If the grommets are ripping, try wrapping the tarp around a small rock in the corners of the tarp and wrap a tie down rope around the tarp wrapped rock to tie it down. I have been using an old Campmore tarp this way for years, all the gromets gave way years ago. (Cliff Jacobson idea)
I am storing my canoe next to the house on saw horse. That side of the house gets direct sunlight half the day. So I made an awning with a tarp. I used 2x4's to create a frame and attached the tarp to it. Problem is, we've been getting 30+ MPH winds and the whole works got wrecked. The tarp acts like a sail and busted up the wood framework. Luckily the wood didnt hit the canoe.
I know UV rays are bad on a 3 ply plastic canoe (Old Town Discovery), but maybe I'm too paranoid about it. What if I didnt cover it??
Just a suggestion: They sell big rolls of Tyvek at most lumber yards. If your at all handy with a sewing machine, make yourself a fitted canoe cover. Seems that might be some protection for a canoe? The thing is, I don't know all these new materials and just what their interaction on each other is or might be, come the time to paddle you might find that the Tyvek has glued itself to the canoe! Or maybe the canoe to the Tyvek?
Where I live we get winter storms with wind that just about knock you over. In my limited experience it's best not to underestimate what the wind is capable of.
I once had a chance to explore a German (WW2) concrete artillery bunker; on some stormy nights I wouldn't mind having a house built like that!
The only thing about a fitted cover for outdoor storage is that you probably want to be sure that you have ventilation. Otherwise you might have mildew problems etc. Wind and tarps don't mix well. It can be done. Needs to be taught and sturdy.
I thought I remembered something about a Tyvek cover: piragis.com in the latest catalog they sent me, has a "canoe storage cover" made of Tyvek. They indicate that it's for indoor storage and short term outdoor storage. Good against 99.8% Uv's. There's more, you might want to check out the web site.
I put in two rows of screw eyes to hold the lacing, which was 1/4" nylon rope. The tarps were laced tightly to the top row. The lacing for the bottom of each tarp went in a zig-zag pattern from one end to the other, which made it easy to tension the tarps. During one or two summers I removed the lower lacing so the tarps could hang down over the boats to protect them from sunlight. The loose edges of the tarps were held down with short ropes and bricks. I should have hung them that way all the time, because the boats faded even though they were in the shade.