It’s been a long time since the last post, because I’ve been through the wringer on the rocker. To review: I tried taking apart the gun’ls to trace them out on paper to get the sheer. I tried running string lines at various stages in the process. I’ve run all the numbers multiple times. I’ve measured, calculated, remeasured, and recalculated. I estimate that more than half of my time building these boats was spent trying to nail down sheer and rocker. But in the end, I think I’ve found a method that works reliably for me.
First, the reason this matters is because unless sheer is right, rocker won’t be right, and if rocker and sheer are wrong then I cannot reliably calculate the lengths of ribs. So here’s how I do it:
- Turn the deck upside down on the horses. For reasons I’ll explain later, it’s necessary to cantilever the canoe a little more than half of its width off of the horses. To do this, I attach rails to the horses that extend the required distance to one side, as illustrated in the pictures below.
- Clamp sticks to either end of the deck, and in the center. I use old window sash weights to hold the center down, though I never actually observed it rising. This just felt like a good way to work against any bowing under the tension of a string.
- Attach stems, and run the keel over them.
- Run a string from stem to stem
- Use a story stick to take the depth, from the string to the keel.
- Transpose the marks by as much rocker as I want. In the case of this boat, it’s 1.5 in.
- Use the story stick to set the height of the keel on the stems, and mark a cut line on the stem, along the bottom edge of the keel.
- Remove the string, remove the stems. Cut the stems.
- Reattach the stems, and re-run the string.
By clamping sticks in (2) I lock in the sheer so it won’t move on me as I pull the string tight and add the weight of the stems. By cantilevering the boat over the edge of the horses I give the string room to run without the horses getting in the way. The string is critical, since it’s used first to set the rocker, and then to take some other measurements later. The story stick is a quick and sure way to set the rocker without fussing with numbers.
Once the stems are set and screwed to the keel, and the string re-run, I use a clever little pair of sticks held with rubber bands to run the width of the canoe at each rib station. The stick that matters is the one that crosses the deck on the bottom of the gun’l. In a previous version of this process I clamped this stick. But moving those clamps got to be annoying.
I’m going to take two measurements, here:
- I measure from the string to the bottom edge of the gun’l (the edge closest to me as I stand over the upside-down deck, the edge that faces the ceiling.
- I measure from the same edge of the gun’l to the facing edge of the keel (top edge, the edge facing toward the floor when the boat is upside down on the horses.
From these two measurements I can derive everything else I need, starting with sheer and rocker. To do this, I use another story stick. As I move along the deck I line up the bottom of the story stick to the string, and I mark it beneath the topmost horizontal stick. Then I use a small square to mark just under the keel. Once I’ve done this, my stick has a series of marks that I can use to take the measurements I need. I record these in my spreadsheet for the formulas to do their work later.
I’ll do a similar process to get the width of the canoe at each station. Marking on a story stick, ill take the measurements off that and record them.
Once all of these measurements are taken, I can get ready for cutting ribs.