That would be difficult. First, you'd need a big bag that conforms very well to the shape of the hull. Second, you need a way to prevent the thin walls of the hull from collapsing under pressure. It would take a lot more than forms every 12". You could use an "equalizer" bag inside the hull, I guess, but you'd have to custom fabricate one. Third, not sure how you figure out how much epoxy is "just enough". Unlike the squeegee method, any excess epoxy is gonna end up on the hull somewhere because there's no place for it to go.

I have a bag and pump if you want to try it (on your boat, of course ;)

Cheers, Pat

My take's a little different from Pat's. I work in a place where we do lots of composite fabrication. Anything that isn't vacuum bagged is hydraulically pressed. No hand layups. I've seen lots of custom single-use bags made up for odd shapes. A kayak would be no problem. As far as it collapsing, that wouldn't happen as long as you only bagged one side at a time. And excess epoxy would be drawn through the breather material, out the tube and into a reservoir.

So it's definitely possible. I don't know if it's worth the trouble when building a single kayak. There's a good bit of expense involved getting all the equipment and materials, potentially as much or more than the rest of the cost of the kayak. When using just a single layer of light cloth, the resin to glass ratio on a hand layup can easily approach or match a vacuum bagged product. Vacuum bags excel for multiple thick layers of glass or carbon fiber, which these boats don't have.

In the boats sold here, you have to have some excess of epoxy to fill the weave and fair the boat if you're varnishing anyway. So while it's possible and interesting, I don't think that it's really useful unless you're making a carbon fiber  boat with no wood core.

Laszlo

I've laid 4 mil pastic sheeting (polyethylene ?) on the wetted fiberglass, and then squeegeed the resin out. Gets a real thin, smooth layer.

I never tried it on something as big as the hull though. Mainly smaller things especially if there were multiple layers.

I didn't say impossible, just difficult. Laszlo, I'm curious about the reservoir you mention. How does that work? Where in the loop is it attached?

BTW, even a single hull side is not flat, so it would need a mold or some other structure to maintain the shape in the bag. Then you would presumably need an extra layer of glass to tie the two sides together, which would negate some of the weight loss.

I'd guess the difference between a good hand layup and an optimal one would result in no more than half a pound of weight savings.

Hey Pat.

Sorry, I didn't express myself correctly, but I didn't think that you were sayting that it's impossible at all. My different take is that one doesn't need to worry about the atmospheric pressure crushing the hull as long as the entire hull is not put into the bag. Instead, one builds up the bag on the surface of the hull from flat pieces of the correct type plastic. That's what the guys at work do (except it's not kayaks).

So for a kayak you could take the hull without the deck and build a bag that is completely on the outside and covers the sides and bottom. The plastic is the top of the bag while the wood is the bottom. The plastic pushes the glass against the wood while the vacuum sucks the resin through the glass.The net air pressure on the wood is always equal, so there's no danger of collapse

The basic setup is a container for the mixed epoxy with a valved hose going to the inlet on the bag and a hose exiting the bag and going into the trap (that's the reservoir I was mentioning). It goes into the bottom of the reservoir while up at the top there's a third hose going to the vacuum pump. The last 2 hoses are not connected to each other. The reservoir is large enough that it will never fill up to the point where epoxy enters the hose to the vacuum pump.

Operation is: Turn on the pump, open the epoxy valve. When the part is completely wet out and epoxy starts dripping into the reservoir, turn off the epoxy valve, but keep the pump runninig.

You're absolutely right about the glass to tie the sides together and the size of the weight difference.

Laszlo