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Osmosis is surely the most important fiberglass' defect but most of the boating fans ignore other fiberglass-related problems such as delamination

Article by
Gino Ciriaci


I have discussed fiberglass' problems several times on Nautica with several articles on osmosis, a chemical phenomenon which is sometimes appointed as the only fiberglass' disease. Osmosis is , in my opinion, the most important fiberglass' defect which can, on the other hand, be easily repaired with expensive but effective cures. Other vices belong to fiberglass (although it is a very satisfactory boat building material) which are sometimes more severe than osmosis itself and which can not always be fixed, or are very expensive to repair; in some extreme cases the cost of repairing could be more than the value of the boat. Osmosis is surely the most important fiberglass' defect, because it can be easily detected (or at least many yachtsmen and professionals think so). This vice is, however, not always easy to find, as some of you can remember from the previous articles on this matter. Most of the boating fans ignore, on the other hand, other fiberglass-related problems such as: cracks, delaminations, star cracking, structures failure, gel coat failure (such as pinholing and wrinkling).

All this highlight a complex reality: just a gel-coat survey is not enough to examine the laminate's condition, and this is a rule often forgotten by boaters. One of the most severe failure of FRP materials is delamination which occur when two layers of fiber separate: it first effect is to improve the elasticity of the laminate (easy to feel by hand) in the effected area, which usually spread to the nearby surfaces so that a hull or a deck become "soft". If, for instance, walking on deck we can detect a clear unusual bending, most likely it is due to delamination which has occurred due to a foam core damage or to the separation of the skin from the core material.

There are four main faults which lead to delamination:

  • core material to skin separation in sandwich structures
  • interlaminar separation in single skin laminates
  • frames and longitudinals separation or deck fittings separation
  • teak to deck separation (on teak lined decks)
Lets now analyze what happen when delamination occur.


More and more boats benefit from the sandwich construction which consist of two layers of fiberglass separated by a core material (balsa wood or close cell PVC foam). The outer layer is usually thicker. The core material do not cover the entire hull surface and, in way of engine girder, deck fittings and chain plates is replaced by single skin laminate. This is done to overcome the low compressive strength of core material, which can fail under concentrated loads, such as bolts. Sandwich delamination occur when one of the two fiberglass layers separate from the core material. This can be due to:

  • overload on the boat structure, like concentrated flexural, torsional or compressive stresses
  • collisions
  • high shear stresses or uncontrolled drilling
  • low resin to fiber ratio.

When sandwich delamination occur, the separated layer become "soft", even under a slight pressure. Delamination can involve the hull bottom as well; micro-cracks can be generated due to the loss of mechanical properties. The cracks may generate water leakage or, even worst, may let bilge water (usually greasy and oily) absorption in the laminate which spreads thanks to the fiberglass's high permeability. Delamination may occur in unusual ways as well. Particular care must be exercised when the boat is hauled for the season: an asymmetrical load on the cradle or an insufficient supporting area may cause serious, and sometimes permanent, damage to the laminate. Those damages and delaminations produce, as mentioned before, a loss in the primary mechanical properties of the shell plate. Because delamination is a progressive phenomenon which spread with time and is responsible for the weakness of the structure leading to cracks on the laminate, it must be repaired as promptly as possible. This is true regardless delamination should occur on the hull or on the deck structure. The deck, in particular, is stressed by concentrated loads and is not always well supported by beams or longitudinals; because it is part of the boat structure and participate to the overall stiffness it is necessary to avoid any delamination problem. On delaminated decks, the most simple repairing method is to drill some holes on the interested surface and inject new resin to glue again the separated parts. If this method should not work, then it is necessary to rebuild the sandwich structure. In this case the inner skin have to be adequately supported, in order to replace the outer skin and the core material. Afterward it will be possible to rebuild the inner skin.

Usually this expensive treatment is omitted and the boat is sold... If we are facing hull delamination, the first thing to do is to dry out the interested area. This operation is not simple and the result is not guaranteed. Then the separated parts will be glued again. On extreme cases, the sandwich structure must be rebuild, following the above mentioned scheme. In this particular operation and for relatively small surfaces, the vacuum-bagged technique is recommended to ensure a perfect bonding. Larger areas will require a vacuum-bagged technique with epoxy resin, which benefit from a longer curing time and better performances. The vacuum-bagged technique is a lamination system which use the atmosphere pressure to scrimp the laminate. A particular film is placed on the surface and bonded with a sealant. A pump is then connected to the film, eliminating the air between the film and the laminate. In this way the difference of pressure between the outer face of the film and the laminate (approximately equal to the atmospheric pressure) will press on the fiberglass layers. Once the resin has cured, the bag is removed and the laminate is ready for finishing.


Most of modern yacht hulls are built in single skin fiberglass, manufactured with glass mat and cloth reinforcements: just like on sandwich construction, two layers of fiberglass can separate thus causing delamination. The main reason for single skin delamination are: - concentrated loads like frequent road transportation or inaccurate storage of the hull on the cradle - poor lamination shop condition, like uncontrolled environmental elements (humidity and dust percentage must be controlled for a proper lamination) - low resin to fiber ratio or inappropriate resin type. A low resin to fiber ratio, in particular, can be easily obtained when laminating heavy plies (say more than 1000 gr./m2) which are difficult to impregnate with the resin; if this is the case, the delaminated ply can be easily divided with a knife, showing dry glass fibers. All this may happen on Kevlar laminates as well. The separation of the first ply of mat from the rest of the laminate is a particular case of delamination, which may occur if the gel- coat is too thick, as shown on picture 8 and 9. Delamination can be easily seen because it highlight a white surface, as shown on picture 10, where the mat ply can be effortlessly separated, as shown on picture 11. The single skin delamination is repaired following the procedures previously described for the sandwich construction. Single skin delamination is, unfortunately, rather frequent, but it has to be repaired only if it involve a large surface; if the delamination process cover just few small spots (say few square centimeters) then it is not something to worry about.


Deck fittings, like chain plates, generate concentrate loads which may cause delamination. Sail boats, due to their rigging, are relatively more influenced by this aspect. Fittings are, on same boats, glassed to the deck and they require an accurate inspection because a delamination can produce the chain plate shifting which can be noticed, on the most severe cases, by the continuous loosing of shrouds. This may not happen on chain plates which are through-bolted on deck as well, but delamination may still occur. On fast power boats, engine girders delamination may occur due to the engine's vibrations, especially in case the engine and the shaft are off-center one another. This case is particularly difficult to repair, because oil, grease and fuel presence in the bilge make resin catalization almost impossible. Both of the delamination cases described in this paragraph are expensive to repair, because they require a large disassembly job prior to the laminate reconstruction; in fact it will be necessary to remove internal fittings (like furniture) or , in the second circumstance, the engines and related equipment.


On teak lined decks, delamination can be found by checking the integrity of the deck structure with a small hammer: as everybody knows, a change in the sound indicate a change in the underneath volume distribution. This rather simple job require, on the other hand, experience and should be carried out by a specialized technician who will check the seriousness of the delamination. First of all, all presumed delamination spots must be highlighted as shown on picture 13, trying to understand if a "delamination scheme" exists (i.e. on some teak planks delamination occurs proportionally in the same area). Then the technician will determine the delamination type: it could be a simple teak to deck separation or a serious delamination in the fiberglass' layers. These two cases have really different repair costs. As anyone could imagine, the cost of gluing some teak stripes is nothing if compared to the cost of removing entirely the teak deck to rebuild the laminate underneath.

In conclusion we should always remember that delamination is not always detectable at first sight, but it has to be found with an accurate survey were a professional opinion is essential because delamination reduce the safety of the boat and can be very expensive to repair.