Here’s a surprise for you; fiberglass boats have wood in them. Older boats, like our project boat, Daydream, have a lot of wood in them. While the hull, deck and other parts may be fiberglass, the interior is often created from plywood and lumber.
Many newer boats are built using a molded interior pan. This pan is bonded to the hull and contains seats, cabinets and other interior features, all molded in smooth shiny fiberglass.
Older boats and some boats built in lower numbers or on a custom basis still have their interiors built in place using plywood bulkheads and other wood components. After the hull is molded, bulkheads are suspended in the open hull and then bonded in place. This process is called tabbing the bulkhead in place.
Sometimes these bulkheads are held in precise location with jigs, allowing many of the interior parts to be cut to size ahead of time. In other cases, they are placed in the molded hull by tape measures and experience.
In most cases the bulkheads do not touch the hull, there being a slight gap between them. This gap is bridged by the tabbing. This served two purposes. First of all, the bulkheads do not need to be cut with a high degree of precision (whack it out with a jig saw, Bubba). Secondly, the gap eliminates a potential hard spot in the hull where the bulkhead meets the hull.
In most production shops, the tabbing consists of strips of fiberglass mat about six inches or more wide. This mat is folded in half lengthwise, placed against the bulkhead and the hull and then bonded in place with resin. In some cases, a strip of foam is placed between the edge of the bulkhead and the hull to ease the corner transition from bulkhead to the hull. A number of layers of mat may be required depending on the strength required of the tabbing.
Once the bulkheads are tabbed in place, the remainder of the interior can then be built in. Cleats, strips of solid wood an inch or so square, are screwed into the bulkhead and additional panels of plywood are screwed to the cleats to form the finished interior.
One of the benefits of a built-in-place interior is that given a power screwdriver and in a half an hour you can get to almost any area on the boat. In a boat with a molded liner, this type of access would require liberal use of a saber saw to cut holes in the pan.
Tabbing can fail from a multitude of causes. Tabbing can be too thin for the application. In Daydream’s case, the tabbing consisted of a single layer of lightweight fiberglass mat. It was clearly too weak for the application and both cracked at the joint and delaminated from the bulkhead.
In other cases, the hull can flex and crack the fiberglass tabbing from repeated bending and flexing. Lastly, an impact against the hull can fracture the tabbing. While tabbing breaks on minor bulkheads may not be critical broken tabbing on major bulkheads should be repaired to maintain the integrity of the boat.
Consider the situation before jumping in and cutting things away. Most major bulkheads are tabbed in place on both sides. Usually some interior wood work will need to be removed to get at the area to be re-tabbed. You want the keep the bulkhead properly positioned while making the repair. This usually means leaving one side of the bulkhead firmly attached to the interior structure while replacing the opposite side’s tabbing.
The old tabbing will need to be cut away and both the bulkhead and hull surfaces ground back to their original configuration. This is a dusty, thankless task; be sure to wear eye and respiratory protection. A Tyvek “bunny” suit will keep those itchy fiberglass fibers away from your skin. Once sanded, clean the surfaces with a solvent to get any traces of dust, dirt or grease off the surfaces.
There are several alternatives to use for the fiberglass component of the tabbing. Fiberglas tape comes in a variety of widths and has a selvaged edge to prevent unraveling. Regular fiberglass cloth can be cut into strips and used for the tabbing. If you cut the cloth on the bias (at a 45-degree angle) it will form better and unravel less. Stitched biaxial mat is also available. This mat is designed to be used with epoxy and has no binder in it. The mat is stitched at regular intervals to hold it together.
A last choice is the common fiberglass mat designed for use with polyester resin. The styrene in the polyester resin dissolves the binder holding the mat together and allows it to drape better. Epoxy will not dissolve this binder and makes the mat harder to conform to curves and corners.
I only use epoxy resin to re-tab bulkheads even though the hull was originally made, in most cases, with polyester resin. Polyester resin only forms a secondary bond with the old, cured polyester resin and that bond is far weaker than the one formed with epoxy.
Two or three layers of reasonably heavy mat will form an adequate tab while more layers of fiberglass cloth are required to build up a sufficient thickness. I usually use three layer of stitched biaxial mat or six layers of 9-ounce fiberglass cloth. If I am using mat or cloth I make each layer slightly smaller than the last, giving a tapered gross section to the tab. This helps eliminate any hard spots formed by the edge of the tabbing.
Once the tabbing is replaced and has cured on one side of the bulkhead, you can remove the structure on the other side and replace that tabbing. No one said it would be easy. However, a minimal investment in materials and some sweat equity on your part will save you a fair bit of cash and protect your investment in your boat. Rest assured that if you found broken tabbing so will the eventual boat buyer’s surveyor.
Details of a typical bulkhead/hull tabbing installation.
An example of the broken tabbing aboard Daydream.
The new epoxy/biaxial stitched mat tabbing.