Cape Falcon 66: To the Mill for Rib Stock

I tentatively was planning to buy my ribs from Brian. But then I called up the guy I bought rib stock from last year to find out if he has anything. He is like a lot of folks out here who have a portable mill and some land, so they slab logs and sell them as a side hustle to support their other hobbies. This guy makes Windsor chairs. So when I tell him I want good bending stock, he knows exactly what to give me. This time he told me he had a freshly felled White Oak hanging around, which he’d been planning to quarter saw because it’s so clear and wide. He agreed to saw out a slice to order for me for 5.00 / bf. So for about $300.00 I should have enough material for all the ribs I need. And since I told him to take out 8′ sections, I should have plenty enough for some kayak cockpit comings besides.

So now the Oak is ready to be picked up. I’ll head out there this Saturday:

As an aside: if anyone is wondering how one finds such a person as this, who has land and a mill, here’s how: you jump on Craigslist or FB Marketplace and search for people selling rough sawn lumber or “live edge” slabs. The other thing you can do is go to the Wood Mizer website and look for folks in your area who have registered their mills. Usually they have contact info, and by my lights it is fairly up to date since some of the people I use are listed there. And don’t be frustrated when the first few folks turn you away. Clear, straight grained white oak isn’t a premium material in a market that favors dramatic grain patterns for bar tops.

Cape Falcon 66: Taking Rib Length Adjustments

Ok, for the uninitiated, this might be complicated. Let’s imagine we have a formula for calculating rib lengths (which we do). The variables in this formula will be: (1) the overall depth of the boat, (2) the depth to sheer, and (3) the width of the boat at the given rib position. We can get all of those measurements at any time, except for (2). The time to get that is now, when we can lay out a gun’l and measure against a straight line.

Brian recommends to do it one way. But, again, I’ve chosen another way. My way is more complicated to work out, and involves way more number munging. But if you’re working in a spreadsheet where you can just copy and paste a formula, it’s not that much more work. And it doesn’t involve having to get down on the floor to snap a chalk line and have to figure out how not to kick your gun’l out of alignment when you trip on it. You can do it suspended in space, with your gun’ls propped up or stretched out over horses, or whatever. Here’s how it works:

  1. Stretch the two gun’ls of the boat end to end, curving outward from one another and secure them at the ends and in the center with spring clamps and battens. (If your sheer is asymetrical, then spread the gun’ls at the end that will be lower by twice the depth you want.)
  2. Measure from station to station, across the two gun’ls to get the distance between the inside edges of the rib at the rib mortise positions. Note: we’re measuring at the marks closest to the center of the boat.
  3. Put each measurement into a table, in a spreadsheet.
  4. Calculate the actual offsets using math (explained below)

That’s the quick rundown. Here’s the tl;dr explanation. Our formula will give us the length of the rib up to an imaginary line drawn through the lowest point of the sheer. We therefore need to ADD length to each rib to account for the sheer. So the marks in red below are the lengths we need, at each rib position:

One way to get these lengths would be to snap a chalk line representing the bottom horizontal in the figure above. Then, we position the gun’l so the middle touches the line and the ends are equidistant from the line (unless we have more sheer in the bow, in which case one end would be higher than the other). Then, we place our ruler between the gun’l and the chalk line at each rib position and record the length.

As I said, this works well enough if you have space and an even floor and you don’t ever kick the gun’l. But there’s an other way to do this. First, observe that each of those line segments above can be extended upward, to another line drawn between the tips of the gun’l, like so:

If I know the lengths of the green lines, I can get the lengths of the red lines, or vice versa, by subtracting the lengths from the length of the line at the widest point (the green line in the center, above). But in this case, it’s the bottom line I want to avoid dealing with. But neither do I want to run a string from end to end to get the top line, since it would be easy to flex the gun’l or deflect the string. So what I did was to lie the two gun’ls flat, nose to nose, with the sheer curves facing out from each other:

Then I measured from the inside edge of one gun’l to the inside edge of the other gun’l, at each rib position. I’m taking special note of the width at the widest point (the deepest part of the sheer). This is the number from which I’ll subtract all the other lengths. The result will be the amount I need to add to each rib to account for sheer.

So to derive the lengths of a given red line r, the formula is: r = (C – g)/2, where g is the length of a green line and C the measurement at the widest point.

As a side note, and just to make things a bit more complicated, you can see that I actually measured from three inches. That just meant that I had to subtract three from the measurement before writing it down.