I had one similar. They are very thin That 21ft cover folded will fit in a shoebox. They are not a cover you can abuse , if you get a tear it will run the full length with little resistance
i just got one like this cover, its alright quality as cheap covers go, though as tony says you will need to support it
I'd seen this chap advertising one that was factory fault or something... says it was purely a packaging fault and the cover was fine condition just a bit of staining, bidding starts at 99p....... cant really go wrong at that price!
i've got one of these covers on my searay and the best answer to the puddling problem that i have found is i light wieght ratchet strap or two from the bow over the screen and down to the blunt end. but with your boat you may find this usefull