Article on this subject by Charles Doan-
FIBERGLASS BOATBUILDING: Internal Hull Structures
Properly installing an interior hull
structure can be very labor intensive. Any economy of scale realized by popping multiple bare hulls from the same mold
can be quickly negated by the attention to detail required to properly finish a hull's interior
. This is probably the one phase of boat construction where builders have tried hardest to streamline their procedures. Their key weapon is the molded hull liner, which is simply another large fiberglass part incorporating elements of a boat's interior that is inserted into a hull.
The larger the part, the bigger the savings in terms of work and effort. A truly comprehensive one-piece hull liner can include not only a structural bilge
grid, but also all major furniture components from the bow to the stern. Bulkheads and partitions in these cases are not bonded directly to the hull, but are fitted and glued into pre-molded slots in the hull liner and overhead deck
liner or, alternatively, are bolted to special flanges in the liner.
Fiberglass hull liner bonds
A liner can't provide much structural support unless it is firmly bonded to its hull in as many places as possible. The usual practice is to lay down beds of adhesive
putty (adhesive "splodges") or thickened resin in appropriate spots, then set the liner down on top of these. This relatively light bond should then be improved by tabbing the liner to the hull with glass tape anywhere there is access to contact points between the two parts
. Such access, however, is always limited, and work spaces are often cramped and awkwardly situated.
In the end, it is never possible to create as strong a structure as is formed when all individual components are bonded piece by piece directly to the hull. If the hull is unduly stressed, the liner may break free in some areas. I have heard more than one tale of mass-produced boats failing like this in strong weather
. Such damage can be difficult to detect and is always difficult to repair. It may involve cutting away and then rebuilding large portions of the liner in situ, which may prompt an underwriter to declare the boat a total loss.
The best practice is to create the hull liner in small sections and install the parts
separately. Ideally, support for the bottom of the hull, usually a grid of some kind, is laid in first. One-piece grid pans are often used, but it is best if the grid is built up in place with each part bonded directly to the hull. Bulkheads and hopefully partitions should also be bonded directly to the hull. Separate interior liner sections can then be laid in place around the bulkheads and on top of the grid. It is easier to create strong bonds between the hull and these smaller, more discrete parts; the bulkheads and bilge
structure will also both offer more support to the hull than would otherwise be the case.
Small hull liner section
Another disadvantage to a hull liner, no matter how it is installed, is that it limits or precludes access to the hull once it is in place. This makes it hard or impossible to repair damage to the hull from within the boat without first cutting away the liner. If the hull is breached while underway, a liner makes it harder to both find and staunch any leak, which is why some cautious cruisers always carry a heavy tool such as an ax or crowbar for quickly tearing away a liner in an emergency