Shearwater sport made from Diviycell

Hi I am thinking of building a very light Shearwater Sport out of Divinycell H80 foam sheet. I have all ready built one out of wood from a kit and it turned out great the problem is my wife would love one but if I am busy with work she can't use it because she can not safely lift it on or off the roof of the car. So my question is has any one here used this type of product to build a kayak or know of any articals on the web that cover this subject as I am not sure how thick the foam would need to be for a kayak.


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RE: Shearwater sport made from Diviycell


i think you bring up an interesting topic and one that i have explored quite a bit....building light boats.   i am currently building a petrel targeted to come in at 33 lbs when complete and outfitted.

a couple thoughts.'s important to establish a target weight of what you are going for.   its not enough to say 'light' as that means a lot of different things to different establishing a weight we can evaluate specific approaches and determine if they can achieve your goal.  

after setting your weight target its possible to see if there is a 'recipe' for the hull design you want that can acheive your target.   for example, once you establish a hull design (like the shearwater sport), you can evaluate materials (okoume, divinycell, cedar strip, etc.) the layup schedule (4 oz, 6oz glass, etc.) and the the corresponding epoxy weight.  and then the extras like seats, rigging etc and their corresponding weight. and see if you can come up with something that works for you respecting other factors like strength (at some point ....lighter inevitably leads to something more fragile).

basically what i am saying is that the final weight is going to be, more or less, the sum of the parts.   

from a weight perspective....without getting into 'exotics' (e.g., using material that are not routinely used in building kayaks or used by home-builders)  for something of the general size of shearwater sport in have the shearwater sport at 48 lbs but you also have the Chesapeake 14 at 36 lbs.   carefully built you can actually achieve these weights or a slight, maybe 5% improvement.  (i have a chesapeake 14 for my wife at 34 lbs)

executing the same hull with cedar strips  you could easily drive the weight of the shearwater in the high 30s and the chesapeake 14 into the high 20s.   

the lightest approach is skin on frame where a similar boats would be executed in the mid to high 20s.

with respect to your particular question on building with foam....a google search of the term 'building a foam core kayak' will bring you back plenty of material.  without getting into the particulars of it, i have stayed away from it....mostly for aesthetic and other issues outside of weight which are important factors in a kayak design.  i would be particularly cautious of the strength issues in slab-sided hulls which generally require a rather strong core material....or if not strong....a very thick core.

hope that helps


RE: Shearwater sport made from Diviycell

I've followed a few small boat Divinycell builds and they have consistently ended up weighing more than the same design built in plywood and glass. According to the designer, nobody did anything wrong, it wasn't a skills problem. Rather, plywood has a higher stiffness than Divinycell, so you need less epoxy/glass on the outsides than you do for a foam build. The foam weight advantage is supposed to really kick in for something that's cruiser sized.

So if you buy all that, you won't find the weight savings that way.

Have you considered skin on frame?

Good luck,



RE: Shearwater sport made from Diviycell

Howard was typing faster.  

RE: Shearwater sport made from Diviycell

   No experience biulding with that foam. The thickness would be in combo with the wieght of glass cloth - it is the sandwich that makes the strength.

You might also consider other technologies - I built a 20 lb, 12 foot solo canoe (undecked Kayak) Skin on Frame. I carry her on one shoulder the quarter mile to the local Mill River. She is a bit heavier one the way home (she is wet and I am tired from paddling). Nick Shade's Nymph canoe, cedar strip, is also very light. He uses very thin strips of cedar to keep the weight down. These technologies could be applied to the Sport design. You even can do a hybrid type design - doing a skin-on-frame deck on any design will take out pounds fast. Two great books on Skin-on-frame are by Morris and another by Starr. An no need to mess with frozen snot (Epoxy)!

You might also consider other loading technologies for your car (although the 20# Skin on frame boat goes on the roof really really easily). There are loader racks that do not require lifting to the roof top.


RE: Shearwater sport made from Diviycell

   Well, if your wife can lift the kayak once down on the ground, but has trouble lifting it over her head, then one thought might be a light, collapsible step stool to help her get higher, and also a kayak loader like Joel mentions.


 Foam core construction, at least in the aerospace context that I'm familiar with, is definitely not the same as wood core in methodology.  And the high strength foam core materials are not cheap. Balsa core is still one of the best light cores in terms of shear and compressive strength, compared to many foams.  The shear modulus (measure of stiffness in bending) of Divinycell H80, one of the ones you are most likely to find from boat building supply houses, is only about 4000 psi compared to the company's balsa core at 24,000 psi, so the same thickness panel is about 6x stiffer in bending w/ balsa, although the balsa is about 2x the density of H80.  Since panel stiffness goes exponentially with thickness but weight goes linearly with thickness, you can get the stiffness of the panel w/ foam up by increasing thickness some and still save weight, but there is engineering to be done there!

RE: Shearwater sport made from Diviycell

   Thanks guys for the advice I will have a look at balsa core and see how much it cost per sheet as I live in Australia getting good light marine ply can be difficult and expensive most of composit sheet is cheeper or the same price as good ply. I was intending to use 5oz kevlar instead of fiberglass to get the strength. The weight I am aiming for was no more than 36lb which I did not think would be to unreasonable. But I think Laszlo might be right if it was easy to build light strong kayaks with foam or balsa core there would expect to see a lot more of them than you do.


RE: Shearwater sport made from Diviycell

I've built a few things with foam cored composites. The fact that almost all competetive performance craft use foam should be a clue to its superiority over balsa for this purpose.

A reasonably accessible build method is to skin one side of a foam panel while laying flat on a table (using a simple vacuum bag setup ideally, but peel ply, absorber, and a couple layers of mdf stacked on top applies plenty of pressure, too), then cut the panels to size and assemble S&G style with the skins on the inside. The foam can be gently bent to the gentle curves of most kayaks, and tighter bends and twists can be accomplished by thermo-forming with a heat gun. Then tape the seams, radius the outer corners, and apply a skim coat of lightweight filler and then wet out your exterior glass just as you would for a ply boat, adding a layer of peel ply if you like.

Using a laminate schedule of 12oz (biax glass would be ideal) outside, 6mm core at 5#/cu.ft., and inner layer of 6 oz (kevlar, or carbon-kevlar mix would be good) gives a panel weight of around .43psf.

Compare a typical ply S&G schedule of 6oz glass over 4mm ply with 4oz glass inside and a total of about .67psf.

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