What is a sandwich core? A sandwich core is the material in between fiberglass mat (CSM). It is used to add bulk and strength to a hull/deck/hull sides without adding significant weight. A sandwich core adds significant flexural strength compared to using skin laminates alone. It also builds thickness to any composite part, which is key to adding stiffness to a vessel. Types of coring materials are listed below.

Nomex® Honeycomb – a sheet of cells nested together to form panels. It is extremely flexible and when expanded is almost entirely open space. Nomex offers good impact resistance and is fire retardant. Among sandwich cores, this material offers the best strength to weight ratio because it is a composite material containing no wood. It is very moisture resistant and will not absorb water into the hull. The hull weight will not fluctuate. Cons include very expensive and mostly used for aerospace applications.

End Grain Balsa – most widely used coring material. It is constructed using small blocks of wood each bonded in succession to a light scrim backing. As a result, it conforms to almost all simple curves and most gradual compound curves. End Grain offers a high compressive strength due to the density of the blocks. If the fiberglass mat sandwiches the balsa on both sides, then the chemical bond is complete. This will create a solid block that is strong and has great vibration resistance. On rough seas, the pounding of the boat will transfer less into the hull and have a smooth ride. Cons include susceptibility to water intrusion. If any material pierces the fiberglass mat into the balsa core it will create area for water to seep in. This will rot the coring material, which is a common problem amongst older vessels.

Foam – closed cell foam is able to resist water, gas, and oil due to it being “closed”. This means that foam capillaries are closed on both ends. It delivers added strength, stiffness, and insulation without adding significant weight. Foam is easy to handle, provides excellent floatation, and can be easily carved or shaped. Foam selections include vinyl foam sheet, divinimat flo-media, mix-pour polyurethane and stiff poly-iso sheet foams as well as a few others. Cons include foam degradation if water saturates the core long enough, it will start to separate the fiberglass laminates it is sandwiched in between. Foam cored laminates are also vulnerable to impact damage and can be prone to core separation.

Travis Palmer SAMS®, ABYC®
Principal Marine Surveyor
Corsica River Marine Surveys
Annapolis, Manyland