Lagoon 440: reinforcing the mast bulkhead, the heart of our refit

Of all the jobs in myCloud’s refit, the mast bulkhead is the least visible and the most important. Here is what that bulkhead carries on a Lagoon 440, the signs that should get your attention, and what we did aboard in 2025 and 2026.

What the mast bulkhead carries

On a Lagoon 440 the mast does not run down to the bottom of the hull as it does on many monohulls. It is stepped on the coachroof, above the saloon, and its whole compression load travels down a compression post and is then spread into the structure by a large transverse bulkhead: the mast bulkhead. On the Van Peteghem Lauriot-Prévost construction drawings it sits at station C4, 8.5 m forward of the stern.

That bulkhead does several jobs at once. It takes the vertical push of the mast, which rises sharply as soon as you are sailing and the rig is loaded. It also ties the two hulls together under the bridgedeck and stops them spreading or twisting against each other when the sea catches the boat at an angle. With a mast carrying more than 17 m of luff, those loads are not trivial.

The catch is that you need to move around the boat. So the bulkhead is cut with doorways into the hulls and with portlights, and every opening interrupts the natural load path. The corners of those openings concentrate stress, which is exactly where cracks like to start.

Why the topic keeps coming up on big production cats

Structural bulkheads are a recurring topic among owners of large production catamarans of this generation. A catamaran flexes more than a monohull, and builders consider some flex to be normal. The real question is where acceptable flex ends and fatigue begins.

Several things make it worse: years of heavily loaded sailing, rig tension set above the recommended values, a boat that has spent a lot of time beating into short, steep seas. A 2009 boat like ours has also been through a great many load cycles. None of this is alarming in itself, but it is a good reason to look closely at the area, especially when buying.

Signs to watch for

None of these is a diagnosis. They are clues that deserve a competent outside opinion.

• Cracks in the gelcoat or paint around the bulkhead doorways, especially at the corners.
• Doors that rub, close less easily than they used to, or whose clearance changes between the dock and sea.
• Visible deformation of the coachroof around the mast step, or water pooling where it used to drain.
• Movement, chafe marks or damp at the foot of the compression post.
• Interior joinery that seems to have shifted, or rigging that goes slack for no obvious reason.

If you see any of these, get a marine surveyor or a composite yard used to multihulls to look at it before any hard sailing. A structural opinion is not something to improvise, and this article is no substitute for one.

What we did on myCloud

It all started with careful, unglamorous work. In 2024 Patrick, our marine joiner, took down the interior lining that covered the area. Carefully is the key word: each panel came off so that it could go back on, which avoids adding a joinery job on top of a structural one. With the bulkhead bare, we could finally see it as it really is.

The reinforcement itself was carried out in 2025 and 2026 by a composite yard. It includes:

• strips of unidirectional carbon laid in T shapes and diagonals around the bulkhead doorways;
• laminate around the portlights cut into the bulkhead;
• an associated mini-bulkhead that takes part of the load;
• the forward technical compartment repainted, with an inspection hatch at the foot of the compression post;
• extra stiffeners in the bottom of the hulls.

It is by far the largest item in our structural file, accounting for more than half of what we spent on structure during the refit.

Why unidirectional carbon

In a unidirectional cloth every fibre runs the same way. It is very strong along that axis and has almost no strength across it. That is exactly what is wanted here: stiffness placed precisely along the load paths without adding much weight. The T-shaped strips work along the uprights and crossbars of the doorways, while the diagonals deal with the corners, where stress builds up.

A hatch to look rather than guess

The inspection hatch at the foot of the compression post is a small detail that changes a lot. Checking that area used to mean taking things apart. Now we open it, shine a light and take a photo. We do it after any lively passage and before each season, and compare with the previous pictures.

Documenting structural work

A bulkhead reinforcement disappears under paint and then under the lining. Two years later nobody can say what was done, where or how, unless it was recorded at the time. That matters for the owner, for a future surveyor and for a future buyer.

This is the method we follow:

• dated photos, filed by zone, taken from the same angle before, during and after, including just before the area is closed up;
• quotes and invoices attached to the job itself, not buried in a folder called “invoices 2025”;
• the location on the builder’s drawings, here station C4, so everyone is talking about the same spot;
• a short summary: what was found, what was done, by whom and with which materials;
• every later check, with its date and a photo taken through the inspection hatch.

Put together, these make a genuine structural file. When a boat is sold, that file is often what separates a buyer worried by the words “reinforced bulkhead” from one reassured by a job that was well done and followed up. We keep all of it, zone by zone, in the myCloud boat management app, which grew out of this very refit.

What we take away from it

A mast bulkhead reinforcement is not a job to do halfway, or blind. It starts with careful stripping, it is decided with a composite professional, and it is only really worth it if it is documented and monitored afterwards. For a catamaran based in Noumea that regularly heads up to Vanuatu, it is also about peace of mind: knowing what holds the mast up, and being able to check it in two minutes.