A boat cover serves several purposes: It keeps leaves and dirt outside your boat, where they're supposed to be. It keeps animals out of your boat when your boat is laid up for the off season. A good one can keep mould and mildew from forming in your boat. Its most important function, though, is to keep the inside of your boat dry. The best thing is, you don't have to buy one; you can make one yourself.
Cotton canvas is rapidly being replaced as the material of choice for boat covers. Natural materials aren't naturally waterproof, and eventually you must re-waterproof them. This need for maintenance is one of the reasons that "store-bought" boat covers are made from synthetic materials. Sold under a variety of trade names and in varying thicknesses, the material is woven, just like cotton canvas, but doesn't require you to apply a coat of wax every so often to maintain its waterproof qualities.
While some materials are simply rubberised cotton, guaranteed to promote the growth of mould and mildew by trapping moisture in your boat, and some are little more than a sheet of plastic just waiting to crack after long exposure to direct sun, the woven synthetics help prevent off season moisture problems and stand up to any environmental condition.
Before your homemade boat cover can take shape, you have to cut the material and sew hems around the cover. A heavy-duty sewing machine will make short work of running a double seam around the edge of the cover.
The shape and size of the cover is as individual as your boat. There are differences in winter and "weather" covers. A weather cover is close fitting and attached to the rail, whereas a winter cover comes closer to wrapping the boat. A winter cover comes about halfway down the side of the boat and can be laced onto the boat with rope that runs under the boat's bottom or attached to the frame of the trailer the boat is on with bungee cords.
Whether you use rope tie-downs, bungee cords, Dzus fasteners or snaps, incorporate a fastening system that gives you the flexibility to allow bow-to-stern air circulation in the long winter months when your boat is laid up, or secures completely to prevent a driving rain from soaking your boat's interior. Mechanical fasteners like snaps, "dot" fasteners or turn-and-lock posts like Dzus fasteners have a post, permanently attached to the top rail of the boat's hull. The female part of the fastening system comes in two parts, each set in place around a hole (which you poke into the fabric) with a sharp thump or the clamping motion of a pair of pliers.
If you plan to use rope tie-downs or bungee cords, you'll have to sew fabric or metal grommets into the edges of the cover. The grommets will prevent the hooks on the bungee cords from tearing the cover fabric and give tie-down ropes a point to thread through to anchor the cover. Fabric grommets must be sewn in; metal grommets are installed by piercing the material and then clamping or hammering a two-part ring into place around the hole. This type of fastening is critical if you're making a winter cover.