How to Order a Custom Sailboat Mainsail Cover: A Step-by-Step Guide

A well-fitted mainsail cover protects your sail from UV damage, rain, and debris when you're not on the water. But getting the right fit requires accurate measurements and the right material choice. Here's everything you need to know before placing a custom order.

Step 1: Measure Your Boom — Length, Height, and Width

Accurate measurements are the foundation of a great-fitting cover. You'll need three key dimensions:

• Boom Length — Measure from the front of the mast (where the boom connects) to the end of the boom. This determines the overall length of the cover.
• Boom Height (Depth) — Measure the vertical distance from the top of the boom to the lowest point of the furled sail. This accounts for how much sail is stacked on top of the boom.
• Boom Width (Girth) — Measure the circumference around the widest point of the furled sail on the boom. This ensures the cover wraps snugly without being too tight or too loose.

Pro tip: Always measure with the sail fully furled and in its normal resting position. Measure at least three points along the boom and use the largest measurement for width and height.

Step 2: Note Your Mast Connection Style

The way your boom attaches to the mast affects how the cover's forward end is designed. Common styles include:

• Goose-neck fitting — The most common; the cover needs a cutout or flap at the front to fit around the fitting.
• Slab reefing vs. in-mast furling — These systems affect how the sail is stored and how much bulk the cover needs to accommodate.

Provide your sailmaker or cover manufacturer with photos of your mast-boom connection to ensure a precise fit.

Step 3: Choose Your Material

Material choice directly impacts durability, UV resistance, and water resistance. The two most popular options are:

Sunbrella

• Industry-leading UV resistance — won't fade or degrade quickly in strong sunlight
• Solution-dyed acrylic fabric; color goes all the way through
• Breathable, which helps prevent mold and mildew buildup underneath
• Water-resistant (not fully waterproof), but sheds rain effectively
• Heavier and slightly stiffer; very long lifespan (10+ years with proper care)
• Best for: boats in high-UV climates, long-term cruisers, premium builds

Gelay

• Lighter weight and more flexible than Sunbrella
• Good UV resistance, though typically rated slightly lower than Sunbrella
• More affordable price point
• Available in a wide range of colors
• Best for: budget-conscious buyers, boats in moderate climates, racing sailors who prioritize weight savings

Step 4: Specify Closure and Fastening Style

How the cover closes affects ease of use and weather protection:

• Zipper closure — Clean look, easy on/off, but zippers can corrode in saltwater environments; choose marine-grade YKK zippers
• Snap fasteners — Durable and corrosion-resistant; slightly slower to open and close
• Velcro + tie-down straps — Simple and low-maintenance; best for calm-weather storage

Step 5: Key Details to Prevent Leaking and Water Ingress

Even a well-made cover can let water in if these details are overlooked:

• Seam sealing — All seams should be double-stitched with UV-resistant thread. Ask for taped or sealed seams for maximum water resistance.
• Overlap at the mast end — The forward opening should have a generous overlap or flap that wraps around the mast to block rain from running in.
• Drain holes or vents — Small grommeted vents allow condensation to escape and prevent moisture buildup under the cover.
• Tight hem at the bottom — The lower edge should sit snugly against the boom with elastic or a drawstring to prevent wind from lifting the cover.
• Avoid pooling — The cover should be shaped or tensioned so water runs off rather than pooling on top, which adds weight and stress to the fabric.

Step 6: Submit Your Order

When placing your custom order, provide:

• All three boom measurements (length, height, width)
• Mast connection style and photos if possible
• Preferred material and color
• Closure type preference
• Any special features (logo embroidery, extra straps, etc.)

A reputable manufacturer will often send you a measurement template or checklist to fill out before production begins.

Final Thoughts

A custom mainsail cover is a long-term investment in protecting one of your most expensive pieces of sailing equipment. Taking the time to measure carefully and choose the right material will pay off in years of reliable protection. When in doubt, go with a breathable, UV-rated fabric like Sunbrella — your sail will thank you.