Ken Raybould on Structural Cores—Part 1

Now retired (well, mostly retired), Ken Raybould has had an impressive career in the marine industry that can be said to have begun with raceboats. In the realm of power, he helped develop the first Ray Hunt deep-V offshore designs to compete in Europe. In sail, Raybould did the structural engineering for yachts that won world championships in the three-quarter-, one-, and two-ton classes. Power and sail alike, those vessels featured innovative FRP construction and served as significant test beds, over a period ranging from 1958 through the 1970s. By 1977 Raybould felt confident enough of his experience and expertise in applied marine composites to start a Southampton, U.K.–based consultancy called MarTech, for which he was principal.


Illustration on cored construction drawn by Hugo du Plessis for his best-selling Fibreglass Boats, a book first published in 1964 by Adlard Coles (London) and still in print. Du Plessis has continually updated the text to keep pace with FRP technology. A review of the latest edition appears here on this website.

By 1992, when a young Professional BoatBuilder magazine launched its trade show and seminar program, known by the acronym IBEX, Raybould could count among his clients American conglomerate DuPont, maker of Kevlar reinforcements and Nomex honeycomb cores—often described as “exotic” materials. During the mid-to-late ’90s, PBB regularly enlisted Raybould to be a panelist or moderator in seminars addressing composite problems and solutions. For example, at IBEX 1996, he moderated a seminar titled “Core Wars”; Raybould’s mild manner kept it from becoming a battle royal among technical reps on the panel, whose companies were fighting for market share. (One brand missing at the time, however, was Corecell, a SAN foam product under development then.)

Raybould’s podium notes for that session are still quite relevant, considering how many boat builders—today, two decades later—remain skeptical of or apprehensive about cored construction.

Although there were reps on the “Core Wars” panel whose products were neither PVC nor end-grain balsa, Raybould, to get the session rolling, focused on those two types, since they dominated the marine market in the ’90s. As I recall (IBEX sessions are not recorded), he didn’t get to pose all the questions—marked by bullet points, below—to that IBEX ’96 panel. But those are questions asked by production builders who still need to be convinced of the advantages and efficacy of cored composites installed correctly.

Raybould’s podium notes regarding PVC cores

  • Please comment on the stability of your product in high service temperatures (as well as high humidity) such as occur, for example, in a Florida or Mediterranean summer.
  • What influence does core density and type, or the thickness of the FRP outer skin, or gelcoat color have on the laminate?
  • Why does outgassing seem to be a recent phenomenon, and how can this problem be avoided?
  • Do you advocate that the space between rigid panels (“boards”) and/or between individual blocks in contour cores (“flexible boards”) be filled with bonding paste or resin? In practice, it sometimes seems many voids are apparent.
  • What are your thoughts on selecting a polyester- or vinylester-based adhesive bonding paste rather than a “wet mat” or similar bedding system?
  • What are the typical variations in density to be found in your most commonly purchased cores?
  • Regarding product storage at the boatshop, or possibly in-transit during delivery to the boatbuilder: has humidity registered on the surface of your PVC panels during adverse climatic conditions? If yes, how should the condition be prevented or treated?
  • What advice would you give to ensure that the interface resin in a bonding compound (putty or wet-mat types) achieves full cure? Is priming—that is, allowing the interface material to gel before integrating it into the structure—advisable?
  • If vacuum-bagging, what degree of vacuum would you advise? Have you seen detrimental effects in applying too high a vacuum?
  • What properties do you consider ideal for a bonding putty or resin? What type of bonding system has proven to give prime mechanical properties?
  • What is your view regarding how well PVC works in a resin-infusion process? Which core-priming system do you recommend for infusion processing?

Questions regarding end-grain balsa cores

  • What is the resin type applied by your company to seal the face of the balsa panel? What is the effect, if any, of the cured sealing resin [applied by the core manufacturer to the end-grain balsa surface] on a secondary bond of a polyester-based laminate?

[Raybould’s remaining questions for balsa-core suppliers are basically identical to those above for PVC cores—except for the following on resin infusion. I’ll note that Raybould was the first person to describe the SCRIMP process, in print, to a trade audience.]

  • What is your view on how end-grain balsa works in infusion processing—especially at the laminate/core interface?

Question regarding peel or zipper effect

[Raybould has long believed that too few builders take into account what happens when hydraulic action compromises some portion of the hull bottom, at speed, via a damaged area in the outer skin near or below the waterline. Accordingly, he’d prepared the following lengthy pet question for the entire panel regarding general design criteria for both balsa and PVC cores.]

  • Has any thought been given by your company regarding how to prevent “peel effect” in an underwater section of the hull, when hydraulic pressure reacts with a damaged skin? In these circumstances, is it valid to rely on a core adhesive or a wet-mat resin to help retain hull integrity? Should reinforcements be integrated “thru-core” in specific areas to help distribute loads and perhaps minimize an outer-skin failure?

[Raybould-as-moderator planned to close the 90-minute seminar with this statement: “For both technical and economic reasons, we need to be more aware of the requirement to implement not only refined construction methods, but also quality control-and-assurance procedures devised to eliminate misunderstandings on how FRP materials might best be used. This does not mean more control; rather, it implies a different approach.” We’ll present precisely that—Raybould’s own approach to cored construction—in Part 2.]

About the Author: Paul Lazarus is Professional BoatBuilder’s senior editor.