Fiberglass Fabric Basics


So you’ve got a quarter guard that got torn up by a piling?
Or you need to plug a hole left by an extracted through-hull?
Fiberglass fabric’s the answer.

Fiberglass fabrics

Recently, in my ongoing quest to buy another boat, I had occasion to contemplate the removal of an old sanitary-system’s defunct seacock. The job, had I opted to buy this particular boat, would have entailed a haulout, the grinding away of two roughly circular, inwardly-tapering depressions around the hole left by the seacock’s removal, both inside and outside the boat. Then both the hole and the inwardly-tapering depressions would have been glassed over. A simple job? Yes, and with an estimated price tag of roughly $500 (according to the boatyard I usually deal with), not outrageously expensive. But exactly how would the yard’s “fiberglass guy” have gone about the task, or any similar fiberglass-related task for that matter, using the three basic fiberglass fabrics you see here?

Chopped-Strand Mat
After using varnishing tape (several layers, stretched tight to resist isotherm heat) or some other material to backstop the hole from inside the hull, a layer of chopped-strand mat (CSM) would have been applied from outside and wetted out with resin. CSM is relatively waterproof, tends to absorb a great deal of resin (for a solid, glue-like bond to other fabrics) and also, because of its makeup, sands smooth quite easily for a nice exterior finish.

Chopped-Strand Mat

Woven Roving
Woven roving is thicker and heavier than CSM—its texture is almost like what you’d see in wicker furniture. Would such a material have been used in this instance? The answer depends on the depth and size of the patch required (roving builds thickness quickly as it’s layered in), the shapes of the surfaces involved (roving resists complex shaping and does better on flat or slightly curved surfaces), and the strength needed (of the three fabrics shown here, roving tends to be the strongest) for the job. Woven roving probably works best for securing delaminated bulkheads, repairing transoms, or doing other jobs that call for strength but not necessarily a sweet finish.

Woven Roving

Fiberglass Cloth
For a small, through-hull related repair, the fiberglass guy would likely have abjured woven roving in favor of a layer of fiberglass cloth (like the medium-density e-glass here) and laid it over the CSM once it had cured and the tape had been removed. The cloth would have been cut slightly larger than the layer of CSM beneath it. Layers of e-glass and CSM would have then followed, each layer larger than the one beneath, until the exterior depression was full. CSM would have constituted the last layer because of its waterproof, easily sanded nature. Once the repair’s exterior was complete, the process would have been replicated inside the vessel. Bottom paint would have finished the job.

Fiberglass Cloth

This article originally appeared in the September 2015 issue of Power & Motoryacht magazine.