How much weight can the bed rails support?

I have a 2011 Taco 4-door short bed.
I want to transport an aluminum canoe that's about 18' long. It's a beast.
I'm in the planning stages of building a canoe rack out of PVC pipe.

I'll have to do up some sort of diagram but it'll look similar to this...



The way I'm thinking of mounting it is to attach a pipe the length of the rail with an elbow at each end right to the rail. The bolts would go through the elbows and the pipe would be more or less for stability.
Same thing on the other side.
Then two lengths of pipe coming up out of the elbow, another elbow at each end and and then two more lengths connecting the two sides.

My concern is the weight of the canoe being supported, partly, by the rails. The other part of the weight would be on the roof.

Do I need to worry about the weight of the canoe causing any damage to the rails?

I'm open to suggestions but I think this is a pretty good idea in theory.

Right now I'm just trying to figure out the best thing to use to bolt it. Right now I'm figuring using washers wide enough to fit inside the track. The most ideal thing would be something like this, although I'm not sure what these are called or if it's a common item in the hardware store.



If done right I think I could slide the whole rack right into the rail channels as one piece and it wouldn't be too difficult.

Not sure how expensive this would get either... I'm going to shoot out to the hardware store to get an idea... I have a Sears Hardware gift card I haven't used from Christmas that should cover the majority of the cost.

 

Run the pvc to the bed for weight transfer maybe, use the rail to stabilize the pvc from shifting.
Alternate plan, use the bed side top to hold the weight like the rack in the photo does. For further ideas run a photo search for "pvc kayak rack" and adjust the parameters for more photos.
Ironicly I got the xporter from thule , looks almost the same as the one in the photo.

 

How much weight we talking for the canoe? You didn't give a number.

That will also have to be one hell of a sturdy rack. You sure PVC's gonna be strong enough, not only to support the weight of the canoe, but also to withstand the headwinds pushing the canoe backward, and flexing the rack? At 40-50-60mph, that's one hell of a lot of pressure. Or will you also tie the canoe off to the front of the vehicle?

 

I found pvc to be too flexable for a previous kayak i had so i went to some good used 2"x4".

As far as the original question, rail strength, Im not sure. I think with the load spread over several bolts , the rail will be quite strong, I'm hesitant to say anything like 150lb per rail, I just dont know, but I never heard of one failing.

 

i wouldnt use pvc. not only is the kayak weight an issue, but also how much force the wind will load down on the rack. build it out of steel. the rack slide is one inch wide. use flat stock, drill a hole in it and copy the locking design of the tie down cleets. thats what im doing this weekend