HOW TO COPY AN EXISTING BOAT AND/OR DESIGN YOUR OWN HULLTHE SIDES OF THE BOAT AND BOTTOM ROCKERImagine the following sequence of steps: drill and pound a verticle dowel into the shop floor, so it points straight up. Place a long straight board upright on its edge so one face is flush against the verticle peg--in the middle of the board. Then pull on the two ends of the board so it bends slightly around the peg. Then, while still maintaining the bend in the board, tip the top edge of the board outward. The bottom edge of this board now defines a shape much like the chine edge of a dory. Imagine stepping backwards a few steps, while your assistant holds the bent board in place. From a side view, looking staight ahead, you see the front and rear ends of the board are raised up from the shop floor, while the edge of the board--at its middle--is still flush against the shop floor. This upward curving at the ends of the board becomes more pronounced as the board is tipped outwards, and less so as the angle formed by the face of the board and the shop floor gets closer to ninety degrees. Dories are made this way. Their sides are flat in one dimension and curved in the other. The greater the outward slant from chine to gunwale, the more the ends of the boat are lifted upwards. In other words the more the sides angle outwards, the more rocker you create in the bottom of the boat. A moderate amount of rocker in a driftboat is an essential characteristic. The right amount of rocker in a driftboat enhances end-to-end stability in white water waves. Too little rocker makes the boat too hard to turn. But too much rocker makes the boat "hobby horse" back and forth in tight waves. Lots of outward slant in the sides gives the boat better side to side stability in turbulent water. But too much outward slant (in the sides) can create a bottom with too much rocker. This is an unfortunate tradeoff. With a chine edge defined by the straight line edge of a board or a sheet of plywood, it's all to easy to end up with too much rocker. There are ways around this design contradiction, which I'll get back to later. THE BALANCE OF FLOATATIONAny boat with a lot of rocker in the bottom, --as all driftboats have to varying degrees--is subject to leaning awkwardly forward or backward if its payload (fishermen and their gear) shifts or changes. To design a well balanced hull you have to think about the way the boat is to be used. If you plan to drift fish with spawn sack and bait rods, or to backtroll diving plugs in deep water, and especially if you expect to fish this way in heavy water, the best seating arrangement by far is the traditional steelhead setup, where the rower's seat is positioned slightly aft of the middle of the boat and both passengers are seated on the front seat. Putting both passengers on the front seat brings the center of gravity closer to the middle of the boat, which makes it quicker to turn in dangerous water. And when you're trolling bait or diving plugs you have to fish off the side of the boat anyway. But if you plan to have two passengers fishing and casting simultaneously with fly rods, then you have to put one assenger behind the rower.These alternate seating arrangements are optimized by slightly different hull shapes. For a traditional steelhead boat, where both passengers sit on the front seat, you need to build a slightly pear shaped hull that is wider in the front (where the two passengers sit) and narrower in the rear. For a fly fishing boat you want to build a more symmetrical boat, with a wider tail to hold up the rear end of boat. Fly fishing boats don't turn as quickly as steelhead boats and they aren't as well suited to heavy white water. But they do handle a rear-seate passenger far better than a narrow-tailed steelhead boat. You can also try to have your cake and eat it too, by building a slightly pear-shaped hull that can seat two front end passengers while traversing heavy water, and still carry a rear passenger while gliding by juicy undercut brown trout banks. If you want to build a boat, think about how you most want to use it. Then find a way to row as many different boats as you can. Try a few wooden boats, like a Don Hill a Greg Tatman or a Montana Riverboat. Find a way to row a few aluminum boats too, like Alumaweld or Willies. And definately row a Lavro, SlideRite and/or Clackacraft boat. You'll find they all have strengths and weaknesses. Some of these varying characteristics are due more to the materials used than the hull shape. But the more boats you row, the more confident you'll be about the kind of boat you want to row. Let's say, god forbid, you really like the shape of the 16' Lavro, and you want to build a stitch-and-glue plywood copy. Here's how to do it: Find someone who owns a Lavro who will allow you to walk around the boat with clipboard and a tape measure. It helps to take the boat off its trailer, but you don't have to. Measure the bottom width, beam, side height and side angle of the boat at the mid point between front and back. Try to measure to within a 1/4". You don't need any more accuracy than that. Now choose a spot 24" forward of the mid-point and take the same measurements there. Then 24" behind the midpoint. Continue in both directions out to the ends of the boat. Buy your friend a beer and then head for the shop. ADJUSTABLE RIBSNow that you have the rough measurements for the boat you want, the next step is to build some adjustable, temporary boat ribs: ribs that hinge around a carriage bolt at the chine corners and splined together over a sliding groove across the bottom. With adjustable ribs you can quickly attach the side pices of the boat and then play with side angles and bottom widths at will. Temporary angles and bottom widths are held together with C-clamps. To make 7 such ribs (one rib across the middle, and 3 more for and aft of the middle) you'll need 14 1x6" boards approximately 30" long for the side pieces, and 14 1x6" boards approximately 20" longer than one half the bottom width at each measurement station. Each one of these bottom pieces needs a 3/4" wide by 3/8" deep groove routed down the middle (with table saw and dado blade or a router and 3/4" bit). For each rib you'll also need a 3/4" x 3/4" spline that snaps into the grooves you cut into the matching. You also need two 1/2" or 5/8" x 2" carriage bolts and wing nuts for each adjustable rib. At this point the assembly should be almost obvious: Each adjustable rib is made from two identical pieces that hinge at the corners and snap together over a sliding spline across the bottom. The only catch is that the hinged corners on one side of the boat need a 3/4" spacer added between the bottom cord and the side piece to keep the opposite side pieces opposite eachother, rather than being offset 3/4" by the splined bottom cord. If I didn't explain this well enough, study the photos below and think about what would happen to the relative positioning of the side pieces without the spacer on one side. 
Once you have built 6 or 7 such adjustable rib sets, you cut out the side pieces and mount them on the ribs, attaching the side pieces with drywall screws, at the layout marks on the side pieces that correspond to the measurement positions you took on the original boat. Unless you want to build a high-sided white water boat you can get both side pieces out of one 4x16' sheet of 1/4" marine plywood. Most driftboats are higher in the front than the rear. They break waves better that way (waves that you will always hit head on if you know how to row well). Traditional steelhead boats are higher in the front than fly fishing boats. To split a 4x16' sheet into two side pieces, snap a line at 20" and 28" from one edge, so the chalk line splits the plywood slightly diagonally (or use the dimensions you wrote down for the boat you want to copy). Then cut a triangle of wood away from the font and rear ends of each side piece, to make transom flare out at the rear and the stem to flare out at the front. To form these triangles I like to come in 10" at the transom and 14-16" at the stem. But when designing a new boat, make these cuts a little smaller at first. You can always cut more off later. Your side panels will look something like this:  So you now have the adjustable ribs on hand and side pieces cut out, complete with layout marks for the ribs. Now start at the middle and attach one side to the middle rib. Then attach the other side to the same rib. Move forward and backward, attaching ribs from side to side until all ribs are attached. Use drywall screws to attach a transom and a stem. At this point the whole business starts to look a hell of lot like a boat. Turn the boat upside down so you can sight carefully along the chine. It's doubtfull you have a smoothly curving line along the chine at this point. But you need one. Play with widths. Loosen a few C-clamps and spread or shrink the bottom widths as needed. Start at the middle of the boat and work slowly out toward the ends to make the smooth, gently curving chine you want, without deviating much from the measurements you took off the boat you are copying. Then, once you have the angles and widths just right, clamp or screw everything together and straighten up the form. Then install the bottom and finish off the boat according to the rest of these instructions. Using any existing hull shape and turning it into a one-off stitch-and-glue plywood boat really no more difficult than building a boat from blueprints. You just need to know what to do and how to proceed before you get started. The hard part--if there is a hard part--is doping all this out on your own. Good luck. I hope that explains how to build and use adjustable ribs, to help design your own boat. |
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