foam core question

[cossi]

I am thinking of building an ultralight kayak.
My normal building method is hand lay up carbon with epoxy on foam core and then vacuum bag (female mould). Now i want to eliminate all excess weight. Normaly i would prepare the pvc foam with a mix of epoxy and microballons to fill the pores and get a good adhession to the skin.
What would happen if i leave this step out? Can i lay the dry foam on the wet carbon and still get a good solid adhession to the outer skin after vacuum bagging? I use airex c70 .75 3mm and 1 layer of 160gr carbon for the outer skin.

[Submarine Tom]

I don't know what would happen.

Are you considering skipping this step to save weight?

If so, do a calculation for me. What is the weight saved compared to your total weight - all in?

That is: your body weight, the paddle weight, your clothes, footware, lunch, kayak, sunglasses, watch, GPS, cell-phone, everything.

Now what is the percentage of saved weight by skipping the step?

-Tom

[cossi]

I would save almost 1 kg without that step. that is not bad, if the adhession is as good as with the prep-work. (and less work too)
This boat is not ment to win the olympics or having commercial goals with it. It is just simply a project for myself testing out my limits. I know it wouldn't make me any faster, but if it can be built ligter without affect stability, i would do it.
The reason why i came to this idea is, because the surface of that airex foam is very fine, no big cells that have to be filled, like on the lower density foams.
So my question is: is the amount of resin in the outer skin enough for a good bonding? When i layed up the outer skin (about 40%fiber ratio)and i press it with my finger it seems to have enough resin for a good bonding. But i am not sure.
Before i make my decision to do or not to do i wanted to have some replies, if someone had any inputs or experience on this.

Thanx

[AndrewK]

Normal practice with epoxy is to laminate straight on to the foam.
There will be no weight saving.

[tunnels]

For ultra light use Nomex!! Thick in the bottom and thin on the sides and a little thicker on the upper surfaces .combined with Crabon and kevlar in the bottom and part way up the sides where it will bag against things

[Herman]

If using nomex, you would like to use prepreg as well, and you will need a bonding layer. Guess what? On 3mm, you will not gain anything.

As for hand laminating + vacuum: One of my clients makes boats that way, also with 3mm foam, and he does not use anything as a bonding layer. He does perforate his foam, to get rid of excess resin from the bottom laminate. He actually usually uses 2 layers of cloth (carbon or carbon aramide mix) on the outside, but his canoes are heavily abused during use.

For still water canoeing, I guess you could even get away with a 100 grams biax carbon. The biax will also introduce less resin, as the bundles are kept straight, and there are no "knots" in which resin just sits, doing nothing.

I have no idea of what is available or not in your area, but over here I sell the 100 grams biax for roughly the same price as 200 grams woven (carbon).

As for adhesion: make a small test piece, and you will find out...