Support your boat cover with a PVC frame.

A boat cover support prevents sagging.
Why Use a Boat Cover Support?
To keep water and snow from collecting on your boat cover, you will need to use some type of boat cover support. This support system will keep the cover from sagging and will prevent excess moisture buildup.
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You can either purchase a ready-made boat cover support system, or you can make your own with materials from your local hardware store. Making your own boat cover support is fairly easy, and you can easily customize the fit for your boat.
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Tips on Building Your Own Boat Cover Support
1 1/2 inch PVC pipe is a popular choice for a boat cover frame. It’s sturdier than one inch PVC pipe, so it’s less likely to break under the weight of snow and ice.
Use one long pipe from bow to stern and add several pipes as cross braces from side to side down the length of the boat. Arched PVC pipes can help prevent puddles from forming on your cover. Moisture will slide right off the boat cover and onto the ground.
Here’s another tip to protect your boat cover and boat: If the windshield or other edges are too sharp, cover them with padding, such as foam insulation tubes slit down the middle. This keeps your cover from tearing on sharp edges.
Learn the difference between storage, mooring, and trailerable boat covers.
There are three basic types of covers for boats. Which type you need depends on how you plan to use your cover. Storage covers are designed for indoor or outdoor storage out of the water. Trailerable covers, or travel covers, are designed to protect your boat on the road. Finally, mooring covers are designed to protect your boat while it is anchored in the water.
Storage Covers
Unless you store your boat indoors in a controlled environment type of storage facility, you will want to choose a thick enough material to provide complete protection for your boat. If you store your boat outdoors, choose a heavy duty material that will protect your boat from rain, snow, moisture, wind, and UV rays. Even if you store your boat in a shed or garage, moisture could be a problem. To prevent moisture damage, choose a cover that is waterproof and breathable.
Trailerable Covers
A trailerable cover must be highly durable to withstand the wear and tear of highway travel. The wind whipping at your boat cover can rip the material if it is too thin or pry the cover loose from your boat if it is loose. Your trailerable marine cover should be tight enough for a snug fit and should include tie downs to secure the cover to the boat and trailer. The material quality should be heavy duty, such as 100% marine polyester, to hold up against harsh weather and high-speed travel.
Mooring Covers
Mooring covers provide in-water protection for docked boats. The best material for mooring covers is a cotton poly blend. The combination of cotton and polyester gives best combination of durability and breathability that you can find in a mooring cover.
Conclusion
If you use your cover for both trailering and storage, look for a trailerable boat cover that is also approved for storage. This will save you the expense of buying two separate boat covers.