I thought I'd just stress that, if a boat is being left covered, how vital it is that the cockpit cover is taught enough and sloped enough over a high boom so that no rain water can pool in it. Sadly I can illustrate with two recent examples.
First, a neighbour covered his boat at his jetty just down river from where I am. Like so many people around here who have bought cheap second hand boats (often off ebay) he has little boating experience. He used a sheet of plastic "tarp", of the sort you get in B&Q, to cover the boat. The photo shows the result. I assume that some water seeped through the cover into the boat and a lot more water pooled in the cover. As a result the boat capsized. Admittedly, once heeled, the boat may have caught on the jetty to finally capsize it.
Second a Seafly racing dinghy which, sad to say, some years ago belonged to me. I part exchanged it for a new Seafly and the boat builders sold it on to a family in Scotland. Eventually it has ended up abandoned on its trolley in the Royal Forth Yacht Club boat park. Under the weight of water pooling in the cover the GRP/foam composite hull has been bowed in and on one side has cracked. Repairable, but I suspect no one will think it worth the effort and the boat will be broken up. The Yacht Club are presently trying to find someone who might want it. It was a fine racing dinghy when I had it, sad.
Peter
ps (advertisement!) my Seafly web site:
http://seaflymemories.uk