More on Cores

We have created videos, instructions, and Epoxyworks articles about repairing cored laminates,
so why another article about repairing cored laminates? I thought it would be good to summarize
some important aspects of cored laminate design and how to make the best repairs.

Balsa Core

First, let’s discuss the most popular and dreaded topic about cored marine laminates in boats, wet balsa core. Many boat owners have dealt with repairs from wet balsa, and pre-purchase surveys often uncover wet core issues that complicate sales. Boat owners often wonder why balsa is so popular, even after dealing with issues from wet balsa core replacements. Valid reasons exist for its popularity, but poor installation practices can lead to problems.

When we refer to balsa core, we are describing end-grain balsa core. This is an important distinction. Orienting the core with the grain perpendicular to the skins provides high shear strength when the laminate bends. The nearby article “Why Cored Composite Laminates?” explains how high shear strength parallel to the skins enhances the stress and properties of cored laminates. The compression strength perpendicular to the face of the laminate is also high because of the vertical grain, which improves resistance to point loads on the laminate such as walking on a deck. These high properties are attained with a relatively low weight.

Foam Cores

Foam cores are available in many different materials and densities. Still, generally speaking, they do not have a weight advantage when compared to balsa IF the strength and modulus (stiffness) properties are equal. Foam is available in very low densities, but its properties decrease compared to higher-density, stronger cores. A significant advantage of foam core is the ability to tailor the properties to the application. Use lower density materials in areas where properties can be minimized, such as in the topsides of a sailboat towards the stern. Reserve higher density materials for the keel sump. Several different types of honeycomb cores can achieve high shear strength with low weight.

Water Intrusion

In all cored laminates water intrusion is a problem.In foam cores, water doesn’t cause rot, but it poses issues in freezing climates and can lead to delamination when flexing during operation, forces it through the laminate. While balsa wood deteriorates when water penetrates the laminate, water intrusion can also affect any core material. This issue typically results from manufacturing defects, improper hardware installation, or lack of maintenance.

Unfortunately, many manufacturers do not follow the recommendations of the balsa core companies. It is always advised to fill in the scores or cuts in the entire sheet of balsa with epoxy. This process not only strengthens the laminate by filling in the gaps that create stress concentrations when bending but, if water does get into the core, it is isolated to the one compromised block. This same technique should be used with foam cores. Sealing all the cut lines in the core, when combined with vacuum bagging of the core during lamination, is an excellent way to seal every block and prevent widespread water intrusion.

Another technique a builder should use is “closing out” the core where there will be a hole. This process requires you to remove the core material around a thru-hull fitting. Bevel the core in this area using triangular pieces of core called fillet strips. Once you’ve done that, build up a thick, solid laminate in the area, using a plywood backer if necessary. This adds significant labor to the building process and can often create a minor cosmetic issue in polyester resin-constructed boats due to uneven resin shrinkage but significantly reduces the risk of water intrusion. A boat that showcases these features demonstrates the use of good lamination techniques.

Some manufacturers make an effort to seal the kerfs and provide closeouts if modifications in the field do not follow similar practices. A new transducer, adding a new windlass, or even installing snaps for a new cover can allow water into the laminate. ALL penetrations into a cored laminate should be sealed with epoxy resin.

Maintaining and Repairing Cores

Maintaining a boat involves more than just caring for the mechanical machinery. As a boat ages, you should re-bed the hardware depending on the environment. This involves removing the hardware and existing sealant followed by the application of new sealant and reinstallation. My personal boat is trailered, used in freshwater, and stored indoors. As it approached its 25th birthday, I removed the trim tabs and the swim platform supports. The sealant had deteriorated noticeably and was causing a slight amount of movement in the swim platform. I was very glad I took the time to re-bed them, and I can even tell the swim platform brackets are stiffer now.

During my 25-plus years in the marine industry, I have observedmany debates over core materials like balsa, foam, and honeycomb. However, everyone agrees that installers can poorly install any type of core material. With proper installation and maintenance, we always suggest using the core the builder selected. Using WEST SYSTEM® products and our recommended techniques, your installation may be better than new.