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.

Step 2: Modifying the Bed Extender for Roof Top Use.

I started with the standard bed extender, but I wanted to be able to swivel the cross bar to allow easier loading of the canoe. I found a galvanized pipe at the hardware store fit the socket on the end of the "L" bar perfectly so I drilled it to lock it into the "L" bar socket.

Step 3: Now for Some Height

I then put the 2" square tube over the pipe and drilled them both at once to accept the lock pin. Then I cut the 2" tube to the correct height for my van.

Step 4: Setting in the Cross Bar

then I put the cross bar in the top of the 2" tube. It is set at the same level as the front cross bar on my van luggage rack, I should put a pin in the tubes to lock the top bar down but the lower leg of the "T" is so long that it can't jump up out of the 2" tube, so I havn't.
As you can see in the picture this allows me to load and unload the van without having to remove the 2" tube section, the tailgate comes up to high to open it with the canoe on the roof though.

Step 5: Now Comes the Hard(est) Part

this is the only hard part of using this loading system,
You need to pick up one end of the canoe and put it on the cross bar.
I then tie the end of the canoe down using the rope that is attached to the inner "ears" on the cross bar to keep the canoe "locked" onto the cross bar at the correct length.
I then tied the canoe down with the rope that attaches to the outer "ears" on the ends of the cross bar to keep the canoe from sliding from side to side.

Step 6: The Rest Is Easy!

now lift the other end of the canoe (one handed) and walk it around to the front of the vehicle. I install the cross bar on the van luggage rack only when I am putting something on the roof as the rest of the time it just kills your gas milage, the rest of the time I keep them under the floor in the "trunk". (those of you with grand caravans will know what I mean) If you don't have a roof rack, canoe blocks (foam block with a groove in it that snaps over the canoe rail) would work for the front.
I make sure that the nose of the canoe falls just at the peak of the windshield, this keeps the drag from the canoe down and doesn't slow the van down noticably. I can do this because the bed extender is considered part of the frame of the vehicle, so the overhang only starts aft of the cross bar.
This is the point when you put in the upper lock pin to keep the cross bar and 2" tube secure to the pipe. Then I throw an endlless cargo strap over the front of the canoe and strap it to the roof of the van (open the doors first so you don't have to do the dukes of hazzard climb in the window trick) the buckle goes in the middle between the front seats which makes it easy to keep an eye on the strap tension. Make sure you twist the strap at least 3 times on each side of the canoe so it doesn't hum like a giant hornet on the roof of the car.
I tie the forward rope of the canoe over the top of the van, and over the carry thwarts in the canoe (to save the paint) and down to the trailer hitch, this rope is to stop the canoe from sliding forward. I tie the aft rope from the canoe to the reciever hitch also to keep the canoe from sliding backwards, so far this has worked for me for almost 2 years on very bumpy back roads and at highway speeds with no problems. I like this way of rigging the tie offs as there is no rope chaffing my hood or the plastic front bumper, and I don't have to lie on the ground to feed the rope through the grill holes in the bumper to find something solid to tie off to.
Yakamia and/or Thule both make similar products that swivel for kayaks and canoes (without the wide cross bar) they both rise straight up from the reciever hitch though so clearance for a tailgate will be a problem, and they are both expensive. This set up will also work for ladders and lumber (within reason), something the expensive ones won't do as well.