10/16/2025
Why Four Watertight Bulkheads Matter on Your Fiberglass Offshore Boat
Hi, I’m Captain Levi, and if you’re serious about heading offshore in your fiberglass boat, there’s one thing you absolutely need to know: your boat should have at least four watertight bulkheads. Why? Because they’re your lifeline when things get rough—maybe even life-saving after a collision.
Let’s start with the first bulkhead, the collision bulkhead, right up in the bow. This compartment isn’t just an empty space; it often holds your anchor rope. But more importantly, if you slam into something head-on and that bulkhead holds firm, your boat stays afloat. That’s a big deal.
Next, moving aft, you have a pair of bulkheads that frame your engine compartment. Below the waterline, there are shafts and stuffing boxes that keep the engine connected to the propeller, but they’re also points of vulnerability. If a leak develops or the hull gets breached, these bulkheads help contain the water inside the engine room. Besides keeping water out, they act as sound, heat, and even fire barriers, protecting not just your boat's structure but your safety as well.
Finally, at the very back, there’s a bulkhead making up the lazarette, the area where your rudders live. Dropping a rudder offshore is every captain’s nightmare—no matter how seasoned you are. So having that bulkhead strong and secure really matters for your boat’s survivability and for your peace of mind.
But here’s the catch: bulkheads take a beating. Every wave your boat slices through causes flexing and vibrations that stress these key structural parts. Moisture in the bilge and heat from engines and generators all chip away at their integrity over time. Bulkheads are what give your hull transverse strength—they minimize that flexing and hold everything together.
Before the late ’90s, most fiberglass boats had bulkheads made from marine plywood, laminated to the hull with polyester resin and fiberglass mats—a process called taping-in or tabbing. When done right, it gave a strong bond. But decades at sea take their toll. Sometimes the fiberglass delaminates away from the plywood, or the plywood itself degrades. I’ve seen this firsthand: a 1977 Bruno/Stillman 35-footer came to my shop in 2003, and her bulkheads were falling apart after 25 years of offshore service. The wood was gone.
So, what do we do now? We build new bulkheads tougher than ever. The new bulkheads are two inches thick, made from two layers of ¾-inch marine plywood laminated with fiberglass mat and biaxial fabric on both sides. This isn’t just tougher; it’s over three times stronger than the original factory construction. Around the edges, we apply ARJAY 2001, a specialized polyester bonding and filling material. We carefully create a 4-inch radius at the joints, which corners the full strength of the laminate—that’s the kind of detail that turns a patch job into a bulletproof fix.
One of my team, Clarice, prepares to tab in these bulkheads using 1708 biaxial fiberglass fabric—that’s high-strength stuff. She laminates it in layers, starting narrow and getting wider, creating a smooth taper—because transitions matter. For resin, we use GP-200: a general-purpose polyester resin favored by boat builders across the country. It cures at room temperature and bonds layers without needing sanding between coats, making the repair stronger and quicker.
You’ll often see her carefully cutting the fabric with heavy-duty shears—these are no ordinary scissors. When it comes to Kevlar fabrics, she’s got special serrated blades, and trust me, no one else touches her shears—not even me!
Ronnie, another member of the crew, applies ARJAY 2001 to old structural parts we’re keeping—like under the aft deck. We’re raising that deck eight inches to make the boat more seaworthy; the old cockpit was just too deep, making things rougher for the crew. By reinforcing these parts and strengthening the transom with bonders like ARJAY 2001, we bring both strength and longevity back to these classic boats.
So here’s the bottom line, matey: always pay close attention to your bulkheads. They’re more than just walls inside your boat. They’re your allies when floating becomes fighting for survival. With proper maintenance and upgrades, your fiberglass boat can take on the sea—and come home safe. Because in the end, it’s not just about building a boat but about building confidence to head out, knowing she’ll stand tough through whatever waves come her way.
Keep your bulkheads strong, keep your boat safe—and happy sailing.
