The hull to deck joint is where the fiberglass pieces from hull and deck molds combine. The joint is a vulnerable area for leaks on older boats. Generally, three types of joints are inward flange, outward flange, and shoebox. Ways of securing the joint include bolting, chemical bonding, and fiberglass tabbing.
Inward Flange
An inward flange occurs when the fiberglass hull curves inward. The deck is flat or curves outward over the inward curve of the fiberglass hull and downward. The builder then attached the hull to the deck via screws or 5200. Hylas uses 6″ centers and adhesive to connect the inward flange of the hull to the deck. Then, they fiberglass over the joint i.e. “tabbed in.” The screws secure down the toe rail as well covering the joint.
The inward flange is the prefered, quality way to secure a hull deck joint. Builders use other methods to save cost. To create the inward flange using a female mold, builders use an extra fitting to support the inward flange while laying up the hull. Builders must add and remove this extra piece during production adding extra work, time. They must remove to piece to free the hull from the mold.
Outward Flange
An outward flange is where the hull curves outward. The deck may be flat or curve down and outward. The builder fixes the hull and deck together through this outward flange of the hull. They then cover the flange with a rub rail or metal toe rail (terminology is caprail if wood). Production boats like Hunter, Catalina use outward flanges and quickly bond them with aerospace adhesives with screws only for alignment. Using an outward flange simplies the layup with no extra pieces. They hull comes out of the female mold or male plug without worrying about removing a fixture. Builders naturally create the outward flange.
While less expensive, the outflange is easier to access and repair. The joint protrutes outward gives a worker more space and access to stop leakage along the joint. The outward flange is noted to be more vulnerable to hull stress and flexing forces.
Shoebox
A shoebox joint is noted as the simpliest and worst joint. The deck molds in such a way to create a downward flange that fits over a simple flangeless hull. The builder screws horizontally through the deck flange and into the hull. The forces on this arrangement pull the deck away from the hull popping nastily the screw away from the joint.
Conclusion
The hull-deck joint is mainly made of an outward flange, inward flange, or shoebox arrangement. Builders choose mainly based on build costs. Often production boats have outflanges while quality builds use inward flanges. The joint is a common source of leaks and can be a serious factor in the value of a boat.