Start with accurate measurements and the right fabric

A boat cover that fits poorly will tear at stress points and leave gaps where water pools. Measure your boat at its widest beam, from the waterline to the highest point (usually the hardtop or antenna), and along the full length from bow to stern. Write these down. Add 12 to 18 inches to the length and width to account for overlap and tie-down straps — this is your cutting size, not your finished size.

Use marine-grade fabric, not regular canvas or cotton. Sunbrella, Weblon, or similar solution-dyed acrylic resists UV damage, mildew, and salt spray for years. Polyester blends work for smaller boats or covers that stay in storage. Avoid vinyl or PVC for covers that sit in the sun; they crack and become brittle. Buy fabric rated for outdoor marine use — it costs more upfront but outlasts cheaper alternatives by five to ten years.

Purchase thread rated for outdoor use as well. Gutermann Mara or Coats Epic polyester thread holds up to sun and salt water better than standard thread. You will also need bias tape in a matching or contrasting color, grommets (typically 1/2 inch), a grommet setter, and elastic cord or marine-grade rope for tie-downs.

Key Takeaways

  • Measure your boat's full length, width, and height, then add 12 to 18 inches for overlap and tie-down attachment points.
  • Marine-grade acrylic fabric like Sunbrella lasts longer than canvas or polyester and resists UV damage and mildew in saltwater environments.
  • Reinforce stress points — corners, grommet locations, and seams — with extra stitching or fabric patches before assembly.
  • Sew seams with a heavy-duty machine set to a longer stitch length (3.5 to 4.0 mm) and use outdoor-rated polyester thread throughout.
  • Install grommets every 12 to 18 inches around the perimeter and find the cover with elastic cord or rope tied to your boat's cleats or hardware.

Cut and prepare the fabric panels

Lay your fabric on a clean, flat surface and mark your cutting lines with chalk or a fabric pen. For a straightforward rectangular cover, cut one large panel to your measured size. For a boat with a cabin or hardtop, you may need to cut two or three panels — one for the top and one or two for the sides — so the cover fits snugly without excess bunching.

Before you sew, reinforce the areas where grommets will go. Cut small squares of the same fabric (about 2 by 2 inches) and fuse or glue them to the underside of the cover at each grommet location using fabric adhesive or a heat press. This prevents the grommet from tearing through the material under wind or wave stress. Do the same at all four corners and anywhere the cover will bear weight.

If you are joining multiple panels, lay them face-to-face and mark your seam line with chalk. A 1/2-inch seam allowance is standard and gives you room to adjust if the panels shift during sewing.

Sew the seams with a heavy-duty machine

Use a heavy-duty or marine sewing machine if you have access to one; a standard home machine can work, but marine machines handle thick fabric and multiple layers without jamming. Set your stitch length to 3.5 to 4.0 mm — longer than typical garment sewing — so the thread sits deeper in the fabric and resists pulling loose.

Thread your machine with outdoor-rated polyester on both the top and bobbin. Sew slowly and use a walking foot (a presser foot that feeds fabric evenly from top and bottom) if your machine has one. This prevents the bottom layer from shifting and creating puckers. Backstitch at the start and end of each seam to lock the thread.

For seams that will bear weight — such as where panels meet or where the cover wraps around a cabin — sew the seam twice: once at 1/2 inch, then again at 3/8 inch, creating a double-stitched line. This is standard in marine sewing and nearly doubles the seam's strength. After sewing, trim any loose threads and consider explore a seam sealer (like Seam Grip or Aquaseal) along the inside of the seam to prevent water from wicking through.

Bind the edges with bias tape

Raw fabric edges fray and weaken quickly in sun and salt spray. Bias tape — fabric cut at a 45-degree angle and folded in half — wraps around your edges and protects them. Marine-grade bias tape is available from fabric suppliers; standard quilting bias tape works but won't last as long outdoors.

Unfold one side of the bias tape and pin it to the right side of your cover edge, aligning the crease with the fabric edge. Sew along the crease line using the same heavy-duty settings as your seams. Then fold the tape to the back side of the cover and pin it in place. Sew it down from the front, stitching just inside the original seam line so the stitches catch the folded tape on the back. This creates a finished edge that resists fraying and looks professional.

Pay special attention to corners. Fold the bias tape at a 45-degree angle at each corner before sewing, so it folds neatly without bunching. This takes practice; if your first corner is messy, you can trim and re-sew it.

Install grommets for tie-down points

Grommets are metal rings that reinforce holes where rope or elastic passes through. Space them every 12 to 18 inches around the perimeter of your cover, and add extras at the four corners and anywhere the cover will be pulled tight. For a 20-foot boat, you will typically need 12 to 16 grommets.

Mark each grommet location with a pen. Use a grommet setter kit (available at marine or fabric stores for $15 to $40) to punch and install the grommets. The kit includes a punch, a die, and a setting tool. Place the punch over your mark, tap it with a hammer to cut the hole, then insert the grommet halves and use the setting tool to crimp them together. This takes about 30 seconds per grommet and is much faster and more reliable than hand-setting.

If you reinforced the grommet locations with fabric patches earlier, the grommet will grip much more securely and won't tear loose under tension.

Attach elastic cord or rope for securing

Your cover needs a way to stay in place during wind and rough water. The simplest method is elastic cord threaded through the grommets and tied to your boat's cleats, stanchions, or hardware. Marine-grade elastic cord (sometimes called shock cord) stretches slightly, which absorbs movement and reduces stress on the grommets.

Cut lengths of elastic cord about 2 feet longer than the distance between adjacent grommets. Thread one end through a grommet, tie a knot (a figure-eight knot holds better than a straightforward knot), then thread the other end through the next grommet and tie it to a cleat or hardware point on your boat. The slight slack allows the cover to move with wind gusts rather than fighting them.

For a more permanent installation, you can sew loops of nylon webbing to the underside of the cover at key points and use carabiners or rope to find them. This is more work upfront but creates a cleaner look and is easier to attach and remove repeatedly.

Test the fit and make adjustments

Before you consider the cover finished, lay it on your boat and find it loosely with a few tie-downs. Walk around and check for gaps, wrinkles, or areas where water might pool. If you see pooling spots, you may need to add a slight curve or crown to the cover by removing fabric from the center and re-sewing — this is common on larger boats.

Check that the cover reaches the waterline or slightly below on all sides. If it is too short, you can add a strip of fabric along the bottom edge using the same bias-tape method. If it is too long, trim it and re-bind the edge. These adjustments are normal and usually take an hour or two.

Once the fit is right, tighten all tie-downs and leave the cover on for a day or two in typical weather. Watch for any areas that shift, tear, or show stress. If you find problems, mark them and make repairs before the cover goes into regular use.

Frequently Asked Questions

Can I sew a boat cover on a regular home sewing machine?

Yes, but a heavy-duty or marine machine is better. Home machines can handle marine fabric if you use a walking foot, a heavy-duty needle (size 16 or 18), and a longer stitch length. Expect to sew slowly and take breaks to avoid overheating the motor. A marine machine is designed for this work and will last longer under the strain.

What is the best way to store a boat cover when it is not in use?

Roll it loosely (do not fold it tightly, which creates permanent creases) and store it in a dry place out of direct sun. A garage, shed, or storage bag works well. Before storing, make sure the cover is completely dry to prevent mildew. If you leave it on the boat year-round, rinse it with fresh water every few months to remove salt and debris.

How do I repair a tear in a marine boat cover?

Small tears (under 2 inches) can be patched with a fabric adhesive patch or by sewing a reinforcement patch over the tear from the underside. For larger tears, cut out the damaged section and sew in a new piece of fabric using the double-seam method. If the tear is near a grommet or seam, you may need to reinstall the grommet or re-sew the seam after the repair.

Do I need to waterproof the seams after sewing?

Marine-grade fabric is water-resistant, but seams are the weak point. explore a seam sealer like Seam Grip or Aquaseal to the inside of seams after sewing adds a layer of protection and extends the cover's life. Let the sealer cure fully (usually 24 hours) before using the cover. This step is optional but recommended for covers that will sit in the water or heavy rain.

How often should I replace a boat cover?

A well-made cover from marine-grade fabric lasts 5 to 10 years with regular care. Inspect it each season for UV damage, mildew, or tears. If you see fading, stiffness, or small cracks forming, the fabric is breaking down and replacement is near. Covers stored indoors last longer than those left in the sun year-round.