Join Date: Feb 2000
Location: Annapolis, Md
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Historically bulkheads and flats were fiberglassed into the boat with strips of fiberglass called 'tabbing'. Properly done, tabbing is continuous and made up of several layers of glass and resin. It is ideally tapered so that the loads are distributed out onto the hull or deck. It is the primary means by which bulkheads and flats are jointed to the hull and deck integrating these elements into an ideally coherent structural framing system. As SD points out, the bulkhead and flat itself is actually slightly isolated from the hull to prevent hardspots which over time can lead to increased fatigue at the bulkhead.
Now then, all of that is the ideal way of joining bulkheads and flats, but it’s not the way that it’s often or even typically done. From the beginning of fiberglass boat building, poorer build quality boats used what is colloquially known as 'skip tabbing', which is essentially short lengths of tabbing and which often not tapered. In the short run, skip tabbing holds the bulkhead into place but over time lacks the strength, stiffness and durability of continuous and tapered tabbing.
Modern production boats often have glued in bulkheads and flats. This is a complex issue. There are modern glues are so strong that the glue joint is stronger than the laminate its glued to. Off hand that sounds like a good thing, but its not so simple. With traditional tabbing the faying surface (the area that is glued to the hull and bulkhead) was typically 4 to 8 inches in width. With glued in bulkheads and flats, the faying surface is the thickness of the plywood (typically 3/8" to 5/8"). Since the loads since the loads don not change with the attachment method, glued in joints subject the laminate to much higher stress levels and so are more likely to cause the laminate to break down over time due to fatigue and delamination.
Another substitute for tabbing is gluing or mechanically fastening bulkheads and flats to the pan or liner system. This is a mixed bag. It requires careful engineering of the pan and liner system so that the loads are properly transferred without making the pan and liner excessively heavy.
Boat builders will often combine systems gluing or tabbing bulkheads to the hull and then attaching the bulkhead to the overhead liner.
For offshore work, nothing beats continuous tabbing tying the hull to the deck. It produces a stiffer, more robust hull. But continuous tabbing is very labor intensive to install and finish so few boats actually use continuous tabbing. As a result, continuously glassing bulkheads and flats is one of the first steps in beefing up a boat to go offshore.