Fiberglass sailboat under construction

We’ve already discussed how a fiberglass laminate is created: what fabrics and resins are used, molds, the problem of blisters, and how cores can be used to make a laminate both stronger and lighter. Now we’ll consider how a simple fiberglass boat hull can be reinforced and strengthened by the structural elements within it. This is necessary, because in fact the molded glass hull of any boat much larger than a dinghy is not normally rigid enough to withstand much abuse. Without internal structures to help stiffen it, a large hull’s laminate would otherwise have to be unreasonably thick and heavy.

The most basic sort of structural reinforcements are called floors and stringers. Floors are transverse sills in the bottom of a hull on which cabin soles are traditionally installed. Besides stiffening the bottom of the hull, floors provide critical support to the root of a sailboat’s keel where it meets the hull. Stringers, meanwhile, are lateral fore-and-aft beams that are installed along the bottom of a hull. Instead of traditional floors and stringers many modern shallow-bilged boats have a unitary grid, sometimes called an “egg crate,” which consists of structural beams running both laterally and transversely across the bottom of the hull.

Interior fiberglass hull structures
Structural elements found inside fiberglass boat hulls

Bulkheads, partitions, and other structural components of a boat’s interior accommodations and furniture also play an important role in stiffening a hull. Bulkheads are particularly critical, as they can simultaneously provide support to the deck overhead, the bilges below, and the sides of the hull as well.

A primary concern with internal structural components is how they are attached to the hull. On some modern vacuum-bagged or resin-infused boats an egg-crate grid is molded into the bilge as part of the primary hull layup, which is an excellent practice. The traditional procedure, however, is to bond, or tab, internal components in place with strips of fiberglass tape after the hull has been molded. These secondary adhesive bonds are weaker than primary chemical bonds. To create a superior secondary bond the surfaces involved must be properly prepared. In many cases the component being tabbed to the hull is made of wood (often it is plywood), in which case the wood grain must be sealed beforehand or it will suck resin out of the tabbed joint when the fiberglass tape is laid down and wet out. Surfaces on both the structural part and the hull itself should be scratched with sandpaper or a grinding disk to give the resin texture to bite onto; they should also be wiped down with solvent before any glass or resin is applied.

The area of the bonded surfaces must also be large enough to absorb loads on the joint. The rule of thumb is there should be at least a 2-inch margin of tabbing either side of any joint, though a minimum of 3 inches is better, particularly on bulkhead joints. Discrete parts such as grids, floors, and stringers located in the bilges of a boat should be completely glassed over so they don’t absorb any oil or water. Limber holes should also be cut through structures in the bilge so water can flow freely and easily to the lowest point in the hull, where a bilge pump can evacuate it. It may also be necessary to cut access holes through these parts to accommodate wiring or plumbing. The interior surfaces of all such holes must be carefully sealed so they don’t absorb any water or oil passing through them.

Bulkhead bonds in fiberglass boats
Bulkheads bonds are stronger if radiused with a fillet (right)

Particular care should be taken with any bonded joint that forms a sharp right angle. The danger here, especially with parts like bulkheads or lateral partitions that transfer loads all the way from the deck to the hull, is that so called “hard spots” will be created. These are areas where abruptly imposed structural support within a hull amplifies the total amount of stress created when the area is subject to load. Even where hard spots are created by isolated minor structures such as interior cabinetry, significant stress can result if there is an abrupt impact or collision in the area. The best analogy is that of a stick broken over a knee. The narrow fulcrum of the knee focuses stress in a single area and greatly decreases the load required to break the stick. Bend that same stick over a wider surface–a barrel, say–and there is much less stress. A greater load can be imposed without the stick breaking.

To avoid hard spots it is best if any perpendicular structure bonded to a hull does not actually meet it. Instead, there should be a small gap filled with a softer material like foam, balsa wood, or putty. The joint should also be nicely radiused with a wide fillet. This serves both to reduce stress in the area and to strengthen the bond generally, as the transition from one bonded surface to the other is more gradual. The wider the radiused angle, the stronger the bond will be and the less stress it will experience.

Hull liners

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
Hull liners simplify interior hull construction, but may compromise structural integrity

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
Smaller discrete hull liner sections can be bonded to a hull more easily

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.

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3 Responses
  1. Amazon

    Thank you very much, lovely bit of info, I have just bought 23′ Clovic Watson (with no GRP or boating experience) and need to put a floor in the shower/ toilet room. I had a feeling there would be issues with bonding to the hull …onwards … more importantly … must look at engine mountings before engine arrives, thanks again, S xxx

  2. Dan

    I’m preparing a Reliance 44 for offshore sailing, and part of that included replacing the cabin sole. Once I got a good look at the “grid” holding the current sole I became concerned. Now reading this article I realize that my concern is well placed. The “grid” has been so chopped up and modified over the years that it’s practically non-existent. By the time I’m done with this boat I will have practically built it from scratch!

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