Most of the structural damage to a boat is inflected by most common element in the very environment in which it spends it's life- water and moisture. As a boat spends it's life "keeping the water on the outside", many owners fail in realize the importance of also keeping it out of the vessels structure, in this case cored decks and cabin tops. A "spongy" deck can be as detrimental to the vessel's overall value, affecting re-sale prices, as it is to the vessel's structural integrity. As a surveyor, problems with cored decks is one of the most common defects that I encounter (next to problems with rudders) while conducting a survey.

Many boats are built utilizing cored decks and cabin tops as a means of reducing weight and construction costs while adding stiffness and strength to the structure. Various materials have been used as core material such as end grain balsa wood, marine grade plywood, and PVC foams (Areix), PVC/polyurethane foam (Klegecell, Divinycell, and Termanto) and other more exotic high tech materials. The core material of choice for the most part has been end grain balsa first pioneered by C&C Yachts, a Canadian boat builder.

Coring involves laminating the material wetted out with resin between two layers of fiberglass reinforced plastic (FRP) thus providing both a mechanical and chemical bond. With modern vacuum bagging techniques the panel is covered with plastic and the air is sucked out using a vacuum pump which helps remove air bubbles and promotes satiation of the core material. The structure, or panel as it is sometimes called, acts in much the same way as an I beam, with the FRP skins acting as the two horizontal elements and the core as the vertical member. Areas subject to high loads such as around winches, stantions, cleats, and chocks, should revert to solid FRP laminate. This prevents the crushing of the core material, one of the causes of delamination. While cored decks are a viable alternative to utilizing hanging knees, carlins and deck stringers in construction to provide need structural strength and stiffness, problems can arise when the core material becomes wet and starts to degrade and rot, or if the structure is subject extreme loading in lightly built vessels.

Obviously the easiest way to prevent the core damage is to keep the moisture out. The exposed edges of core material should be sealed with resin to prevent the ingress of moisture, none should be left exposed. It is common to find exposed core material at the cut outs for doraid vents, hatches, port lights, chain plates, and anchor winch chain/hawse pipes among others. Any areas where the FRP skin has been penetrated by fasteners and screws should also be sealed. This can be accomplished by drilling oversized holes filling the with a thickened resin mixture and then re-drilling an appropriate size hole. A pipe cleaner can also be used to apply resin to the exposed core in existing fastener holes. All deck hardware, port lights, hatches, and fittings whose fasteners penetrate the FRP skin should be well bedded and then rebedded with an appropriate bedding compound about every 5 years or so. I personally do not like to use polyurethane compounds such as 3M's 5200 as the adhesive properties make hardware extremely hard to remove for rebedding. And they will need to be rebedded eventually. The polysulfides, or traditional bedding compounds are my first line of defense.

When the FRP skin separates from the core the strength of the overall structure suffers dramatically. Often a crunching sound will be heard when walking on the decks, and percussion testing along with core sampling will confirm this. The deck/cabin top is a integral part of the vessels overall strength and when the vessel rides over the crest of a wave, the bow and stern are partially unsupported. Sometimes when beating into a steep sea, the bow for a long distance aft is totally out of the water. Under these conditions the deck is in tension. In the trough of a sea, the reverse applies. The ends of the hull may be deeply immersed, with tremendous buoyancy lifting the vessel, while the middle is partly unsupported. This condition also occurs when the bow plunges deep into the face of a wave. Now the deck is in compression. These reversing loads can be tremendous. Now this is not taking into account the loads associated with both keel and deck stepped masts and chain plates on sailing vessels which adds a whole new set of load factors. If you were to think of the hull and deck assemblies as a shoe box with it's top firmly secured to the sides (the deck to hull joint) you will realize how important a structurally sound deck is to the overall structure. Remove the shoe box's top and see how readily the sides (Hull) will flex in and out!

When delamination has occurred repairs can be costly and time consuming. In the simplest cases where the skin has separated from the core, and the core material is dry, it is sometimes possible to re-bond the skin by injecting epoxy resin mixed with a slow catalyst to prevent an exothermic reaction into closely spaced holes drilled at about 3 inch intervals. The mixture can be injected using a large syringe with the needle removed. The use of epoxy resins is recommended over that of polyester and vinylester resins due to the superior adhesive properties of epoxy. In this case where the core is dry, the delamination may very well be caused by structural weakness and strain and thought should be given to the addition of hanging knees, carlins, and deck beams to add strength and stiffen the overall structure. If crafted of fine hardwoods this addition can be quite beautiful as well as adding a great deal of strength to the deck structure.

In cases where the core material is saturated the core must be dried out completely and all rotten material removed. Rotten core material should be replaced with the original type used, or one of similar structural properties. If a material is used that is either stiffer or more flexible that the original material it should be scarified in at 12:1 ratio allowing for a gradual transition in flexibility and stiffness in the materials. This procedure will therefore eliminate the possibilities of "hard Spots" in the panel and the potential for damage and further delamination of the FRP skins.

Capt. Scott Thompson, AMS