Measure overall length and maximum beam, then add drop on both sides and both ends so the tarp can tie below the gunwale. Support a ridge so rain drains off instead of ponding in the cockpit.
Boat covers fail from three things: too little fabric to secure, too much unsupported fabric that ponds water, and chafe on sharp hardware.
What to measure
- Overall length of the boat, including motor if it must be covered
- Maximum beam, usually at the widest point of the hull or trailer guides
- Height from the highest point to the intended tie-down line
- Windshields, towers, rod holders, and other projections
Simple sizing method
For a basic open boat:
Planning formulas
Tarp length ≈ boat length + 2 × end drop
Tarp width ≈ beam + 2 × side drop
Examples
| Boat | Approx. size | Planning tarp |
|---|---|---|
| Small skiff / jon boat | 12 to 14 ft long, narrow beam | Often 12×16 to 16×20 ft class |
| Runabout | 16 to 18 ft with windshield | Often 16×20 to 20×20 ft class |
| Pontoon | Wide beam, long deck | Measure carefully; width grows fast |
Fit and airflow
Leave a way for moisture to escape if the boat will sit for weeks. A sealed wet cover can trap condensation against seats and carpet. Pad sharp corners, antennas, and windshield edges so the tarp does not saw through itself.
Material choice
Light PE works for temporary trailering or storage. Long outdoor storage favors stronger coated fabric and better UV stability. In every case, a supported ridge matters more than buying one size larger and hoping the water runs off.
Frequently asked questions
Should a boat tarp be longer than the boat?
Yes. Include bow and stern overhang plus room to tie below the gunwale. A cover that ends exactly at the hull edge is hard to secure and easy for wind to peel back.
Can I use a rectangular tarp instead of a custom boat cover?
Yes for short-term storage or trailering protection, especially on simple open boats. Custom covers fit windshields and hardware better for long-term outdoor storage.
How do I keep a boat tarp from collecting water?
Support a center ridge with a pole, pool noodles, or a board so water runs off. A flat tarp on a cockpit becomes a pond and can stretch or tear.
Does the motor count in the length?
If the cover must protect the motor, yes. Measure the full protected length, including the motor or lower unit profile you want under the tarp.
What material is best for a boat cover tarp?
For frequent on-off use, a manageable PE or light coated fabric is practical. For long outdoor storage, stronger coated fabric and better UV resistance usually last longer.
How this guide was prepared
We separate geometry and unit conversions from product claims, use exact standards where available, and flag specifications that vary by manufacturer. See our editorial policy for the review process.