One of the requirements for building the hull has been the following: “...low-tech assembly that can be carried out DIY-style on a riverbank or a beach.” Easier said than done! How can one build a hull that’s true and fair without having so much as a true horizontal surface for a reference?
Until now, the interim plan (for lack of a real one) was to build the hull upside-down, starting with the deck. Once the two layers of plywood that make up the deck are screwed and glued together, a frame is erected over the underside of the deck. Then the plywood panels that make up the sides are screwed and glued on, then the bottom. Then the entire bottom of the hull is sheathed in fiberglass, and the bottom covered in copper sheets. Finally, the hull is flipped right-side-up, and the upper surface of the deck and the superstructure (deck arches, cockpit, dodger and hatches) are completed. Then the hull can be splashed and the interior work finished with the boat in the water, preferably at a dock or in a slip at a marina.
There is a problem with this plan: assembling the deck requires a perfectly flat surface, or the deck, and then the frame, will end up warped, making the rest of the hull impossible to assemble. But where can one find a perfectly flat and level surface on a riverbank or a beach? Having to first erect a 20 by 40 foot platform makes the project take longer and cost more.
But there is an alternative: start by assembling the frame, still upside-down, and save the deck for last, to be assembled once the hull has been flipped right-side-up. The frame is built up in “stations,” which are vertical slices of the hull going from the transom to the bow. Once each station is in one piece, it is stood up vertically and joined to the previous station using longitudinal frame members. The stations can be propped up on bricks, cinderblocks, rocks, timbers, bits of driftwood or whatever else is readily available. The height of these makeshift supports can be adjusted by digging under them to lower them or by piling up soil or sand under them to raise them up. After each station is joined to the previous one, plywood panels that make up the sides are screwed and glued on immediately. To fine-tune the vertical alignment of each station of the frame, wedges can driven in under frame members.
Once the entire frame has been assembled and the sides installed, plywood panels that make up the bottom are added. Some amount of tugging at the frame left and right may be required in order to counteract the frame's trapezoidal tendencies and to align the frame with the bottom panels. This can be done using a Spanish windlass: a loop of rope and a stick with which to twist it. Once the bottom has been assembled, the sides and the bottom are sheathed in fiberglass. Finally, the bottom, topsides and transom up to the waterline are covered in copper sheets and the topsides above the waterline are faired and painted. Now the hull can be flipped right-side-up. At this point, a bit of foresight is called for, to make it easy, when the time comes, to roll the hull into the water over logs, dragged over skids, or whatever other arrangement can be contrived for the splashdown. Ideally, all that will be needed is to knock out some chocks and let gravity do the rest.
Once the hull has been flipped and until the deck is assembled, it is open to the sky, forming an above-ground swimming pool, and so the plan should be either to complete the deck in short order, or to erect a tent over the unfinished hull, because having it fill up with rainwater would be bad for it. Installing the deck involves laying down, screwing and gluing two layers of plywood, sheathing the top surface with fiberglass, and installing aluminum diamond hatch panels over it.
Once the deck is complete, the hull is ready for water and can be splashed immediately, although I suspect that most people would prefer to assemble and install the deck arches, the cockpit seats, the dodger and the hatch cover prior to launch. In any case, some amount of hardware needs to be added beforehand: gudgeons for the rudder, engine bracket, deck cleats, etc. Since the lifelines attach to the deck arches, and may be required for launch as a safety consideration, the deck arches need to go on first. Masts and sails can be added at any time later, or even not at all if sailing turns out not to be in the cards. The motor (Yamaha t50 long shaft is still the favorite) can be dropped into its engine well either before or after launch, or not at all if all that's required is to float in a marina slip or at a mooring ball.
This assembly procedure fulfills the promise of “...low-tech assembly that can be carried out DIY-style on a riverbank or a beach.” Plywood panels are installed using brushes for brushing on the epoxy and screw guns for driving screws through the plywood (where indicated) and into the frame members. Fiberglass sheathing is added by draping fiberglass cloth and saturating it with epoxy using squeegees and rollers. Copper for the bottom and aluminum diamond hatch for the deck are laid on using specific glue and caulk, but the procedures are simple. Fairing and painting the topsides is done using spreaders, sanders, rollers and brushes. All of this is very much within the skill set of anyone who is moderately handy and can follow instructions.
But what about frame assembly? Well, here is where there has been a recent breakthrough. Now the only tools required for assembling the frame are a wooden mallet and a ratchet with a hex socket. The frame can be hammered together and will hang together by gravity and friction.
The frame consists of brackets and frame members. The brackets are welded together out of aluminum square tubing, 3½ by 3½ inches if you build to imperial measurements, 100x100mm if to metric. The frame members are made of Douglas fir timbers, also 3½ by 3½ inches or 100x100mm. The ends of each timber are precisely machined: the last six inches on each end are taken down by the thickness of the aluminum tubing for a tight press-fit into a bracket and to compensate for any twist along the length of the timber; the very tip on each end is tapered for ease of insertion into the bracket; the ridges that are six inches from each tip are milled to set a precise length from ridge to ridge and therefore from bracket to bracket. Two sides of each timber, as needed, are planed flat to compensate for any bowing and twist. Once it is milled, each frame member is coated with penetrating epoxy, making it immune to short-term humidity fluctuations and preventing any further bowing or twist.
Building up the frame involves driving frame members into brackets using a mallet, then fixing them in place by screwing in a single self-tapping screw for each joint. The screw does the additional job of making the already tight, press-fit joint even tighter by compressing the grain at each end of each frame member. Since the tips of all frame members are confined within an aluminum tube, it doesn't matter if the screw splits the grain; that frame member isn't going to go anywhere. Then, once the boat is in the water, osmosis causes the water content of the frame members to increase, causing it to swell up slightly and further tighten the joints. The use of softwood will prevent the frame members from bursting the aluminum brackets, as would happen if hardwood were used.
Some two dozen different bracket designs are needed for the entire frame. One set, used throughout the frame, is quite generic: the brackets are designed for a variety of joints, but since the hull is square, all of these joints are at 90º.
Another set of brackets is used to join frame members along the bottom. These are fabricated in a variety of specific angles, to accommodate the shape of the bottom.
Here is the hull frame in longitudinal view with the brackets omitted and the locations of the transverse frame members indicated in magenta. Note the rounded plywood panel at the bow; there are four of them, and they are used to create the rounded shape of the bow, which is the only curve in the entire hull. There are four of these panels, located at Y=8, 1.5, -1.5 and 8 feet, where Y=0 is the centerline. Also shown is the inner layer of the plywood panels that make up the sides, which are screwed and glued into place as the frame is assembled, starting at the transom. All of the plywood panels will be marked with the locations of all of the screws.
And here is the top view of the horizontal cross-section of the frame corresponding to Z=8, which is the deck (horizontal cross-sections start at Z=0, corresponding to the flat part of the bottom, to Z=8 at the deck). This view shows all of the brackets that are located immediately underneath the deck and the inner layer of plywood panels that make up the deck. The plywood panels are staggered between the layers to maximize overlap while minimizing scrap.
At this stage, the end of the project’s design phase is starting to come into focus. At this point it is a matter of completing the mechanical drawings, numbering all the parts, generating tool paths for milling them out and, very importantly, generating a bill of materials in spreadsheet form for producing precise cost estimates and assembly time estimates. The project is now at a point where new design ideas are not necessary for completing it. If all goes well, detailed study plans and a cost estimator will be made available before the end of this year.