Is your bike rack full of clamps, claws, and cleats that might get bent while carrying your sunfish? Or, is the free boat you found on Craigslist likely to develop another crack if tied down against your rack? Tie a couple of 2x4s to the rack with innertubes and head to the beach!
Step 1: Tie 2x4s to the Rack
Find some old 2x4s and tie them to the rack with innertubes.
Step 2: How to Tie With Innertubes
When you start wrapping, just wrap around the end. There's no need for a knot. The friction of the innertube on itself will hold it in place. When you get the end, tie a half-hitch. If you tie any more complicated knots, you'll never get them undone. Instead, you'll end up cutting your innertubes off every time. This only results in a bucket full of useless 6 inch long innertubes and everyone running around the shop madly asking each other "where are the innertubes? Did you go to the bike shop's dumpster?!?!"
Step 3: Foam
Put some foam on the 2x4s and dump the boat on top. Make sure to figure out if your rack can handle the weight of the boat before you put it on. My rack can take 75 lbs. This sunfish weights exactly 75 lbs according to its owner, who convinced me to take him and the boat to the beach in the first place.
Step 4: Tie the Boat on With a Trucker's Hitch.
Yes, it's a little fast; he'll be sure to make a proper trucker's hitch Instructable soon.