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Old 13-05-2008, 02:38   #1
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multihull build options.

From one of the pioneers of foam sandwich construction for sailboat hulls, Derek Kelsall gives his view on the new line of plastic honeycomb products. He also does not hide his dislike for balsa core products as a build material for a boat.


Hi Boat Builders,
Our friend Rob Denny raised the topic a couple of weeks back. I have had a few inquiries talking about the pros and cons of the much cheaper honeycomb and pvc foam.
To quote my answer to one where impact damage was of concern, and the belief that honeycomb would be best in that case -Thin skins are more prone to damage to the outside skin, which takes the form of a tear as in clothing.
I have never seen damage that has gone right through both skins of foam sandwich. That is due to the toughness of foam. Linear Airex is the toughest and a layer of kevlar could be added between glass skins if that was of major concern.
If water can be kept out of individual cells, what would happen if damage coincided with a join in the sheets of honeycomb?Honeycomb has other handling disadvantages apart from infusion (which isstill a huge advantage in being so much cleaner etc.)- cutting, sanding,routing, bevelling, joining sheets and joining into blocks of foam forshaping are all part of the job.

What about that finishing task?One more. Take a plastic honeycomb sandwich and hold it to the sun. To be comfortable you will need extra insulation.
If honeycomb showed overall benefit I would use it. I have no connections with any foam producers (they all sell balsa, which I condemn, so do not always make friends with them either).What I would advise is to test samples before making that finaldecision. Take a sledge hammer to both foam and honeycomb sandwich, break it, cut it, sand it, soak it, boil it - and let us know what you find.No doubt there are ways to avoid some of the potential problems I mention here. I do not have lots of honeycomb experience. I do know that PVC foam has stood the test of time, which has included a few situations where foam has proven itself to be exceptional against impact damage in particular. Much better than solid glass.
It took me five minutes to get water through a typical skin to Nidacorecells using a hair dryer and water from the tap. Nidacore is a finecore for interiors.Enjoy your boat building - more with KSS resin infusion.Ballotta boat builders were mentioned also. Ballotta is KSS, resin infusion and pvc foam oriented. Their success is down to efficient method, access to our range of designs and our custom design service but above all to good management and high standards. They can quote for a very wide range of sizes and types, without further investment in tooling.Do not expect any compromises from Ballotta or Kelsall, to save a dollar on materials.We enjoyed the Denny/KSS proa project we did in Australia last year and would be happy to do more similar - for KSS foam build.Derek.-- Derek Kelsall, FRINA.,Kelsall Catamarans ltd.Tel 00 64 7863 3332.More information about
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Old 13-05-2008, 03:07   #2
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Polycore has a film that can handle 1 atmosphere without problems, guaranteed. I do not understand the hair drier test. All cores have some problems with joining. Stick the panels together with a bit of hot glue and if there are too big a gaps, fill them with microballoons. I don't see that this is any different from any other core joining. There are difficulties in infusing both sides at once but not insurmountable and it is possible to buy 20' panels heat pressed with pre preg, as Pacific multihulls are offering in their kits Rebates can be done with a heat press, or gluing a thinner strip of core. Cutting is a breeze. Bevelling can be done with a heat press, Routing is not a problem, neither is preshaping using pressure shaping. Rob Denney has just tested this out on a scale model of a large Harryproa. Tests on resilience such as bashing with a hammer has polyprop ahead of the other cores as it absorbs energy better without damage. This also makes it a good noise absorber. I suspect it would be tougher in the case of tearing. I haven't seen many boats built with transparent skins. The certification is underway for Australia. Much as I admire Derek's work, I do disagree with him on this one
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Old 15-05-2008, 04:28   #3
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Spoke to Polycore, and they told me about their film. It is semi permeable to allow for a bit of air movement when it heats up, but is impervious to liquid water and resin. Polycore is not Nida plast. They claim superior properties to corecell. They also offer epoxy glassed panels heat pressed at a very competitive price. They are about to publish their tests on impact resistance compared with other cores. They do not have any problems with the core below the water line.
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Old 15-05-2008, 08:34   #4
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PVC honeycomb is fascinating stuff and invites controversy on a par with epoxy vs vinylester. Those who use it love it. Those who do not would not touch it. My cat is built almost entirely of Nidaplast so I can share some experience with it.

The hulls are made of 12mm Nidaplast "marine" which is scored into 50mm squares so it can be molded around curves. The hull was vacuum bagged into a female mold using glass/kevlar/epoxy for the skins. Unfortunately, the PO had neglected to exclude the core in way of all the through-hulls, then bedded them with silicone. When I hauled the boat I was able to spin the through-hulls in their holes with my hands. Of course they all leaked and the water was free to roam along the cuts in the Nidaplast. When I removed the through-hulls water leaked from the hulls for days. The hulls are fairly flat on the bottoms and in the port forward compartment where the leaking was worst the core had delaminated. I attribute this to water in the core combined with pounding and lack of stringers to decrease panel size in this area although other factors may have been at work. Not good. How would another core have fared? I can only guess that Corecell or Divinycell would have done better by limiting water migration. Balsa would likely to have been much worse. Even a solid laminate would have suffered under these conditions, although to a far lesser extent. However, this was a relatively small area and the rest of the hulls had no delamination. Impressive in it's way and I believe the delamination could have been avoided had proper boatbuilding technique been followed. Regardless, we removed the honeycomb from beneath the waterline and replaced it with Corecell, but in my heart of hearts I feel this was probably not necesary.

The cabin was made of 25mm Nidaplast. I was amazed to find that the cabin remained relatively cool in the tropic sun. At midday I could touch the overhead and without any additional insulation it was only slightly warm.

When considering technical data, honeycomb comes up short in shear strength and excells in impact strength and insulative properties when compared to other cores of similar weight. There are differences between different brands of honeycomb as well. On another site a poster reported that his group had done testing on an American product, Nidaplast (French), Quingdong Tubus (China) and Hexacor (China as well, I believe this may be the same product as Polycore in Aus). The French and American stuff did well, The Tubus however suffered delamination on some panels. The Hexacor did best of all. This is of interest as Hexacor is substantially cheaper than the French and American stuff.

During the rebuild we have used both foam and honeycomb, incorporating each where it's strengths can be put to best use. The main crossbeam and hull repair were done in foam, whereas the bulkheads and cabin will be done in honeycomb. The cabin top will be done in 50mm honeycomb for rigidity and insulation. The real kicker is cost. In Thailand a sheet of 25mm Divinycell goes for $212 US as opposed to $53 for Hexacor.

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