As the days lengthen and the first hint of spring arrives, thoughts turn to launching. For catamaran owners, the pre-launch period is a golden opportunity to get up close and personal with both hulls. A catamaran offers stability and space, but its twin-hull design means twice the surface area to inspect, and unique structural considerations. A few careful hours now can save you a season of trouble.

Why catamarans demand a different pre-launch routine

Catamarans sit on their keels or hull bottoms differently from monohulls. They often have shallow draft, which means keels, rudders, and saildrives are more exposed to grounding and debris. The bridge deck can hide problems, and the stress of sailing on two hulls means you need to check not just for impact damage but also for flex and fatigue around bulkheads and keel roots. If you keep your cat on a drying mooring or trailer, the hulls take extra punishment. Before you splash, walk around both hulls with a notepad and a critical eye.

Hull and keel inspection: looking beyond the antifouling

Start by cleaning off any loose antifouling or grime so you can see the gelcoat clearly. Work in good light, ideally with a torch for shadows. Run your hand along the hull; your palm will catch blisters, cracks, or depressions that your eyes might miss.

• Impact damage: Look for gouges, crazing, or deep scratches, especially near the waterline, keel roots, and bow sections. Catamarans often take knocks when berthing or on moorings.
• Keel and skeg attachment: Check the seam where the keel or skeg meets the hull. Any cracking, rust weeping, or movement is a red flag. Tapping with a small hammer (gently) should produce a clear ring, not a dull thud.
• Stress cracks: Fine hairline cracks around bulkhead tabs, chainplates, and keel floors can indicate flexing. On a cat, these areas work hard. Note any that are new or growing.
• Hull-to-deck joint: Though not strictly hull, this joint can leak and cause hidden damage. Check for separation or sealant failure.

Osmosis: spotting the early signs

Osmosis is a chemical reaction that draws water into the laminate, forming blisters. It's not usually structural, but it can be costly and unsightly. Early detection is key.

• Blisters: Small raised bumps under the gelcoat, often filled with acidic fluid. They may be isolated or clustered. Press them; if they weep, that's a sign.
• Moisture readings: Use a moisture meter (or hire a surveyor) to check hull moisture levels. Compare readings across both hulls and above/below the waterline. High readings in one area warrant further investigation.
• Dull gelcoat: A chalky or dull finish can indicate early osmosis, especially if combined with small pimples.

If you find blisters, don't panic. Small ones can be opened, dried, and filled. Widespread osmosis needs professional drying and re-lamination. Plan this early—drying can take weeks or months in the UK climate.

Rudders, steering, and underwater gear

Catamaran rudders are often vulnerable, especially on beach landings. Check each rudder for play in the stock, cracks in the blade, and corrosion around the bearings.

• Rudder play: With the boat ashore, try to move each rudder up and down and side to side. Any movement beyond a millimetre or two suggests worn bearings or a bent stock.
• Blade condition: Look for chips, cracks, or water ingress. Tap the blade gently—a dull sound means water inside.
• Steering cables and quadrants: Inspect for fraying, rust, or stiffness. Lubricate and adjust as needed.
• Propellers and saildrives: Check for dings, bent blades, and corrosion. Replace anodes if they are more than 50% depleted.

Through-hulls, seacocks, and anodes

These small fittings can sink your boat if ignored. Work through every through-hull on both hulls.

• Seacock operation: Open and close each one. They should move smoothly, not stiffly or with a crunch. A seized seacock is a serious safety issue.
• Hose condition: Check for cracks, hardening, or leaks at hose clamps. Double-clamp below the waterline.
• Anodes: Inspect all anodes—hull, saildrive, rudder, and prop. Replace if they are eroding unevenly or have lost more than half their material.
• Bonding wires: Ensure they are connected and free from corrosion.

Planning repairs and your launch timeline

Once you have your list, prioritise. Safety-critical items—seacocks, rudder play, keel damage—come first. Cosmetic osmosis can wait if the boat is otherwise sound, but if you plan to sell soon, address it.

Consider the UK weather. Epoxy and antifouling need specific temperatures and humidity. Drying out osmosis blisters may require a dehumidified tent or heated shed. Book a surveyor early if you're unsure; their moisture readings and tap tests are invaluable.

Finally, keep a record. Photograph everything, note dates, and track repairs. When you launch, you'll have peace of mind—and a catamaran that's ready for coastal cruising, from the Solent to the Scottish islands.