When fiber­glass isn’t...

SAIL - - Contents - By Don Casey

KNOWHOW Core Is­sues: un­der­stand­ing how your boat is built

Does your boat suf­fer from CD? A mul­ti­tude of ag­ing sail­boats do, and it can be ru­inous. If you are a sail­boat owner, you need to know how to pre­vent CD and per­haps how to treat it. What the heck is CD? I just made it up (in­flu­enced no doubt by in­ces­sant phar­ma­ceu­ti­cal ads). It stands for core dys­func­tion. It means some­thing is go­ing on that has com­pro­mised the stiff­ness and strength of a lam­i­nate re­in­forced with a core ma­te­rial. How­ever, be­fore ad­dress­ing CD, let’s get up to speed on core con­struc­tion.

The use of cored lam­i­nate has un­de­ni­able ap­peal for sail­boat con­struc­tion. Com­pared to solid fiber­glass, core con­struc­tion is lighter and/or stronger. That trans­lates into a boat that is more eas­ily driven, mak­ing it quicker or, with smaller sails, eas­ier to han­dle. The stiff­ness of cored lam­i­nate re­duces the need for ad­di­tional sup­port com­po­nents, de­liv­er­ing more in­te­rior space with fewer lay­out lim­i­ta­tions. Most core ma­te­ri­als also pro­vide sig­nif­i­cant in­su­la­tion, which in turn, makes cabin tem­per­a­tures more com­fort­able: cooler when the out­side tem­per­a­ture is hot, and warmer when it is cold. Th­ese same in­su­lat­ing prop­er­ties also re­duce or elim­i­nate in­te­rior con­den­sa­tion and dampen sound trans­mis­sion, mak­ing the boat qui­eter. Add to that lower cost, par­tic­u­larly where stiff­ness is the pri­mary ob­jec­tive, and it is no won­der that vir­tu­ally all fiber­glass sail­boats in­cor­po­rate core in some part of their con­struc­tion. If your sail­boat has a cabin, you can be 99 per­cent sure that the deck is cored. Many pro­duc­tion sail­boats also in­cor­po­rate a core in the hull con­struc­tion in pur­suit of bet­ter per­for­mance.

It is im­por­tant to un­der­stand that core rigid­ity is not what de­ter­mines the stiff­ness of the lam­i­nate. Balsa core, for ex­am­ple, is of­ten made of small blocks of balsa ad­hered to a scrim, sim­i­lar to mo­saic bath­room tile. So how do two flex­i­ble skins and a core as floppy as a door­mat cre­ate a stiff deck? The same way three thin metal pan­els cre­ate a bend-re­sist­ing Ibeam. The par­al­lel flanges of an I-beam are joined by

the per­pen­dic­u­lar com­po­nent called the web. With­out get­ting too deep into the weeds, bend­ing stress im­poses com­pres­sion loads on one flange and ten­sion loads on the other, cre­at­ing a shear load on the web, which the steel re­sists might­ily. The re­sult is a com­bined stiff­ness much greater than the in­di­vid­ual stiff­ness of com­po­nents.

Un­for­tu­nately, cored lam­i­nate com­pares to an I-beam in prin­ci­ple only. While the I-beam is typ­i­cally formed from a sin­gle steel blank, the “web” com­po­nent of a cored lam­i­nate is usu­ally wood or plas­tic foam sand­wiched be­tween fiber­glass skins. This lack of ho­mo­gene­ity makes the as­sem­bly frag­ile, even more so than solid fiber­glass, where lam­i­nates are bonded to­gether on a molec­u­lar level. In a com­pos­ite lam­i­nate, the core is at­tached to the skins via an ad­he­sive bond, not chem­i­cally. That bond is the weak link. If it fails, the three de­tached com­po­nents be­come more leaf spring than I-beam.

WHY BALSA IS GOOD Wood is by far the most com­mon core ma­te­rial used in boat con­struc­tion. Nearly all pro­duc­tion sail­boats man­u­fac­tured in the last 50 years in­cor­po­rate a wood core in the deck struc­ture—some­times ply­wood, but mostly balsa, thanks to the fact it is light, stiff and in­ex­pen­sive. In ad­di­tion, with its grain sealed against the skins, balsa does not read­ily trans­mit mois­ture, lim­it­ing the po­ten­tial dam­age of wa­ter pen­e­tra­tion, and ex­hibits ex­cel­lent shear strength, per­haps the best of all com­mon core ma­te­ri­als. It also re­sists be­com­ing de­tached from the skins bet­ter than other cores, which is good for main­tain­ing I-beam like stiff­ness and pre­vents the mi­gra­tion of wa­ter across the core. If mois­ture does pen­e­trate, balsa main­tains much of its in­tegrity wet and can of­ten be fully re­stored by dry­ing. It also re­sists rot, so the win­dow of opportunity for sav­ing the core af­ter a leak oc­curs is much wider than with other woods. Lastly, balsa has good in­su­lat­ing prop­er­ties.

Ply­wood is also found sand­wiched in sail­boat decks, but it is a poorer choice. Its ap­peals are low cost and strength, but ply­wood is heavy, and the mul­ti­ple bonds be­tween lay­ers ex­ac­er­bate the dis­ad­van­tages of a glued-to­gether com­pos­ite. Worse still, the lam­i­nar con­struc­tion of ply­wood makes it ex­tremely sus­cep­ti­ble to wa­ter mi­gra­tion, and the woods used are prone to rot when wet. The real-world near in­evitabil­ity of wa­ter pen­e­tra­tion ar­gues that there is lit­tle to rec­om­mend ply­wood core be­yond low ini­tial cost. That said, em­ployed just at hard­ware mount­ing lo­ca­tions, ply­wood does of­fer greater re­sis­tance than balsa to the com­pres­sion of fas­ten­ers or rig­ging loads, so used this way, it can serve well, if kept dry.


Be­yond that, plas­tic foam core ma­te­ri­als are more com­mon in hulls than decks. Foam core comes in three flavors: styrene acry­loni­trile (SAN), lin­ear polyvinyl chlo­ride (PVC) and cross-linked polyvinyl chlo­ride. Th­ese have vary­ing en­gi­neer­ing prop­er­ties, but from the owner’s view­point all are es­sen­tially the same. The pri­mary ad­van­tages of plas­tic foam is that it is lighter than balsa, its closed-cell con­struc­tion re­sists wa­ter mi­gra­tion or sat­u­ra­tion, and it does not rot. This re­sis­tance to wa­ter dam­age has made plas­tic foam the dom­i­nant choice for cored hulls.

A third core pos­si­bil­ity is a honeycomb struc­ture be­tween skins. This cre­ates the light­est com­pos­ite pos­si­ble, be­cause most of the space be­tween the skins is just air. A honeycomb com­pos­ite can also be im­pres­sively stiff, al­though be­cause the bond is lim­ited to the edges of the honeycomb it is also rel­a­tively frag­ile. Honeycomb core has long been used in one-off race boat con­struc­tion where win­ning trumps dura­bil­ity. How­ever, in re­cent years plas­tic honeycomb has also been find­ing some em­ploy­ment in pro­duc­tion boat hulls and decks. The long-term out­come of this prac­tice has yet to be de­ter­mined. How­ever, there is lit­tle doubt of the ben­e­fit of us­ing a honeycomb core for in­te­rior bulk­head con­struc­tion, where it can re­duce the weight by more than half com­pared to ply­wood.


Which brings us to core dys­func­tion, or CD. Be­ing in­vis­i­ble to the boat owner, the sub­stan­tial ben­e­fits of cored con­struc­tion are sim­ply a fea­ture of the boat—un­til some­thing goes wrong or it stops work­ing, at which point it can quickly over­shadow all else.

The pri­mary causes of core fail­ure, or CD, are im­pact, wa­ter in­tru­sion and man­u­fac­tur­ing de­fect. Im­pact needs the least ex­pla­na­tion. When a com­pos­ite lam­i­nate ex­pe­ri­ences an im­pact, the im­pacted skin takes a con­cave shape while the op­po­site skin is forced into a con­vex shape. That puts heavy shear stresses on the core, most acute at the core’s outer sur­faces, hence at the bonds be­tween core and skin. The re­sult is likely to be de­lam­i­na­tion. With the core sep­a­rated from the skins, the stiff­en­ing func­tion of the core is largely lost. The vi­o­lent flex of an im­pact can sunder the bond over a large area, but even a small area of de­lam­i­na­tion can be de­struc­tive. That area will flex un­der pres­sure, tear­ing at the edges of the bond and over time in­creas­ing the area of dam­age. De­lam­i­na­tion can be hard to dis­cern un­til large enough for the skin and core to spring apart.

Wa­ter in­tru­sion is the real de­stroyer. The bond be­tween skin and core can be re­stored, but not if the core is wet. Sat­u­rated ply­wood quickly de­lam­i­nates, los­ing all func­tion. Sim­i­larly, de­spite claims to the con­trary, balsa also rots. True, it re­sists for a while, but in the con­stant pres­ence of mois­ture, de­cay is in­evitable. What­ever the core ma­te­rial, trapped wa­ter also ex­erts a hy­draulic pres­sure that will fur­ther dam­age the core and bond. If a boat ex­pe­ri­ences freez­ing tem­per­a­tures, the ex­pan­sion of the wa­ter be­com­ing ice also does dam­age.

How does the wa­ter get in? The tini­est skin breach from im­pact will jet in mois­ture, with the crush­ing force of the im­pact typ­i­cally hav­ing left an invit­ing void be­tween skin and core. Even un­dam­aged fiber­glass is not 100 per­cent im­per­me­able, so where core is em­ployed for a sub­merged por­tion of the boat, mois­ture might sim­ply mi­grate through the ex­te­rior skin. For that very rea­son, pro­duc­tion boats typ­i­cally core hulls only above the wa­ter­line. By far, the most com­mon source of de­struc­tive mois­ture in cored lam­i­nate is via holes drilled in­ten­tion­ally through the oth­er­wise pro­tec­tive skins. This can be es­sen­tial hard­ware, or a teak deck (hun­dreds of holes!), fac­tory in­stalled or an item mounted by yard or owner.

In a per­fect world, even with an open­ing in the skin, wa­ter would have no place to go given that the space is filled en­tirely with a closed-cell core. How­ever, the re­al­ity is that core is typ­i­cally laid into a fe­male mold dur­ing con­struc­tion, mak­ing the com­plete­ness of the bond in­vis­i­ble on the un­der­side of the core. Add to that a core ma­te­rial con­fig­ured

in blocks or with deep kerfs to al­low it to con­tour to the shape of the com­po­nent it re­in­forces, and there are likely to be am­ple chan­nels and reser­voirs for mois­ture to mi­grate and ac­cu­mu­late.

CD can also oc­cur due to man­u­fac­tur­ing de­fects. A com­mon ex­am­ple is cored con­struc­tion of in­ad­e­quate thick­ness to re­sist flex­ing, which soon de­lam­i­nates. Some plas­tic foams are un­suit­able for spe­cific core ap­pli­ca­tions due to me­chan­i­cal or ther­mal prop­er­ties. De­fec­tive or in­com­pat­i­ble resin can be the source of fail­ure. And then there is just plain poor work­man­ship.

How do you know if your boat or one you are think­ing of pur­chas­ing has CD? Even­tu­ally the signs are ob­vi­ous—springy decks, the sound of crack­ing or squish­ing un­der­foot, even wa­ter squirt­ing or ooz­ing from cracks or un­der hard­ware. Less ex­ten­sive de­lam­i­na­tion may be lo­cated by tap­ping the lam­i­nate with a brass or plas­tic mal­let or the plas­tic han­dle of a screw­driver. In­tact lam­i­nate sounds sharp, de­lam­i­na­tion or sat­u­ra­tion sounds duller. The ex­act sound is not im­por­tant; it is ac­tu­ally the change in sound you are lis­ten­ing for.

A ma­rine sur­veyor may ar­rive with a mois­ture me­ter, but such me­ters are scan­dalously un­re­li­able for de­ter­min­ing core con­di­tion, no mat­ter how con­fi­dent the user. The unerring test for core wet­ness is ex­plo­ration. For a prospec­tive boat, you may have to be sat­is­fied with­draw­ing some hard­ware fas­ten­ers and prob­ing their holes. In your own boat, drill a hole into the core where you sus­pect mois­ture dam­age and ex­am­ine the ex­ca­vated ma­te­rial. If you are still not sure, an un­der­used tac­tic is to mon­i­tor a square of clear plas­tic placed over the hole, sealed com­pletely on all four sides and ex­posed to the sun. Seal a sec­ond piece ad­ja­cent, not over a hole, as a con­trol. If mois­ture forms on the un­der­side of the test square, it is com­ing from the core. A core sam­ple taken with a hole saw can elim­i­nate all doubt.

Con­sid­er­ing there is no pill for CD, how should you pro­ceed? Core is­sues in a hull should never go un­treated but for a springy deck, liv­ing with it is one choice, some­times the best one for older boats where the skin is suf­fi­ciently strong. Stiff­ness might be re­stored by adding stringers or other ex­ter­nal re­in­force­ment. Where de­lam­i­nated core is dry, restor­ing the bond by resin injection is an oc­ca­sional pos­si­bil­ity. Per­fo­rat­ing the top skin can al­low un­com­pro­mised wet core to dry nat­u­rally (shielded from weather) or the process can be has­tened with de­hu­mid­i­fi­ca­tion, heat or vac­uum. Fill­ing a pat­tern of drilled holes with re­in­forced resin that bonds to both skins can res­tiffen a deck by pro­vid­ing a colum­nar sub­sti­tute for the de­cayed core. How­ever, when the core ma­te­rial is dam­aged, the best so­lu­tion and of­ten the eas­i­est in the fi­nal anal­y­sis, is to re­move one skin, re­place bad core, then re­bond the skin or lam­i­nate a new one. Done prop­erly, this re­stores the struc­ture to full in­tegrity.

Bar­ring man­u­fac­turer de­fect, CD is eas­ily pre­vented by avoid­ing im­pact dam­age and keep­ing mois­ture out of the core. The ubiq­ui­tous boat term “rebed­ding” should clue you in that sealants are in­ad­e­quate for the lat­ter. Sealants in­evitably fail, let­ting wa­ter into the core long be­fore you see a trickle in the cabin. Ev­ery hole into or through com­pos­ite lam­i­nate needs to be drilled over­size, filled with re­in­forced epoxy, then drilled again to seal the open­ing away from the core ma­te­rial. Larger cutouts in the skin must have the core around the hole ex­ca­vated and re­placed with re­in­forced resin.

In the end, vir­tu­ally all soft decks are due to ne­glect. Be­ing fore­warned is be­ing fore­armed. If you main­tain the seal that pro­tects the core, your boat is un­likely to ever suf­fer from CD. If you don’t, ma­jor re­pair and/or sig­nif­i­cant ex­pense or loss is surely in your fu­ture. Be vig­i­lant. s

Var­i­ous types of closed-cell foam, and end-grain balsa: your boat is al­most cer­tain to have one or more of th­ese cores in its layup

Cores in top­sides re­duce weight and add strength and in­su­la­tion

End-grain balsa is still the core of choice for many builders

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