Getting to the core

Ten-thousand dollars, how can that be?!” This was the response from a customer who sat in my office at the boatyard a few years ago. I had just explained the repairs and the associated cost to correct the damaged fiberglass core on his 15-year-old 40-foot cruising sloop. Saturated core material in the cabin top caused extensive delamination.

Ugly, tell-tale brown stains running from under the heads of several canvas snaps used to secure the dodger foretold a sad but all too common story. These holes, eight in all, each less than an eighth of an inch in diameter, drilled into the cored laminate when the boat was just a few months old, had allowed water to enter insidiously over the following years.

Moisture in the core is a big problem. Another core-related repair carried out just a few months ago involved the correction of leaking cabin ports on an eight-year-old 44-foot cutter, built by a highly regarded U.S. manufacturer of offshore voyaging boats. The bedding around each port failed, as all bedding will ultimately, allowing water to leak into the cabin. In the process, however, water also entered the improperly sealed core material, exposed when the ports were installed. Moisture meter tests indicated that water had penetrated the core for several inches around each port opening, and subsequent disassembly confirmed this. That problem was caught comparatively early (the savvy owner, a full-time liveaboard cruiser, was well acquainted with core failure issues) and thus the repair bill was relatively modest at something under $6,000.

Lest the reader draw the conclusion, from the above anecdotes, that only voyaging sailboats are subject to the malady of core failure, one of the largest U.S. production powerboat manufacturers grappled recently with a number of owners (just how many has not been disclosed) of their 40+ and 50-foot cruiser range, whose cored bottom laminates suffered various degrees of water saturation.

What do all of these cases, and many others like them, have in common? The short answer is, cored fiberglass laminates that failed to perform as designed, expected or advertised. The long answer follows in the text below and delves into exactly what cored construction is, what its advantages and disadvantages are, how it should (and should not) be used, as well as why it fails.

Chances are good that some part of your boat, or the next boat you purchase, has or will have cored construction. The more you know about this construction technique, the better prepared you will be to critically assess its performance both before and after purchase, prevent misuse and errors during service or repairs, and notice existing and potential failures, hopefully before they become serious.

Cored construction takes off

New technology is often slow to catch on, and FRP core or sandwich construction was no exception. A few production-boat builders were using cored FRP as early as the mid-1950s (this core was longitudinal or long-grain structure balsa). Catalyzed into action by their competitors as well as a later increase in resin prices, many once-conservative boatbuilders welcomed core as a solution to all their resin-induced woes (see Fiberglass construction basics, page 54). In a number of ways, their expectations were met and exceeded. Core construction was indeed at least as strong as considerably heavier conventional laminates. However, the learning curve for the use of this boatbuilding technique and its associated materials has been steep.

The process and attributes of cored construction are relatively straightforward. Essentially, a core material (more on specific types of core material later) sandwiched by two skins of fiberglass laminate has often been likened to a girder or an I-beam. Viewing a cored laminate in section, the skins are represented by the horizontal top and bottom panels, with the core material as the vertical section of the I. The inner and outer skins absorb loads that are tensile and compressive (pulling and pushing, essentially). Provided the core material is resistant to shear (movement that is in the same plane but in opposite directions) and compression or crushing, and the skins are exceptionally well bonded to the core, this makes for an extremely strong structure.

Ensuring a strong and lasting bond between inner skin, outer skin and the core material is part of the challenge for every boatbuilder who chooses to use this construction technique. In brief, the sequence for construction of a typical cored panel, be it a hull, deck or cabin top, is as follows.

Gelcoat is sprayed into a female mold and allowed to cure. Fiberglass fabric is then laid over the gelcoat and then saturated or wet-out with resin, usually polyester or vinylester, the latter being preferable because it is more resistant to osmotic blistering (see ON issues 136 and 137 for an in-depth look at the blister phenomenon). The wet-out consists of just enough resin to saturate the glass fabric completely, but no more. Grooved steel rollers are used to force the resin into and air out of the glass laminate.

In some shops, short strips of glass filament are mixed with resin and sprayed onto the mold, rather than laying out and wetting the fabric by hand. This is called chopper-gun construction. It is useful and efficient; however, it is also frowned upon by some in the industry because the fibers are very short, randomly oriented (not as strong as continuous-length fibers), and precise thickness control is difficult to achieve. As a result, a chopper-gun lay-up usually isn’t as dense as a proper hand lay-up, so more of it is required to achieve the same strength. While chopper-gun construction isn’t high tech, it does serve a purpose and is less expensive than hand lay-up techniques. Recent studies show that, contrary to once strongly held popular belief, chopper-gun hulls may actually be more resistant to osmotic blistering than their hand lay-up cousins, the shorter strands presenting dead ends to water migration into the laminate.

Once this layer or outer skin &mdash be it hand lay-up or chopper-gun applied (additional layers of different fabrics often are applied over this initial outer skin) &mdash has cured, it may be sanded smooth where necessary (any irregularities in the surface may create core bridges or voids and ultimately core bond failure) and wiped with solvent. Absolute attention to detail and contamination avoidance is required for all stages of FRP boatbuilding, particularly where the installation of core material is concerned. A laminator’s wet, oily or sweaty hand placed on a laminate that is about to receive core material could represent a ticking time bomb that will not manifest itself for months or years to come. Conscientious FRP shops always use a vacuum rather than compressed air to blow dust and debris off molds, parts and other equipment. Blowing with compressed air simply redistributes contaminants and debris around the shop and onto FRP skins awaiting additional laminates or core application, and any oil or moisture present in the compressed air supply will further contaminate the process.

Adding the core

Up to this point, the FRP boatbuilding process is no different than solid laminate construction. Now, however, the process deviates with the application of one of two materials: a resin-rich (this is the only time additional resin is called for) layer of chopped strand mat (CSM, dispensed from a roll, not a chopper gun) or a proprietary thixotropic product used for bonding core to FRP skins.

The core material is supplied in sheets of different dimensions, but it is typically 2 feet wide and between 0.5 and 1 inch thick (different shapes, contours and lengths are available). To conform the core to curved surfaces, it is scored or cut into small blocks about 1 inch square. The blocks are glued, using a material that is soluble in resin, on one side to a thin fiberglass fabric known as scrim. The core sheets &mdash scored blocks already glued to the scrim &mdash are supplied by the core manufacturer as a single, ready to apply unit.

When the core sheet is laid onto a concave structure, such as the inside of a hull laminate, the scrim is placed up or in, facing the applicator. This allows the core blocks to spread apart, absorbing the curvature. The gaps between the blocks, known as kerfs, are filled with the resin-rich CSM or the proprietary core bonding material as the core is pressed into place.

The final step involves application of the second or inner skin over the core. This is done in much the same way as the first skin, right down to a final application of finishing gelcoat. The combination of different glass fiber fabrics and core material, known as the laminate schedule, is determined by the naval architect or boatbuilder. A military patrol craft may use a substantially heavier laminate than a racing sloop.

Small details

The process described above sounds relatively easy; however, there are numerous small details that must be followed in order to achieve the desired result &mdash a strong sandwich structure that will last long and resist water absorption or delamination. After the outer or first fiberglass skin has cured, for instance, sheets of core should be cut and fit carefully, jigsaw-puzzle-like, before they are applied with resin. This process is critical in preventing voids, gaps between core sheets and resin puddles. A good laminating shop will prefit and cut these sheets, then number them for installation later. Once the resin or bonding putty has been applied, the clock is ticking. If it cures before the core is applied and positioned properly, the laminate is ruined and often must be discarded in its entirety.

Nearly all core manufacturers specify prewetting the core with resin, often called hot coating, before it’s applied to the hull (two applications, several hours apart, may be specified). Some cores are available in a prewet or primed state; otherwise, the hot coating procedure is critical. Some cores, particularly balsa, are especially absorbent. When placed against wet resin, they will draw the resin in through capillary action, creating what’s known as resin starvation where the core is pressed against the outer skin, which often leads to weakening and delamination.

Proprietary core bonding adhesives, by virtue of their thixotropic nature, mitigate this to some extent. However, most core manufacturers recommend hot-coating cores that are not precoated at their factory, regardless of the adhesive or bedding process that’s used. Cores being applied to curved surfaces are often draped over round jigs (sometimes nothing more than a trash can lying on its side) to expose and open the kerfs; resin is then brushed, rolled or sprayed into these gaps.

Failure to fill the gaps

One of the most incipient and potentially disastrous flaws that can creep into the core lamination process is a failure to fill the gaps or kerfs between the core blocks. When core first was popularized and for some time thereafter, it was generally assumed that the kerfs did not require filling. In fact, it was believed that filling the voids only added weight, precisely what the naval architect and builder were, presumably, attempting to avoid by using cored construction. As it turned out, it does add weight; however, it is a necessary evil. Subsequent analysis of failed cores has shown that fractures often occur where the resin or proprietary bonding adhesives stopped in the kerf. Thus, unfilled kerfs make for a weaker structure with a higher tendency toward delamination and structural failure.

Additionally, the voids left by unfilled kerfs present an ideal path for water to follow in the event of hull damage or a punctured skin (remember the aforementioned dodger snaps). A 40-foot cored hull whose kerfs are not filled may possess as much as 5,000 feet of linear voids. When water enters and travels along these valleys &mdash a process called channeling &mdash the result may be a considerable increase in vessel weight as well as costly repairs (often exceeding the value of the boat). Simply put, regardless of whether the cored laminate is part of the hull, deck or cabin structure, every effort must be made to fill these voids. Toward this end, some core manufacturers specify the use of vibrating rollers during the lamination process to aid in displacement of air from laminates and kerfs.

Unfilled kerf voids represent a liability, regardless of the core material used. While it is true that closed-cell, synthetic foam cores are less likely to absorb water than balsa cores are, the channeling process may occur regardless. If the voids are present, the type of core material is irrelevant.

A final consideration that must be borne in mind during any FRP lamination program is the regulation of, or accounting for, temperature and humidity variations. The temperature of the lamination shop as well as all of the laminating materials (including core, resin, glass fabric and mold) should be between 65� F and 85� F for several hours before lamination begins and for at least 24 hours after its completion. Low temperatures are particularly harmful to FRP lamination, inhibiting proper catalyzation of the resin and thus ensuring a plasticized or flexible laminate. High ambient temperatures will generate additional heat within the laminate, causing it to become brittle (made worse by the insulating nature of the core). If less catalyst is used in the core bonding material, as is commonly done in warm weather, it may lead to incomplete curing and poor adhesion.

While temperature may be controllable within a laminating shop, few are humidity controlled. As a result, boatbuilders must store materials in cool, dry locations until they are ready to be used.

Up next

In part two of this series, we’ll examine different core materials and options for their installation, as well as failure modes and their prevention.

By Ocean Navigator