In my Instructable "how to row a canoe" I show my big heavy coleman canoe, after using a load extender with my pickup truck for many years, I had to figure out how to put my canoe on top of my new minivan without scratching it up. I didn't want to have to worry about lifting a big heavy boat onto the roof of my van after wearing myself out rowing. so here is what I came up with.
Step 1: The expensive parts
The most expensive part you will need is a reciever hitch for you car or truck. I got mine from JCWhitney.com for $100 dollars it is a class V (5) hitch and is rated for 700 lbs tounge weight, since my truck is only rated for 200 lbs I figured it wouldn't hurt to have a strong hitch and since I plan on driving the truck into the ground no one else will over load my truck. trailer hitches are rated by the weight they can pull, a class I hitch is the simple flat bar that you put on a car to support the hitch ball. Class II is a 1 1/4" square tube that is rated higer than a class I, it is the lowest of the reciever type hitches. Class III and up are 2" square reciever type hitches, the hitch has a 2" square hole in it to take the draw bar that the hitch ball is mounted on.
You can get a hitch and mount it yourself, or a Uhaul, Sears automotive department , or similar will sell you one and mount it for you.
The next part you will need is a "truck bed extender" I got mine from Harbor Freight.com (p# 39168-2VGA) for 30$, it now lists for $39.99.
The third part you will need for a minivan or car is a 2" square steel tube that fits into a 2" reciever hitch, I got mine as a left over from construction at work.
The last part you will need is another draw bar pin like the two that come with the bed extender. I got mine cheap at Home Depot, but harbor freight, jcwhitney, or any trailer hitch type place will have them.
This picture is from the Harbor Freight catalog, I used to haul my canoe upside down on the bed extender just like the boards are shown in this picture.