Sailmaking

I slathered a layer of epoxy on the interior sides of some panels the other night, but the really big news here is that I've begun to make my sail.  I got the fabric today from Rockywoods, and by the end of the evening I'd measured and cut out the two halves of the sail.

The fabric (a 4-oz ripstop polyester) is attractive and lightweight.  One side is coated with a urethane treatment of some sort, so there's a slight color difference between the sides.  But appearance isn't high on my priority list, and anyway, it'll look way better than a blue tarp would have.

Squeezing 57 sqft of sail into 5 yards of fabric.

I rolled it out and followed the instructions in the plans for squeezing the entire sail into 5 yards of fabric.  (The plans discuss doing this with 6' wide fabric; my fabric was about 5' wide, but it fit.  Just barely....)

For a while, I was at a loss as to how to get the shape from the plans to the cloth.  The plans list the length of the curved head and foot of the sail, and they list the length of the center of the sail, but there was no list of offsets at various station lines like there had been for the plywood.  There was also some confusion as to how I could get the proper curve for the hollow at the open end of the crab claw.

I had learned in my navigation class how to use an engineer's scale to read a measurement from a nautical chart, and I had heard somewhere that you can do the same thing with boat plans.  So I got the engineer's rule from my nav gear and confirmed the width of the sail at the top, which allowed me to determine that the sail curves inwards one foot from the tips to the center.  Once I've joined the two halves, I'll draw that curve in with a batten.

If you look closely at the full-sized version of this pic, you can see that the plans read 6'7" (or 79") along this line.  The plans are at 1:10 scale, and I've laid the 1:10 edge of my engineer's rule along the line.  This allows me the determine that the length shown is indeed 79".  It's unnecessary for this sort of situation, but this technique is useful if you want a dimension that's not given in the plans.  As long as your rule has a scale that matches the scale of your plans, it will work.

I decided the best way to get the shape of the long curve onto the cloth would be to carefully mark the three corners, then mark the length of the curved side on a batten, and then use exactly that much batten to draw in the curve.  My boys helped again - they're getting to be pro's after the plywood drafting job.

I soon had the shape of the two sail halves drawn in, but it would be a mistake to cut along those lines.   The center seam needs some overlap, and it takes fabric to enclose the rope that's supposed to strengthen each edge.  So I traced in a second line in a different color, 1.5" from the edge of the sail, and this is the line I cut.

When it was all done, I had two halves of a crabclaw sail.  I couldn't resist the temptation to lay them out together to get a sense of the full size of the thing.  At 57.5 sqft of sail area, it's a pretty powerful rig.  Setting it out in the living room brought that home a bit - it seems bigger than the sail for my El Toro dinghy, which weighs about the same.

Next up in sailmaking: I'll be pinning and stitching the center seam together.  I also need to procure some rope for the edges so I can sew those in.    I may also cut some reinforcement radius pieces for the corners out of my meager scrap pile.  It seems smart - most sails have reinforcement patches there.

Meanwhile, I still have interior epoxy coating to do.  Everything is supposed to get two coats, with a bit of sanding between.  And if I'm close to having a sail, I really ought to consider getting the spars finished.  Plenty to do!

Mystery Solved: A Small Error in the Wharram Melanesia Plans

I mentioned in my previous post that there was one station point that just didn't line up with the curve of the side panels.  I quintuple-checked my measurements of that point and its neighbors, of the station lines themselves, and of all the figures on the plans related to any of these items.  In particular, I checked and rechecked my math, because I'm working in Imperial units, and it can be easy to go awry when adding unlike fractions.  At every step of the way, it all checked out except the location of that point on the curve.

Searching online for phrases like "Wharram Melanesia error" or "Wharram Melanesia station line" turned up nothing obvious, either.  I had just about decided to email the Wharram office and see if they had any ideas, when a thought occurred to me.  The Wharram folks, being Brits, work in both metric and Imperial units, and the plans are marked with dimensions in both systems throughout.  Not having access to a good construction tape measure with metric markings, I've been using the Imperial measurements.  But what if there was an error in the conversion?

I checked, and the numbers didn't match up.  The plans give the depth of the side panel at that point as 13 3/16" (around 13.19") or 351mm.  But 351mm is equivalent to around 13.82", not the 13.19" indicated.  When expressed as fractions, it's easy to see the potential for a typo in the two numbers - 3, rather than 13, 16ths. 

(In case you're reading this after searching for an explanation for the errant point on your own Melanesia, dear reader, the point in question is the side panel curve at the second station line forward of the center joint.)
I checked the position of my curve at the spot in question, and it came in around 13.7" - about an eighth of an inch shorter than the figure I'd just arrived at.  I'll need to redraw those side panel curves with that station point included to make sure I don't have gaps in my hull at those points.  But that's only a few minute's worth of work.  And the discrepancy between the curve as I drew it and the curve it should be is only 0.125", rather than the 0.625" error.

Further, the error would have required a sudden concavity in the middle of a convex curve.  It would be very difficult to render that curve correctly; with the new figures, I need to produce a slightly greater convexity at that point, which is much easier.

So that's a big relief. After figuring out this problem, I traced the stem and stern posts onto the plywood, and figured out a way to sketch in the 13.75" radius curves for the deck beams.  At this point, the transfer of curves from plans to plywood is complete.  I hope to begin cutting the panels out tomorrow.  I need to order epoxy, fiberglass tape, etc., so that I'm ready to begin putting it together when the cutting is finished.