Step 1: Grab Your Notepad and Take Notes, This Video Instructable Is Information-dense!

Here's the list of things to buy/find/scavenge (parentheses is where I got the item)

One Inflatable twin size air mattress (Target)
One 10' x 12' polyethylene tarp (Target)
One 8 foot long 2x4 (Home Depot)
One 2'x4' plywood Project Panel (Home Depot)
Four Schedule 80 gray PVC conduit pipes, 1-1/4" diameter, 10 feet long (Home Depot)
100' poly rope, 1/4" or 3/8"
Marine-grade #8 phillips head screws, such as stainless steel, 12@ 2-1/2"and 4@ 1-1/4" (Home Depot)
Outdoor paint, acrylic or synthetic enamel, from the customer reject color shelf (Kelly Moore)
Craft thread or Dual-purpose thread (JoAnns)

It's a bit of a challenge to get all these materials together if you buy them all at full price. But if you have paint leftover, or can find the air mattress or the tarp on sale, your sailboat will cost under $100. Sweet!

The tools you'll need:

Drill with a 1/2" bit and a phillips head driver bit
Hole Saw
Circle saw (And perhaps a chisel or a keyhole saw)
Scissors
Sewing Machine
Needle
Lighter
Sanding block and sandpaper
Paintbrush

Step 2: Corrections and Clarifications to the Video, and Upgrades

The keel has a notch 1-1/2" tall and 6" wide if you are going to screw it directly to the 2x4. If you are going to make the keel removable for easier transport, then make the notch taller at 3" so you have room to screw in the keel stays.

Since circumference = Pi times diameter, make sure your pole pockets for the battens, gaffe and boom are sufficiently large to accommodate the pvc. If your pockets are 8" around they will be more than adequate. For the battens, simply sew rectangles of tarp material to the air mattress' tarp sack. I cut strips 6" x 26", hemmed the ends to make pieces 6"x24" and then sewed those rectangles to the sack. I folded the edges under and held everything in place with small bits of masking tape. The battens are equally spaced, about 16" apart.

Make your sailboat feel a bit more stable by adding wings. You'll rest your feet on these tiny trampolines and gain a little better control of your boat. Make two of the battens 4 feet long instead of 2 feet long, and instead of sewing simple pole pockets, make a big sleeve 16" wide and 45" long. Replace the two small sleeves at the rear with this one big sleeve. Loop rope through each batten and create a tension structure by tying the rope through a hole in the 2x4 and cinching it up really tight.

Most cars can fit an 8' length but 10' is more challenging. If you plan on taking the sailboat car camping, consider cutting the gaffe from 10' into a 2' section and an 8' section. Spray the end with silicone so you'll be able to take it apart again. Some lubricants will eat the pvc. I haven't made a big sail for this little boat yet, but by splitting the gaffe into two parts, you could make a sail with a 16' gaffe that would probably work well.

Use a real paddle as a rudder.

Add metal pulleys, cleats and use an oar-lock for the rudder

Use a better grade of tarp.

Use Dacron fabric instead of tarp. Get Dacron here: http://gaboats.com/

Step 3: Safety and Liability

Boating can be dangerous. Be smart. Wear a PFD, and make sure someone knows what you are doing.i

Printed directly on the air mattress is a warning that it is NOT a floatation device. You are making a choice to intentionally misuse this product when you use it as the bladder for your sailboat. If you get into trouble with a sailboat made from these instructions, it is unlikely that you or your heirs will be able to recover any sort of settlement against me, the air mattress manufacturer, or Instructables. You assume for yourself all responsibility for your choices, whether you can foresee the consequences or not. Furthermore, I specifically assign to you the responsibility for communicating to your heirs that you entered into the building of this sailboat freely and that you accept any and all consequences that could arise from your participation.

Step 4: Further Exploration

Is there a way to make an air mattress windsurfer?
If the gaffe, mast, and other long parts were collapsible and lighter, is there a way to make a sailboat that is back-packable?