How To Make a Winter Boat Cover

Choose the Right Fabric

Before selecting fabric, think about where and how your boat will be stored.

Our pick for this project: Top Gun® 9 Marine Fabric. Our boat will be stored outside over the winter, and Top Gun® 9 is abrasion-resistant, highly water-repellent, UV- and mildew-resistant, and holds its shape beautifully.

• Full outdoor exposure: Top Gun® 9 or Odyssey® Marine — extremely durable, UV stable, and very water-repellent.
• Seasonal or sheltered storage: Top Notch® 9 — breathable, lighter weight, available in a variety of colours.
• Budget-conscious seasonal storage: Winter Boat Cover Fabric.

Look for fabrics with:

• High water resistance
• UV stability
• Mildew resistance
• Adequate breathability
• A workable width (usually 60–64")

Measure the Boat

Boats are complex shapes with curves, tapers, and hardware. Accurate measurements lead to a better fit and less wasted fabric.

Record:

• Length: Bow to stern, beam
• Width: Widest point
• Height: From deck/windshield to gunwale
• Obstructions: Rails, consoles, motors, raised seats, etc.

Use a soft tape measure and measure twice. A precise starting point helps prevent mistakes later in the process.

Estimate Your Fabric Yardage

Most small boats are wider than a single fabric width, so you'll usually need to join two panels. Typical fabric widths:

• Top Gun® 9 — 62"
• Odyssey® Marine — 64"
• Winter Boat Cover Fabric — 58"

Example calculation — Boat: 21 ft long by 8 ft (96") wide, using Top Gun® 9 (62" width):

• Step 1: Convert length to yards — 21 ft ÷ 3 = 7 yards
• Step 2: A single 62" fabric can't cover a 96" beam, so you'll need at least two panels sewn together
• Total yardage: 7 yards × 2 panels = 14 yards total (before adding extra for hems, shaping, and allowances)

Quick estimation formula:

• Determine how many fabric widths (panels) are needed to cover your boat's beam
• Multiply the adjusted boat length by the number of panels
• Add 12–18 inches to the length for hems, shaping, and overhang
• If you're unsure, add 1 extra yard to avoid running short

Once you've calculated your yardage, cut your fabric into two panels (typically equal lengths) to prepare for merging.

Optional but Recommended: Create a Full Pattern Using Patterning Plastic

Patterning plastic lets you create a precise, custom-fitted template before cutting your real fabric — especially helpful on boats with complex curves, consoles, or asymmetry.

1. Drape the plastic — lay 4–6 mil clear patterning plastic over the boat and let it drape naturally.
2. Secure in place — tape along the gunwale using strapping tape; add Seamstick on top for repositionable hold.
3. Mark key reference lines (sharpie recommended) — centre line, hem line (3–5" below gunwale), vent location, rope exit points, reinforcement areas, full perimeter outline.
4. Make relief cuts — small slits wherever the plastic wrinkles so it can relax over curves. Cut darts if relief cuts aren't enough, to shape around consoles or bumps.
5. Refine the shape — trim excess plastic to match the final shape you want.
6. Remove & flatten — this becomes your full-size master template.

Why this step matters: prevents costly mistakes, ensures symmetry, simplifies sewing, eliminates guesswork, and produces a custom, professional fit.

Merge the Fabric Panels (Before Cutting the Final Shape)

Whether you use the patterning plastic method or not, you'll need to merge your fabric panels before cutting the final shape — your pattern is a single large shape, so your fabric needs to become a single large sheet first.

1. Align panels — lay the fabric right sides together (the UV-protected face ends up on the outside once sewn).
2. Apply Seamstick — run double-sided basting tape along one panel edge to keep layers aligned.
3. Sew the centre seam — a flat-felled seam is strongest and fully enclosed; a top-stitched seam is easier for beginners. Sew the full length as straight as possible — this becomes the centre spine.
4. Press the seam flat — smooth by hand to reduce bulk and help water shed properly.

Benefits of a joined centre spine: stronger structure, better drape, a natural water-shedding line, and the ability to cover boats wider than a single fabric width.

Use Your Patterning Plastic to Cut the Fabric (If Using a Template)

1. Lay out the merged fabric — spread the joined sheet on a clean floor or large table, free of wrinkles or folds.
2. Position the pattern — place the plastic template on top, aligning its centre line with the sewn spine and fabric grain.
3. Secure the pattern — use pattern weights, clips, or painter's tape to prevent shifting while tracing.
4. Trace everything — transfer perimeter shape, hem line, vent location, rope exits, reinforcement patches, darts and relief cuts using a washout pencil or chalk.

Add seam allowances (if not already included): ½"–1" for seams, 3"–5" for the cord hem. Then follow the traced lines and carefully cut the fabric using sharp shears or a rotary cutter — your fabric now matches your boat's exact cover shape.

If Not Using Patterning Plastic: Drape Fabric Directly on the Boat

If your boat is fairly simple and symmetrical, or you prefer traditional draping, you can skip the plastic template.

• Drape the merged fabric over the boat and centre the spine along the middle of the hull
• Allow the fabric to fall naturally over both sides, adjusting until it sits smoothly with no pulling or strain
• Use clips to hold the fabric in place while marking

Marking the hem line, relief and dart points:

• Mark your hem line 3–5" below the gunwale with a washout pencil or chalk — this becomes the fold line for your draw-cord hem channel
• Walk the perimeter to ensure the hem line is consistent and symmetrical bow to stern
• Lightly mark areas where fabric bunches or tents upward as potential relief-cut points
• Mark spots where fabric excess forms a "pocket" around consoles or rail mounts — these may become dart locations

Marking the vent location: while the fabric is still on the boat, locate the highest point (often above the centre console or windshield) and mark it — this is the ideal spot for a boat vent to promote airflow and reduce moisture buildup.

Final check: walk around the boat and confirm your hem line and markings are symmetrical on both sides.

Create the Hem for the Cord

• Cut marine rope long enough to wrap around the boat's perimeter, plus an extra 2–3 ft
• Fold the fabric up to the hem line you marked previously
• Secure the fold with basting tape
• Sew the hem to create a cord channel, leaving a 2–3" opening for inserting the cord
• Feed the cord through the channel using a cord puller or safety pin

The draw-cord hem allows you to tighten the cover securely under the boat's edge.

Install Rope Exit Grommets

You'll need exit points where the cord emerges so you can tighten and tie it off.

• Choose two locations at the stern, usually one on each side
• Reinforce each area with a small square of scrap fabric
• Install marine-grade grommets using a hammer and grommet tool (with hole cutter)
• Pull the cord ends through the grommets

These grommets become your tie-off points for the draw cord.

Choosing the Right Grommet Size and Rope Diameter

The size of your rope determines which grommet size you should use. Choosing the correct pairing ensures the rope slides smoothly through the hem, tightens properly, and doesn't stress or tear the fabric.

Common cord sizes for boat covers:

• 3/16" (5 mm) — good for light/medium covers
• 1/4" (6 mm) — the most common choice, ideal for heavy marine fabric
• 5/16" (8 mm) — heavy-duty option for very large covers

Cord options:

Polyester Neobraid Rope — a durable, dependable option with minimal stretch, strong UV resistance, and resistance to mold, oil and stains. Provides a secure, consistent fit and long-term stability.

Bungee Cord — a flexible option for covers removed frequently or needing quick tensioning. Elasticity pulls the hem inward for easy fastening. Less durable than polyester rope, but useful for small tension points.

Does Fabric Weight Affect Rope & Grommet Size?

Yes. Heavier marine fabrics — like Top Gun® 9, Odyssey® Marine, or any thick PVC-coated fabric — create a bulkier hem, which means you'll need a slightly thicker rope and a larger grommet for smooth tightening. Light fabrics work well with 3/16" rope and #1 grommets, while heavy-duty fabrics perform best with 1/4" marine rope and #2 grommets, which set cleaner, handle tension better, and slide more easily inside a thick hem.

Trim Excess Fabric

After sewing the hem and confirming the drape:

• Trim any extra fabric along the sides, leaving at least 1–2 inches beyond the seam to keep the hem strong
• Avoid over-trimming — always leave a bit of extra material for durability
• Use sharp fabric shears to get clean, neat edges

Install the Boat Vent

Remove the cover from the boat and lay it flat, then follow the vent manufacturer's instructions:

• Use the inner vent piece as a template to trace a circle over the marked location
• Carefully cut out the opening
• Insert the vent base through the hole
• Secure it using screws, rivets, or the vent's locking mechanism
• Ensure the vent sits flush and that water can flow freely around it

Some vents are designed to work with a support pole, which helps lift the cover and further reduce sagging and water pooling.

Fit & Finish

Place the finished cover back on the boat and check:

• Overall drape and alignment
• Snugness around the draw-cord hem
• Smoothness around the vent
• Any loose threads, gaps, or trouble areas

Make any final adjustments:

• Tighten the draw-cord
• Add reinforcement patches in high-stress or chafe areas if needed
• Apply a fabric protector spray for added longevity

Your DIY boat cover is now ready to use!