: I've a customer who's asking about a sail rig.

You have to ask them what kind of sailing they plan on . . . for kayaks and canoes, etc, there are a couple of options. By far the simplest to build and use is a "down wind only" rig. There are plenty of commercial downwind sails available, though they are easy enough to make on your own, and all you really need is a simple mounting bracket on your deck. You can steer with your weight or your paddle, and you can often sail up to a beam reach with them. I think kites are the newest fad . . . .

A rig that lets you sail upwind as well is a lot more complex - you need some sort of leeway prevention (daggerboard, centerboard, keel, bilgeboard, or leeboard), and a dedicated rudder is useful as well. Your mast step and partners will need to be up to the stresses involved, too.
Even then, without outriggers or some way to hike out, you could probably paddle upwind faster than you could sail!

In any case, the definitive book is "Canoe Rig - the essence and the art," by Todd Bradshaw. Much of it is transferable to kayaks, but it's well worth owning no matter what!

Good luck!
Dave