sweet... thanks 69cpu! I've never even felt the boat tip, but I usually sail with a 7yo who isn't the strongest swimmer so I tend to let go of the lines when a gust comes along.
That said, he always wants to go faster, so I hope to add my jib design to the list of existing ones soon!
I'm going to bring a tube and see if I can take him tubing
Update, two things I have learned:
1- the old windsurfer sail I had laying around was indeed too stiff to act as a jib.
2- Using lines that only have 1% stretchability is important, because otherwise when you try to furl the sail the whole system just bends and refuses to function. (If I had gotten on the water, I probably would have discovered a good gust makes everything go splat). Also, you can't jugde stretchability by yanking on the line at the hardware store