This is the kayak rack I built to alleviate any problem of 'oil canning', an affliction common to the popular poly boats when stored incorrectly due to stresses on heated, softened hulls. Though outside, I have not had a problem yet. I had planned this to be a 2 yak carrier, but my city living biz partner needed boat storage so my equipment space underneath is occupied for the time being. I carved foam blocks for her kayak to rest on which still gives some room for equipment that doesn't fit in their respective cockpits.
I started with a a frame of a door which I had used to display painted doors that I made in an earlier existence. It seemed well qualified for the task as the beam sizes of the kayaks are 22.5" and the door it was meant to house was 24". This gave some latitude to slide the kayaks in easily. It was also long enough to stretch the fabric past the bulkheads on both. If I had built this from scratch, I think I would gone a bit wider and used springs to attach the fabric to the vertical yellow pine uprights.
I used artist canvas that I had as the slings since I have moved to painting on wood panels. I probably would have used a heavy fishnet due to the elasticity of the weave if I was going for new. I placed grommets in the four corners after they were measured and seamed all around. They had stretched nicely to give even support to the hull.
The wheels were a high quality trash pick as they are a large diameter and have bearings.
I laminated two yellow pine uprights opposite grain direction to increase strength. I placed eye hooks and s hooks to hang the canvas. They were spaced for adequate clearance from the hull of the boat above to the cockpit combing below.
I always leave the bulkheads open when storing them. I also keep them well out of the sun. They will live in the garage after my studio is finished
I had a pre-existing roof rack on my cap for hauling wood and metal, the materials I use expensively in my shop. I wanted the ability of carrying two kayaks on this rack, but it required some modifications for the particular way I had preferred the kayak to rest to prevent any deformation of the hull. I presume that it would be best to rest the boat on it's beam. The secondary advantage is that I do not need cockpit covers as the angle will not allow rain to accumulate.
The first thing I did was attach two eye bolts on the bottom bracket of the truck rack. I sprayed bio-shield on them to prevent rusting.
I constructed the a frame from cedar because of it's weathering capabilities. The original cedar materials rack has been there eight years with no problems. It is hinged in 3 places for flat storage. I added strips of wood on the base so it mortised into the materials rack. I also secure it with a parachute cord. All of the exposed wood is covered with a closed cell foam. I used waterproof wood glue + stainless screws and brass bolts for joining the wood and hardware(brass).
Nylon web belts with ratcheting mechanisms secure the kayaks' hull, and NRS nylon straps which have a locking cam method of tightening are additional insurance through the bow and stern.
I travelled with 2 boats 1400 miles in early June with no problems.