Boat covers are most effective when made from a sun-resistant and waterproof material that fits snugly around the edges of the boat. A moderately fitted cover can be made with basic sewing skills and will be sufficient at protecting the boat. Natural cotton canvas is a breathable and water-repellent cover material. If a choice of weave is offered, choose duck canvas, which is a tighter weave. Sunbrella acrylic canvas is the most commonly used material because it holds up better in the sun.
Measure the centerline of the boat by stretching the measuring tape from the tip of the bow to the center of the stern. Next, measure the beam width, across the widest point of the boat (including rails if present). Add 1 foot to the centerline measurement if the rails are taller than 6 inches or if the boat has a trolling motor.
Add 1 foot to each of the centerline and beam width measurements. Calculate the amount of material needed based on these numbers. Most stores sell fabric in widths between 4 and 6 feet. Stores carrying marine fabrics may supply larger widths of canvas. Prewash the canvas to avoid later shrinkage.
Sew canvas together to create one sheet that is the size of your measurements. It may be best to cut the length of the fabric into sections that are the same as the width of the boat. This way the fabric width will not be cut but sewn side by side, creating horizontal seams 4 to 6 feet apart, stretching from side to side.
Drape the large canvas sheet over the boat, covering all top surfaces and overhanging past the lip of the deck by at least 6 inches. Clamp the canvas to the sides of the boat to make it snug and secure for marking. Ensure that the fabric is centered and even on all sides.
With a fabric marker, mark areas of excess fabric to be taken in with darts. Mark any sharp or tall areas where the cover is likely to wear quickly. Trace a line where the cover will cinch under the lip of the deck; 6 inches below that line, draw another line, which will be the cut line.
Sew darts in marked areas by pulling extra fabric to the underside of the cover and pinning to secure. The extra fabric in the back will be in a long triangle shape. Sew in a straight line along the edge of this triangle, directly next to the surface of the cover. Cut the extra triangle fabric off after sewing.
For each marked area of increased wear, cut three squares of canvas the size of the area. Stack the three squares together and pin to the topside of the cover, above the stressed area. Sew on the squares in this stack, creating three-layered patches to strengthen these areas.
Fold fabric over 2 inches around the edge of the cover and pin to the underside. Stitch this down all the way around the cover. Bend a wire hanger to form a straight line. Attach nylon line to the hanger with duct tape or tied.
Choose a spot on the cover that would provide easy access to cinch the cover onto the boat as well as remove it. Snip a small hole at this spot. Run the hanger with line through the fabric tunnel, all the way around the cover, and pull it back out the same hole. Run the two lines through the cinching drawstring mechanism, and tie knots at the end of the lines.
The nylon cord that holds the cover to the boat could be substituted for a length of bungee cord if desired. Measure the bungee cord so that it wraps around the boat when stretched out. Run the bungee through the fabric tunnel and either tie it off or use the drawstring mechanism.
When purchasing fabric, be sure to buy more than you need. It is easier to work when there are leftovers rather than not having enough.