First off, the method I describe below is really only necessary when other more traditional patching techniques just won't cut it. I've seen traditional welds work just fine. However, this particular situation called for something a bit stronger. There was quite literally a hole in the bottom of my boat (a result of a poor attempt at a traditional weld and some very thin plastic.)
I first got this idea from Pete G. who has succeeded at patching several boats in a similar manner.
It actually works!
What you'll need:
4-6 Machine Screws and nuts
2 sheets of plastic from an old boat (mine were about 4in by 4in)
First Step: Cut out two sheets of plastic from your buddies old boat. Both should be equal in size and should be considerably larger than the area you want to patch. I found plastic that was the same color as my boat, but that this is not necessary. In fact, plastic of another color would just make it that much classier.
Step 2: Place one of the sheets over the area you want to seal on the outside of the boat. Drill holes at the corners of the sheet strait through the boat.
Step 3: Line up the second sheet of plastic on the inside of the boat and drill 4 matching holes through the inside sheet.
Step 4: Now that you have the machine screw holes all lined up, place the sheets of plastic aside for a moment and cover the inside of the crack (hole) with several layers of duct tape. I used a heat gun to heat the duct tape and it conformed to the crack pretty well.
Step 5: Line the sheets up correctly and place a nice thick layer of aqua seal over the entire underside of the outside sheet. Insert the machine screws. They should reach from the outside of the outter sheet through the inner sheet.
Step 6: Twist on the nuts on the inside and tighten er down snug. As you tighten the screws, the heads on the outside of the boat should sink in create a nice smooth surface.
Step 7: Once the patch is tightened all the way together (like a sandwich), use the sandpaper to sand the edges of the outer patch down. It doesn't need to be perfect, but it helps to have the edges as flush as possible to prevent the patch from 'catching' on rocks.
Step 8: Test your patch on low water Olympic peninsula runs in early October. If it holds up (like mine did), you're stoked.
And a quick photo drop: (fall 2011)
Our humble abode for the weekend. Olympic penninsula overlooking the Puget sound.