Step 1: Lofting, Scarfing and Using a Batten for True Measurements

Note: I never thought to take pictures of the lofting procedure. However, after I lofted my design to full scale, I took the measurements for each station rib, cut them out, and then placed them on the line of my full-scale drawing. I then I built the frame to hold each one in place. Keep in mind that this is a female mold. You would make the opposite framework, a male mold, although you would not be making all of the station-ribs like I had to do.

The following table provides the measurements for each one-foot station (rib), as if you were looking straight down into the boat. Imagine looking straight down into the boat just as two workers placed a wire rectangle straight above the boat and perfectly aligned it with the boat. The wire rectangle has one-foot horizontal wires and a vertical centerline wire. From the centerline, you could measure out on each one-foot station line noting the distance from the centerline to the outside edge of the floor and then the outside edge of the sheer line, which is the top of the side-panel. What follows is a table of measurements and the view I have just described so that you may draw it to full-scale on your garage floor. The rocker measurement is the distance from ground level to where it meets the curvature of the bottom at each station. * These are not true measurements, however. Be sure to read the section on True Measurements using a batten or thin piece of wood to draw the actual curve.

** Keep in mind that the stern-piece extends outward past station-rib #12 for approximayely 8-inches, which gives the total center-line length as 12'-8".

Transom Measurements: Bottom width = ____; top width = ____ height = _____

Stern Piece: Bottom width = ___; top width = ____ height = _____

Unless you are an experienced small boat builder, I recommend purchasing Samual Devlin's book, Devlin's Boat Building (ISBN 0-07-157990-7). Although I was much impressed with Devlin's book, which touts the stitch-and-glue epoxy method of boat building, I was unwilling to risk trying to make a stitch-and-glue boat from a scaled drawing on my drafting board. I lacked the confidence that I could take the model and loft it accurately to full scale and then cut out my patterns. I did, however, loft a full-scale drawing by spreading out rolls of news print paper on my garage floor. Most newspapers discard rolls of news print that still have quite a bit of paper on them when they are replaced with new rolls. Butcher paper works fine too.

Although I used a female mold at considerable expense, using my dimensions and building from a simple plug or male mold will save you both time and money. Lofting to Full Scale: I started out with a 13-foot by 6-foot rectangle drawn full scale on a huge piece of paper in my garage. (Keep in mind that the sheets of paper you tape together needs to be large enough for a top view and a side view. Once the top view is drawn to full scale, you can utilize the same centerline and station lines (ribs) to draw the side-view.) The bottom line of the rectangle was my stern line and the top line became my transom line. The two long, vertical lines forming the outside of the rectangle would be the edges for the sheer line, the top edge of the side panel. Next, I snapped a centerline down the middle. I then drew horizontal lines across the rectangle in one-foot increments.

Using the measurements in the table above, I marked the intersection points on the station lines for both the floor and the sheer line. Using a 14-foot batten (strip of wood), I then bent the batten strip so that it intersected with each point. * Note: This line became my true measurement line for both the sheer line and the bottom line. Scarfing: Read Devlin's chapter 7 on scarfing for tool attachments. Making a scarf joint is perhaps the greatest challenge for an amateur boat builder. Some marine plywood distributors ship 4 x 16-foot sheets, which are already glued together with a scarf joint. Gougeon Brothers sell a jig attachment for a circular saw. Once you have joined two sheets of plywood together, you can draw the bottom piece of the Bitterroot Pram directly on the plywood rather than on paper.

The station lines become potential placement for an interior rib or temporary spreading brace. Since I was working with a female mold, my ribs became reversed or recessed ribs, which would be an accurate cutout of the boat at that particular one-foot increment. You, of course, would make a male-rib, as you would want to be working on the boat upside-down from the outside. In other words, your boat would be constructed by building a hull with the bottom facing up. Two of the ribs will actually be solid bulkhead, rib #2 (two feet from the transom. The second bulkhead was placed four feet back from the point of the boat. I cut out a hole so I could use the area under the top deck for storage). Some ribs will be removed once you have glued your hull together at the seams with cloth and epoxy resin. Instead of a full rib, a small panel will be used instead. (See photograph below.) This will provide unrestricted seating on the floor while hunting ducks from within the boat.

Notice the simple framing bracket for the rowers seat. Just drop the plywood seat in the bracket with a bolted seat of your choice.

True Measurements from a Flexible Batten

Batten: (thin strip of wood measuring 14-feet)

For clarification, permit me to review the lofting procedure.  The table above provides the distance measurement for the bottom of the boat and the side measurement at the sheer.  These are not accurate figures.  Use them to mark each station-rib-measurement.  Now, take a thin 14-foot batten-board.   Be sure your station marks for each rib are accurately drawn.  Double check to see that your rectangle and centerline are true and square..  Each outside, sheer point of intersection should be marked.  (A trim piece or gunwale is usually attached at the sheer line to strengthen and protect the side of the boat, but since I have included a rising deck, I chose not to include a gunwale.  Using a uniformly cut piece of wood, bend the stick so that it intersects each point.  The natural curve of this stick becomes your true measurement, and the resulting line might be a little different from mine. Measure from the centerline to the outside edge of the bottom and again at the sheer.

In his book Devlin suggests marking this distance with three sets of numbers.  (The first number is for feet; the second number designates inches; the third number represents eights of an inch. Example: 2-5-2) Do not repeat the batten line on the other side without first mirroring the exact set of new measurements from the other side.  I made this mistake and was surprised to discover a slightly different curve on the other side.  Use the true measurements from the side that you just completed and mark the intersection points on the other side.  Then use the batten stick to intersect with those lines for a tracing.  Once you have plotted your new numbers on the other side, you can use the batten-stick and draw in the curve.  This curve should be an exact mirror curve of the other side.  It takes at least five hands to hold the stick in place and for someone to draw the outside curve of the bottom line and the sheer.  You now have the most important set of numbers and a drawing lofted to full scale.  With the bottom measurement and sheer measurement drawn to scale on your garage floor, you now need to have the side height measurement for each station rib.  Keep in mind, however, that the bottom curves up 8-inches at both ends.

Side-Panel Measurements:Now that you have a top view of your boat drawn out to scale, which shows the bottom points and the sheer, you need to draw a side view of the Bitterroot Pram.  Using the same station lines on your top view, add the side view right on top of the other drawing.  The centerline now becomes your ground line, which would be the point that the boat would rest on a level floor.  The key to this step is to first consider the curvature of the bottom of the boat.  Using the centerline of your full-scale drawing, measure up from this ground line a distance of 8 inches, and snap a line across the entire distance.  You now have a parallel line 8 inches from the centerline, which represents the bottom rocker.  Use the 14-batten and draw a curved line from the transom to the stern plate.  (If you have actually gotten this far, and you are seriously considering building this pram, write me and ask for a paper pattern of the side panels.  This is a tricky curve.  I'll copy the pattern of the sides, transom and bulkheads on a piece of paper and send them to you for $15.  This charge will cover my time scrounging around for a tube plus shipping costs.)

Steps for Building the Bitterroot Pram