and all this can be further reinforced by adding a compartment in the forward part of the hull. Stems built this way become very durable.
A curved panel is much stiffer than a flat panel and shape can be used to provide strength without penalty of added weight. Round bottom and multi-chine hulls function with minimal internal framing, and the intersection of panels on multi-chine designs is reinforced with epoxy fillets and glass tape to become a fore and aft “stringer” which provides resilient stiffness the length of the hull. Even on single chine hulls the tape and epoxy fillet forms an I-beam structure that extends strength between athwartships supports. Apply tape both inside and multiple lams outside for added strength.
Ply/epoxy construction allows the use of surprisingly minimal thickness components. There is seldom a structural need for vertical bulkheads thicker than four mil in these cartoppers, and flat and sloping decks in lightweights can usually be four mil “no-step” decks, or an upgrade to six mil in decks used for seating and glass cloth can also be applied to decks vulnerable to wear. Decks provide two-dimensional hull support, and no underside support cleats are required when a deck is filleted into a sloping hull side. A large radius fillet will reach out onto the deck two inches or more providing an attractive structural molding; any hatches or screw-out access ports will also stiffen the deck. Compartment edges are fitted with a softwood cleat on the inside edge which serves as a glue surface for the deck, and after gluing down the deck a large diameter round is cut into the edge and taped.
Side decks are an overlooked method of reinforcing the sheerline of the hull with minimal weight. When adding a side deck it’s possible to eliminate all but a single inwale glue strip and after attaching the side deck take a block plane and cut a large radius round on the edge of the hull, then apply glass tape for a clean “sneaker” look that will take a lot of abuse.
Four mil ply is sufficient for narrow side decks, but larger, wider decks may need six mil ply. Glue on a flexible laminated plywood coaming for extra support and to help keep the cockpit dry.
The gunwales or sheer line of these small boats deal with terrific stress from rowing, wave action and the grinding abrasion of laying against gnarly pilings and alongside other boats. My own boats are routinely flipped upside-down for storage and rolled over on their side to form a windbreak or rain shelter for camp-cruising. Multiple strips are laminated onto the plywood sheer line of the hull to form the gunwales, and hardwood strips look and wear better but are much heavier. Lightweight softwood can be used for dramatic weight saving, or use a single strip of hardwood over softwood lams. To add more stiffness separate the inwale and outwale strips with lightweight spacer blocks, or to reinforce the ends of the hull more efficiently a compartment with flush deck provides useful space, and the gunwale structure can often be shortened or eliminated between compartments
I always encourage builders to climb inside the hull as soon as possible during this process for trial fitting before deciding the final location of seats, compartments, oarlocks, foot stops, etc. Stack phone books for seats, clamp-on temporary oarlock positions, use blocks of wood for foot stops and so on until the spaces feel right, because we’re all built different and possess various degrees of flexibility and comfort levels. When the hull is watertight go out in wave action and sit quietly. Some flexing is expected but any section of hull that pants more than a small amount may require reinforcement and fortunately there are multiple options to correct any problem including the addition of compartments, glass tape, veneer patches, etc., and a real beauty of this system is that modifications and retrofits are easily applied as needed.
TECHNIQUE and MATERIALS
Lightweight construction requires quality materials. Here's where you pay more for less but the benefits are realized on many levels including ease of building, looks, and longevity, not to mention the safety factor and resale value.
Start with dry wood. Dry wood is stronger and has superior abrasion resistance with reduced potential for rot pockets. A wood stove-heated shop is ideal as it acts as a dryer to reduce and stabilize the moisture content of wood. Even a preliminary sorting of materials will produce a weight saving, so select all trim wood for optimum grain orientation and moisture content. Keep a supply of Starbucks gift cards in your wallet and be prepared to bribe the guys at your local lumberyard so they’ll let you pick through the stacks for the lightest, clearest, knot-free pieces.
Get the best plywood you can afford. Hardwood ply should be your first choice because it takes epoxy so nicely and is readily available in metric thicknesses and a selection of species and quality grades. The more lams of veneer in a panel the better, and for my own small boats we use mostly four mil and six mil British Standard 6566 or BS1088 hardwood ply. It comes in millimeter thicknesses, and for non-metric users the six mil is five laminations of veneer and equivalent in thickness to a hair under one quarter inch. The four mil ply is three lams and just under 3/16 inch. Okoume plywood is typically as much as 25% lighter than mahogany ply and therefore a good choice for ultralights and lightweights, but it often has somewhat less attractive grain. The topsides in my lightweights are usually four mil, and I seldom have reason to use anything thicker than six mil, even on white water dories for commercial use in the Grand Canyon, some of which are still around over 15 years later.
Ultralight hull bottoms can be as thin as four mil, cloth sheathed on both sides and often with wider than usual tape on the chines and graphite on the bottom exterior. For a standard lightweight bottom, six mil hardwood ply with six-ounce glass cloth on both sides works well, but for hard service drifters and river boats I sometimes use 3/8 inch thick softwood AA marine grade ply for the bottom panels. All softwood ply requires glass sheathing on all surfaces, otherwise the swelling and shrinking of the annular rings in softwood veneer will produce hairline cracks in the epoxy coating. Skegs, keels, and rubbing strips can also be used to reinforce bottoms with minimal weight and can easily be retrofitted if needed. Skegs and keels are intended sacrificial, usually made of four and six mil ply attached and supported by low density fillets—all of which is easily planed off and replaced when they wear down as intended.
An epoxy glue joint is lighter and more efficient than mechanical fastenings, and screws are used only for temporary awkward clamping situations or hardware installation. Screws used repeatedly are stainless steel square drive Robertson head which do not strip as easily as a Phillips or Reed Prince. Wax the screws with beeswax to prevent the epoxy locking them in place and preventing withdrawal and use flat washers to prevent burying the screw and splitting out.
Veneer can be used for laminating very lightweight structural knees and stems, but only slice cut, quarter sawn veneer, and never the cheap rotary peeled veneer. Slice cut red cedar veneer is particularly good because it is very lightweight and glues so well with epoxy. A supportive knee laminated with three or four lams of red cedar is amazingly strong and flexible. Patches of veneer can also be glued to specific locations for reinforcement or to dress up an area.
It’s not practical to completely “mummify” an entire boat in epoxy, although some builders get close, but by pre-finishing separate components before assembly it’s possible to efficiently apply two or three very consistent coatings of epoxy to all plywood surfaces. Pre-finishing components does a dramatically better job than trying to coat all surfaces once the hull is assembled. Pre-finishing ensures that all surfaces are uniformly sealed including traditional rot pockets like the inside of compartments, but the major advantage is that after assembly the hull requires only minor detailing and preparation for varnish or paint. This is huge if you consider for a moment the usual time spent finishing.
Lay the plywood pieces flat on a workbench and dump mixed epoxy on the plywood, move it around with a squeegee to wet the surface then use a foam roller to achieve a consistent coating. Finish by tipping off with a disposable foam brush. Bend down and slight low across the surface to see globs and runs that need more brushing or rolling. To prevent the usual bubbles in the first coating of epoxy play the temperature game by heating the wood thoroughly beforehand, then to prevent outgassing turn off the heat and allow the temp to start dropping when applying the first coating. This really works and saves time, epoxy, and effort. Let it cure overnight and use a sharp flat scraper to remove bubbles, the usual raised splinters and kamikaze bug craters, then apply the second coat which will lay on much smoother.
These boats can be built with a bare minimum of tools and the only thing you really need is a jig saw, block plane, a drill, and various clamps, but scrapers deserve special mention. Nothing else works nearly as well on epoxied surfaces, and don’t even think about using sandpaper except possibly on trim wood. A rectangular scraper provides eight cutting edges and can be sharpened in minutes. They don’t make dust, they level high spots and highlight low spots and can also be used to remove the raised selvage edges of tape, and are perfect for the pre-finishing process.
A fillet is a bead of thickened epoxy applied to a hull seam or an intersection of panels, usually formed into a concave shape by dragging a round shaped plywood paddle over the epoxy bead before it kicks. The tenacity of an epoxy fillet is remarkable! They become structural and also function as a supportive cleat and molding and by using appropriate fillers they can be engineered to match the strength of wood. Mixtures of low density fillers such as West System 410 Microlight and 407 Low Density are ideal. Nothing is gained by adding the considerable weight of a heavy filler when a lightweight low density mixture will work better and provide improved resilience to prevent cracking when flexing, and also remain sandable if necessary.
Fillet paddles with a radius from ? inch to 10 inches will all be useful and are easily made from scrap plywood. Tight acute angles require smaller radius fillets and more open angles require much larger radius fillets. Paddles can also be leaned at an angle to increase the radius slightly, and flexible plastic paddles can also be squeezed to fit the application. When applying large radius fillets it may be necessary to make it a two or three step process to get it right. Make successive passes and clean off the excess with a sharp putty knife and then leave it alone. Don’t pester the fillet! Sanded fillets somehow always look amateurish. If you must sand fillets at least have the decency to paint over them.
The entire hull can be sheathed with six-ounce glass cloth for additional protection, but it’s usually not necessary if using quality hardwood ply. I prefer to tape exterior hull seams, but if cloth sheathing is judged necessary for a yacht-quality finish consider taping the interior seams in addition for strength. Glass tape adds terrific strength to the hull, and I’ve even learned to appreciate the look, but use real 10 ounce tape and don’t cut strips from cloth as there is no selvage edge.
Apply the tape in full length strips, and use the narrowest tape that will do the job. Tape can be applied pre-saturated or use tabs of masking tape to hold it in position while rolling or stippling with a bristle brush. Cut the brush bristles down to about one inch long to provide necessary stiffness. After the tape is saturated go to each end and tug gently to pull out wrinkles, then position the tape over the seam by pushing with the stipple brush. Plastic squeegees are also handy for applying glass cloth and tape as they remove excess resin that may cause the fabric to float and leave an inconsistent surface. Stand back often to eyeball and correct unfairness. To add multiple layers of tape apply a narrower width over wide tape to stagger the selvage edges. Lastly you have the option of sanding with successively finer grits of paper to featheredge the tape for a yacht finish, which is not my idea of a good time, or if the hull is to be painted out use low density sandable fillers then sand lightly. Otherwise just roll on two coats of good varnish and enjoy the natural wood.
The use of graphite has been one of the most successful treatments for my own series of small boats. A graphite coating provides substantial abrasion resistance allowing a lightweight hull to slide easily over gravel and parking lots without damage. The more it’s used the slicker the graphite gets and adds only ounces of weight, and may also be wrapped up around the chines of the hull to provide an appealing waterline. A serendipitous benefit of having a graphite bottom is that the wear pattern reveals unusual areas of stress or wear on the bottom, which can be a clue to retrofit a patch of glass tape to the area for reinforcement. To give the graphite coating more body to fill small scratches or dings mix in a small amount of colloidal silica. Spread with a squeegee, then use a foam roller and finish with a throwaway brush. If it gets so stiff you can’t brush it out, you’ve mixed in too much silica.
Carbon fiber is graphite in fiber form and for certain applications can’t be beat, but it works only in tension mode. It’s great for oars, masts, and spars and allows scantlings to be reduced for dramatic weight savings. Rout a groove in a lightweight oar and saturate with epoxy, then tamp in two or three rows of carbon fiber. Saturate well with epoxy, allow to cure then apply low density filler to fill the slot.
Building small lightweight masts and spars is fun work, and there are weight reducing options not available when building larger masts and spars. I octagon most of my small masts, both to reduce the labor of final rounding and also because I like the utilitarian look. To lighten the mast after shaping, rip it lengthwise with a band saw or jig saw. Leave the blade marks alone, and use a sharp gouge or molding plane to remove wood from the inside of each half. Take care to leave a minimal wall thickness, then saturate each half with epoxy and reglue. Install wiring and crumpled aluminum foil as required. The blade marks will ensure a perfect fit when you glue it back together, and the mast will weigh about half as much.
So building a lightweight is easy—the challenge comes in making the lightweight boat strong enough for safety and to deal with real world conditions. But with a combination of thoughtful design and quality materials the amateur garage builder can produce a structure with strength to weight ratios rivaling contraptions from a NASA lab, and without special tools. Sure I miss the fun of building plank-on-frame hulls with the delightful smells of Alaskan yellow cedar and the burnt oil fragrance of sawn teak to which I had become addicted, but the pleasures of light weight and enhanced performance more than justify the means.