Theme:

Reduction of weight in the manufacture of a kayak with the use of the sandwich material Easy Cell 75 Closed Cell Foam PVC, 3 mm thickness.

 

We have previously used Soric 3mm in a construction of a kayak with one layer of carbon fiber and one layer of Kevlar, then sorric as sandwich and finally a layer of carbon fiber, all produced in a single resin vacuum process.  A strong construction, but a little too heavy.

 

To reduce the weight we have now used Easy Cell 75 Closed Cell PVC Foam, 3mm thickness as sandwich.

In the design we have, with the vacuum resin infusion process, performed the outer carbon and Kevlar layers, and after the final curing, glued with resin, a layer of 3m EasyCell75. After this glue has hardened, we have again, with the vacuum resin infusion process, performed the innermost carbon layer on top of the foam.

 

Unfortunately,the weight is almost the same, and it is apparently a weight gain that occurred during the infusion process in the foam, and the last carbon fiber layer. We know from the past that the carbon layer itself occupies only 50% resin by weight.

 

Why do we have this weight gain?

Wich methode should we use to reduce weight?

Is the infusion process and the foam not suitable?

You could possibly save a little weight by doing the infusion in one go and this would require you to create small holes in the foam at approximately 50mm intervals. I imagine that applying resin by hand when secondary bonding the foam adds a bit more weight than desired.

Hi MarkMk, Thank you for your ansvar.

I dont think that your suggestion will help much, because we only used 200g resin when we applied the foam to the fist layers. I think we have to explain the weigth in each step throughout the process for building the half part (the hull), approx 2 Sqm in each layer. We want this part in total to be max. 4 kg

1. Clear Gelcoat 800g in mould (We think we will try use 2k clear coat or nothing here insted next time)
2. First carbon 200g and kevlar 200g, Infusion
3. The weight at this point after a full cure was 2.6 kg
4. Easy Cell 75 Closed Cell Foam PVC, 3 mm, glued by 200 g resin, by hand.
5. The weight at this point was here 3.6 kg (At this point we expected the total weight to end up arround 4.5 kg)
6. 1 layer of carbon 200g + 0,7 sgm kevlar on strategic points
7. Resin infusion. After full cure, the weigth was 6.5kg Far to heavy we think.

We have made some tests with drilling holes in the foam, and infused all layers in one process with good results. But we thourght it would give us some troubles to hold all layers in place during the layup and vacuum proces. But we would love to do it all in one shot.

Best regards, Keld

Hi FLD, Thanks for your ansver

Jes, it sounds that it could be the reason. The vacuum was nearby 100% during the proces, but the resin flow was very slowly, and perhaps we stopped to early with infucion mesh (8 - 10 cm from the flange).

We will try to clamp off, as you suggest.

The Kayak we bulid is a 44 cm Marathon, our goal is a stiff 8kg kayak.

Best Regards
Keld
 

I think that when you vacuum infuse the inner skin the infusion process fills all of the foam cell surface with resin too and this is why it is heavy.

Hello from me,

I would suggest two approaches:

first: You could try to use prepreg as the inner layer. If you got the oven for this, of couse. Do it all in one shot in prepreg would be an more efficient solution, but kevlar prepreg isn't easy to come by.

second: There is a different kind of soric, called "soric lrc". Lrc means low resin consumption. As you might guess this is sort of a light weight type of soric. The difference is about 700kg/m^3 to 450kg/m^3. This doesn't quite get you to your target, but combined with a lighter kevlar layer, and strict resin measurement you should be able to get there. maybe try to keep the resin pod about a metre below your mould. This prevents most of the resin overflow. I reached around 1930 g/m^2 with the following layup: 200g/m^2 carbon; soric lrc 3mm; 200g/m^2 carbon. Even with 80g/m^2 kevlar this would be a little above your target. But if you swith the inner layer to 160g/m^2 you sould be able to do it.

regards

Hi oekmont, thanks for your reply

Unfortunally we dont have a oven in that size 5.5 meter, it could be worth to make one. We have no experience with prepeg so it must wait untill next project.

Part of your second suggestion seems to be able to solve our weight problem, but i´m not quite sure. Our first test hull with this layup ended up on 6.5 kg.

Gelcoat 800g + 1 layer 200g carbon + 1 layer 200g kevlar + 2mm soric + 1 layer 200g carbon + 0.7 sqm kevlar all infused in one shot... very easy methode, but the total weight was 6.5 kg. I guess that something vent wrong in the infusion process, perhaps we could save 1 kg there, and another 500g if we use 80g kevlar instead of 200g, then it still be to heavy 5 kg. But with soric light 1.5 mm instead of 2 mm i guess we could cut of 500 g more, and 2 k clear coat instead of gelcoat now we are almost there.  

There is a lot of factors to think and test through

Best regards Keld
 

Hi Fasta, thanks for reply

I think you are right about at least some of the foam cell er filled with resin, but i dont understand why, because the water uptake should be less than 1 %

How many sqm are the dinghy you make? I'm an old dinghy sailor (Laser, europe) so i´m a litte intersted in what type that is.

What kind of paint du you use, and do you paint in mould or after?

I think we will give soric lrc 1.5 a try, and a lighter kevlar layer. Combined with a better infusion control as mentioed in other posts, it could help us to reach our target. 

Best regards, Keld