How to Choose a Tarp for Boat Covers: Materials & Fit Guide (2026)

How to Choose the Right Product

How to Choose the Right Boat Tarp

The core problem boat owners face is water sitting on the tarp and trapped moisture under it — both cause sagging and mildew. Solve that by choosing a tarp with the right material, enough thickness, and secure tie-down points, then size and install it to encourage runoff and airflow.

Material: Marine-use tarps are usually polyethylene (poly) with UV treatment. Look for explicit mildew-proof or mildew-inhibiting finishes and UV protection to prevent breakdown in sun and reduce mold growth. Poly tarps are lightweight and waterproof; heavier 12–20 mil options resist punctures better but weigh more.

Thickness (mil): Thicker equals more tear resistance and less stretch under load. For seasonal cover on small boats, 6–10 mil can work if tensioned tightly. For long-term storage or harsh conditions choose 12–20 mil (the 20 mil option reviewed is aimed at this use). Keep in mind thicker tarps need stronger tie-downs and center supports.

Hems & Grommets: Reinforced hems (rope or double-stitch) plus rust-proof grommets every ~24–36 inches let you tension across the hull without tearing. Closely spaced grommets give more secure anchor points and reduce the chance of wind flapping that leads to fatigue tears.

Sizing & Fit: Always allow 12–24 inches of overlap around the hull so you can tie down to trailer rails or eyelets. Too small a tarp forces tight seams and exposes the hull; too large a tarp creates excess fabric that can flap and collect water unless you add anchor points or use a custom fit. For boats with cabins, add internal bows or a center support to create a peak and shed water.

Tie-downs & Sag Prevention: Use straps, cam-buckles, or bungee systems across multiple grommets for even tension. Create a crown (peak) in the tarp with a support pole or cross-bow to guide water off the sides. Avoid tying a tarp so tightly that it pulls grommets out — instead, distribute load across the reinforced hem and consider backing plates on vulnerable points.

Ventilation to Reduce Mildew: Mildew thrives in warm, damp darkness. Add vents, leave aft corners slightly raised, or run a breathable cover layer (like a mesh vent) under the tarp to allow air circulation. Cleaning and drying before storage is essential: saltwater residues feed mold, so rinse and dry both boat and tarp before long-term use.

Budget Tiers

Budget tarps are good for short-term or occasional use — they usually offer great value and waterproofing but may have mixed long-term durability. Mid-range tarps (like the 20-mil option reviewed) prioritize thickness and tear resistance at a reasonable price. Premium, larger tarps provide wide coverage and extra reinforcement for long-term outdoor storage but will be heavier and may need more hands and hardware to install properly.