To those interested parties, sorry about the delay.  First, bear in mind that I already knew 10 sf was a recommended size for a storm jib for our P-15.  Then I messed around with my architectural program to get an idea of the respective dimensions that would yield 10 sf.  That done, I got enough 1/4" dacron yacht braid to go around the perimeter, plus a healthy dose of extra for the loops at the corners.  You see, I had this idea of making a looped, rope perimeter, then filling in the shape with polytarp.  This had pluses and minuses, as you'll soon see.

I took my rope out to the boat, with the mast rigged.  I tied a bowline at the tack, measured off the luff I wanted, then tied in another loop inline where I wanted the sail's top loop (durn it, can't remember what it's called).  I added a healthy, 15" pendant to the tack because I wanted the sail to have at least a little effect on the airflow over the main.  I fastened the the "tack" to the pendant, the "top" to the halyard, and drew the halyard tight.  Next I managed to do something clever:  I added  the dimensions of the leech and foot together, added some for the loop at the clew, and tied the end off at the tack.  By snapping my sheet to the resulting loop, then drawing it tight, I was able to simulate the forces the sail would be under and thus get the leech/foot proportions right because the triangle it formed naturally found the spot where the vectors were equal....eek, hopefully this clumsy explanation is clearer than mud!  The goal was to come up with a triangular shape that set well, and it seems to have worked.  Once I found out where the sheet wanted to be, I tied in another loop to act as the clew.  That completed step one.

I carried my carefully tied border into the house and laid in on a blue polytarp that was being sacrificed for the project.  I put a tiny bit of belly into the sides to give the sail a bit of shape-I think I used 1.5" for the luff and leech, 0.75" for the foot.  For a storm jib, cutting it dead flat might have worked.  I'll not go into great detail here, since much better instruction can be found by searching "polytarp sails" on the net.  Anyhow I got the sail together, and it looked like everyone else's poly sail except it had loops in the corners instead of grommets.  I did put grommets along the luff, and used small, galvanized snap shackles from Lowe's for hanks.

Test day.  Instead of prudently waiting for benign sailing weather, I went out when the wind was blowing the trees about.  The gusts made the rigging moan; I think this means the winds were over 20-25 mph when gusting.  The sail worked beautifully, allowing Amy Ann to point better and sail livlier than with reefed main alone.  I noticed, though, that gradually the pointing fell off.  Closer investigation showed that the storm jib was bagging out because the tarp was sliding up the perimeter rope, giving me more belly just when I didn't want it.  I've since compensated for this by putting grommets in the reinforced corners, then lashing them to the existing loops with 1/8" nylon cord.  I don't know how well this will work, and probably won't find out until fall, when the local winds pick up again.

Hope this helps.  Cheers.

Steve Tyree
P-15 #2098 "Amy Ann"