Hull layups "core" technology vs single skin

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Irish Sea
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Hull layups "core" technology vs single skin

Post by Irish Sea » Sat Oct 31, 2015 11:43 am

Hi Guys,

I was wondering how these newish "core" layups as used by Tiderace or Zegul compare to the more traditional single skin hulls used by the likes as Valley, P&H, NDK.

I understand that the idea behind the core is to keep two, presumably thinner layers of laminate appart from each other for added stiffness.
But how do these layups compare in other aspects as in repairability, puncture resistance, weight?
Also what happens with the core in case the outer skin is punctured? Does it draw water?

Would be great if somebody could shed some light on the topic.
I'd also be interested in some real life experiences concerning Tideraces G-Core and Hardcore layups for "demanding" activities like rock-hopping.

Cheers,

Bjorn

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Phil Hk
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Re: Hull layups "core" technology vs single skin

Post by Phil Hk » Sun Nov 01, 2015 11:56 am

Hi Bjorn

I was demoing a Tiderace Excite last year off coast of Jersey. A large swell dropped me on a vicious, gnarly piece of rock. The sound was horrible and I was convinced I'd put a hole in the hull just forward of the seat. True - it damaged the outer layer and possibly the core to a lesser extent but the hull remained watertight; given the impact the damage was minimal and easy to repair well. There was no star cracking visible. It is also worth considering the weight : strength ratio as former can play a role in damage potential and Tiderace kayaks are both light and rigid.

Phil

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Re: Hull layups "core" technology vs single skin

Post by flat earth sails » Sun Nov 01, 2015 9:41 pm

My pace 17 is the bacic lay up. I think the epoxy resen goes a long way to making it resilant to impact stare craks ect.....
I think the epoxy is more flexabel

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Re: Hull layups "core" technology vs single skin

Post by Jim » Mon Nov 02, 2015 1:31 pm

No experience of sea kayaks but I have recently started slalom paddling and ended up getting a C1 with a sandwich hull, and a K1 with single layer hull - they are from different manufacturers and pitched at very different price/performance points but my experience so far:

The C1 is lighter (well higher price and performance point, but being wider I was expecting both to come in about the same weight) and stiffer. I had a bad impact at Symonds Yat a couple of weeks ago during a practice run (at full speed), I hit the rock so hard I capsized, also spectators on the bank heard it. The result is 3 grooves in the bottom of the C1 the core has been compressed, 1 edge of each groove is sharp where the gel coats is cracked, but it is impossible to tell without chipping off the gel coat if the carbon/kevlar skin is actually sheared right along (must be partly cracked). I didn't notice until later on, the boat remained dry, although I have since fitted gaffer tape over the grooves until I decide what to do next - just in case water could seep into the core.

The K1 is a bit heavier and I can feel it flex sometimes when paddling (manouevring hard). I mostly use it on the artificial white water course, which is not very wide and not very forgiving to a numpty like me. When I hit blocks I can feel the hull flex. At one point I have missed an eddy and hit a block hard just in front of the seat - there is no hole but I can see the resin has cracked under the stress and needs a little patch inside and the gel coat touching up externally. I also seem to have popped the external seam tape in a few places, so it leaks quite a bit (did I mention the C1 has no external seam tape, only internal - except a few sacrificial inches at each end as wear strips).

So bearing in mind that the C1 was damaged by a naturally sharp river rock, and the K1 by rounded plastic blocks (the C1 has hit plenty of blocks too) I would have to conclude that the sandwich construction of the C1 is quite a lot tougher than the single skin construction of the K1, and in my opinion has so far been well worth the price difference (almost double).

That probably isn't the whole story, I am definitely a lot more confident in K1 so probably try more marginal manouevres (i.e. fail and hit stuff more), but the wierd part is I feel as though I paddle much better in the C1, even though I can't always get places I can with 2 blades.

Would I consider sandwich construction for a sea kayak?
I have never managed to put a hole in my carbon/kevlar Taran which I have had for much longer and paddled a lot more than the slalom boats so actually the possible damage problems aren't a big factor, but I think if I was in the market for another one and sandwich construction was an option, I might well go for it.

Water absorption with outer skin damage will depend on the core used and how well it is bonded to the skins - open cell foam or end grain balsa is likely to absorb much more water than a closed cell foam, and if the skins and foams are bonded properly there should be no capillary action along the interface. If coremat is used, this will be saturated with resin anyway (saves about 30% weight over making up with regular layers) so it would be impossible for it to absorb water, I suspect however that kayaks are using more sophisticated core materials.

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Re: Hull layups "core" technology vs single skin

Post by Irish Sea » Mon Nov 02, 2015 5:44 pm

Thanks everybody. Very interesting so far!

Tiderace says they use a "resin supported core material" in all their layups.

Zegul say they use a "Soric core" in their heavy duty layup and a so called "HD LRA core" in their other layups.

Soric core
Soric SR is a special type of pressure stable polyester none-woven core material designed specifically for vacuum resin infusion. Soric will not collapse under vacuum and actually acts as the flow medium meaning no infusion mesh or peel ply are required.

For HD LRA core I couldn't find anything. Anybody knows what that could be?

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Re: Hull layups "core" technology vs single skin

Post by jeremyduncombe14 » Mon Nov 02, 2015 6:15 pm

HD must be high density. Can't find out what LRA means, I suspect it is just a proprietary name, but there is some interesting stuff about it on www.taheoutdoors.com/en/production-of-kayaks-and-canoes. It is their top-end, high performance lay-up.

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Re: Hull layups "core" technology vs single skin

Post by Irish Sea » Mon Nov 02, 2015 6:25 pm

They use the LRA core on all their fibreglass and carbon layups and the soric core only on the glass/kevlar "rock solid" layup.

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Re: Hull layups "core" technology vs single skin

Post by Jim » Tue Nov 03, 2015 2:21 am

Soric is a more advanced material from the same people as Coremat, it should soak up quite a bit less resin than Coremat so making a lighter laminate, but with excellent bonding:
http://lantor.com/lantor-soric/

HD LRA, probably the LRA part stands for Low Resin Absorption, and HD for High Density, neither of which tell us much about which actual product is used, but strongly suggesting it is a closed cell foam of some kind.

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Re: Hull layups "core" technology vs single skin

Post by Irish Sea » Tue Nov 03, 2015 8:31 am

Thanks, Jim very informative as always :)

So would you say the soric core has superior properties to the LRA core, assuming the LRA is indeed some sort of closed cell foam?

And what about TR's "resin supported core material? Any idea what that might be and how it compares?

Cheers,

Bjorn

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Re: Hull layups "core" technology vs single skin

Post by Jim » Tue Nov 03, 2015 4:36 pm

To be honest, I was getting the impression that Soric is more appropriate to infusion moulding than hand layup, but there are a couple of different grades so maybe one is for hand layup and one for infusion?

The difficulty with determining superior properties is that it will depend on what aspect you are considering, bearing in mind that core is a lightweight filler to move the extreme fibres away from the neutral axis and increase the section modulus of the laminate. Fresh air will increase the section modulus, but won't stop the outer skins moving towards each other and reducing the modulus so the core needs to have some compressive and shear stength as well as being light weight. Also if the skins are not bonded to the core they work independently so only their self modulus is effective. To keep weight down you don't want resin to soak into the core, but you do want it to soak in a little at the surface to ensure a good bond, or you can use a core that resin soaks right through ensuring a good bond, but ends up makign a slightly heavier laminate.

With a closed cell foam that has 'LRA', the surface cells are usually open due to the way the sheets are made/cut. Normally you seal them with a resin/filler mix and then immediately apply to the laminate so the filler resn and laminating resin bond chemically, and the filler ensures good mechanical bond to the foam. Everything inboard from the surface is closed cells full of air. HD foam has better shear and compressive properties than LD, but at the cost of a little extra weight.

With Soric there seem to be different sized cells, larger closed cells that are strong enough to support themselves under vacuum (vac bagging/ infusion can put about 1 atmosphere of pressure on the laminate during curing, which from memory I think is about 10 tonnes per square inch) and smaller open cells that allow the resin to soak through. What you get then is a core which has resin contiguous through it and the layers above/below, but with lots of decent sized air pockets to keep the weight down, but I suspect that overall it will work out a little heavier than a closed cell foam for the same thickness of sandwich, but with reliable bonding. The shear and compressive strength in the cured sandwich will be fairly high as the resin and fibres from the fabric make the strength, not just the normal properties of the fabric (or foam in a foam core).

Coremat is an older technology which is still relevant, as far as I can tell it has smaller closed cells distrbuted throughout a permeable fabric. It will work better with hand layup than Soric because you can roll resin into it rather than needing to infuse it by vacuum, but it will absorb a lot more resin than Soric or a closed cell foam, but 30% than making the thickness up with solid laminate.

Looking about there are other manufacturers offering a wide variety of core materials that work in these ways (and others, such as aluminium or kevlar honeycomb) but in terms of a "resin supported core material", I would say that both Coremat and Soric and similar materials from other manufacturers would fit that description. Aled may well interject if my speculation is too far from the mark.

As for which is the best/right material to be using, there are a lot of considerations, including cost. Some core materials are only available in thicknesses too great to be of use in canoes and kayaks (for example, honeycomb types), others like Coremat have been around a long time so customers may perceive tham as being lower technology than some of the newer materials, but it is probably the only core which is suitable for a non-vacuum bagged construction, most others will tend to spring up over time so it really pays to cure in a vacuum bag to make sure everything stays where you put it by the time it has cured, and some, like Soric probably need to be used with resin infusion (similar to vac bag but resin is also drawn through the laminate by vacuum), so it is not simply about what is the best material for the kayak, but which is the best material for the processes the manufacturer uses.

It has been quite a few years now since I used to work in laminating, and I never specified the core, I just used to use what I was told, which was mostly to use closed cell PVC foam, occasionally balsa or plywood. There are quite a few 'new' materials out there, as well as some old ones that I have never used.

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Re: Hull layups "core" technology vs single skin

Post by Irish Sea » Wed Nov 04, 2015 9:24 am

And another impressive writeup :)

Thanks a lot, Jim!

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Re: Hull layups "core" technology vs single skin

Post by Mikebelluk » Sun Nov 08, 2015 9:52 pm

I find my lo tech core material works very well.
Western red cedar.
Natures best.

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Re: Hull layups "core" technology vs single skin

Post by Jim » Sun Nov 08, 2015 10:12 pm

Mikebelluk wrote:I find my lo tech core material works very well.
Western red cedar.
Natures best.
And the looks are equally desirable.

Given a choice between beautiful wood and beautiful carbon fibre, I'd buy both!

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Re: Hull layups "core" technology vs single skin

Post by Irish Sea » Mon Nov 09, 2015 6:37 am

If I could only find the time to build one of these beauties...

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Re: Hull layups "core" technology vs single skin

Post by Aled » Thu Nov 12, 2015 1:38 am

Jim wrote:Aled may well interject if my speculation is too far from the mark.
Not at all! Jim's on the money here...
We use three grades of core which vary in thickness and density. The layups are boat specific as are the reinforcement positioning. The epoxy resin when vacuum bagged gives best strength and allows for differentiated cloth positioning within the layup. Some models have continuous core while others are position specific, some boats have a multitude of core combinations - every model in every construction has an unique layup pattern/sequence. Shame to hide it all away really...

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Re: Hull layups "core" technology vs single skin

Post by gnarlydog » Tue Nov 24, 2015 4:22 am

Mikebelluk wrote:I find my lo tech core material works very well.
Western red cedar.
Natures best.
Have you considered the ultra high tech (but rarely used, because expensive), natural material, cork?
Yep, good old cork that resists compression and springs back if compressed, without leaving a big dimple?
No Asian manufacturer of kayaks uses cork however, to my knowledge.

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Re: Hull layups "core" technology vs single skin

Post by Aled » Tue Nov 24, 2015 11:39 pm

gnarlydog wrote:Have you considered ... cork?
Yes, albeit a few years ago, we also considered basalt woven cloth. There were issues with cork: inconsistent material thickness, varying material density and varying degrees of porosity/voids. Some boats would be heavy, some boats would be 'less stiff' than others, all would have hard/soft spots, all leading to too many production inconsistencies. Basalt cloth had great physical properties but it was an earthy dark brown when laminated. I still look forward to using more natural materials in future...

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Re: Hull layups "core" technology vs single skin

Post by gnarlydog » Thu Nov 26, 2015 11:09 pm

so, cork is not used in large scale production since it's hard to source it in a consistent quality/density?
I guess the balance for cost vs benefit is not that favorable and there are other materials more widely accepted, to make cork a niche market material.
I just like the idea to have a core material that does not deform under pressure and then not return to it's original shape.
I have a kayaks where the core compressed and a visible dimple resulted...

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Re: Hull layups "core" technology vs single skin

Post by Irish Sea » Fri Nov 27, 2015 8:21 am

Hi,
Willy Neumann in Germany is (optionally) using cork as a core material for their boats. Don't know how they deal with the natural inconsistencies of the material.

http://www.willyneumann.com/index.php?o ... &Itemid=38 (scroll down a bit)

rough translation for the non-german speakers:

Back to the nature - The revolution in boat building- cork meets high tech

the brilliant natural material with it's unique properties:

high elasticity
quality from nature
a sustainable resource
unsinkable
excellent adaptabilty
low specific weight

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Re: Hull layups "core" technology vs single skin

Post by flat earth sails » Fri Nov 27, 2015 9:28 pm

I wonder about the sustanability, cork is a slow growing tree, are ther meny plantations comershaly growing the stuff?

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Re: Hull layups "core" technology vs single skin

Post by Irish Sea » Fri Nov 27, 2015 10:21 pm

I think in europe the majority of the cork comes from portugal. As far as I know the cork (which is basically the bark) can be harvested every couple of years from the same tree without destroying it.

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Re: Hull layups "core" technology vs single skin

Post by Jim » Mon Nov 30, 2015 11:10 am

gnarlydog wrote:I have a kayaks where the core compressed and a visible dimple resulted...
Try gently wafting a heat gun over the area for a few minutes, often the foam will pop back to shape and push the dimple out, or most of the way out - this has worked on my slalom boat, although I still need to do the gel repairs on the 3 that were gouges from hitting a rock at full pelt....

Obviously success will depend on the type of foam core so results cannot be guaranteed!

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Re: Hull layups "core" technology vs single skin

Post by gnarlydog » Mon Nov 30, 2015 9:44 pm

Jim, thank you for the tip: I will give it a try and I will emphasise the "gentle" part ;-)

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