I have a 25' Kingfisher with an arch. I covered the boat with shrink wrap one year and had mildew problems. What I do now is build a kind of roll cage looking frame. I run four 1 1/2" pvc scedule 40 tubes from the bow rail (I just use electrical tape to fasten the conduits to the bow rail) over the arch, spaced equally along the width of the arch and then 45 them down to end up on the swim deck. I use a couple of pieces of stapping and conduit straps cross ways to brace the individual conduits. I then get one of the large silver tarps that are say $90 from Northern and cover the boat, I tie it off well by bringing the lines under the hull - I tension them with shock cord. In the bow I leave an opening where the trap sits above the bow rail (think big scoop) maybe two feet wide. The wind can then circulate through bow to stern. Things stay dry underneath and ventilated. I get three or four years out of a tarp and for me it works well. I live in the N.E. we don't get loads of snow where I am but it has stood up fine to snow loads, we do have lots of wind however and the tarp has only blown off once - in a snow storm, winds were 90+.