How does one vacuum-bag a hull?

Okay, so reading that subject line out loud makes it sound kind of silly. But I'm serious as a heart attack. I've seen pictures and videos of guys vacuum-bagging an entire hull section (say, the starboard bottom, or the port hull side). How is that done? I mean, understand the concept of vacuum-bagging, but how do you completely encapsulate and entire hull section in the vacuum bag?

Do they lay plastic on the mold frames before laying up the wood? (I'm considering a cold-molded wood hull, by the way. Likely double-diagonal planking with a top layer of fore-n-aft.) As a completely unexperienced vacuum-bagger wannabe, I would guess that's the routine: Put vacuum bag plastic on mold frames, then lay up first layer of planks, then roll on epoxy, then lay up second layer (partial layer?) of planks, then put on the outer vacuum bag half, then vacuum until cured. Remove outer half of bag, continue planking, lather, rinse, repeat. Use vacuum bag to install fiberglass after final planking (fore-n-aft) layer is on.

But that's all just a wild guess. Somebody enlighten me, please.

 

hi adt2,
There are a lot of different ways to use vacuum bagging. You can bag stuff down to a concrete floor, for example by sealing the edges of the bag to the floor with polyurethane caulk or with "dum dum" or something. You can bag onto a hull the same way, but the way you are talking about doing it with strip planking would be near impossible and a lot of work.

You may be interested in this description of a building method that uses v bagging.

http://www.multihulldesigns.com/pdf/cm/CYLINDER MOLD MULTIHULL CONSTRUCTION.htm

 

I think the "bagging stuff down to a concrete floor" routine is for laminating large panels that are then installed elsewhere, no? What I'm talking about is actually using the vacuum bag technique to laminate the layers of the hull together as it's being laid up. Something like this.

 

he is bagging the laminate to the corecell foam core in that video. The corecell is held onto the timber frames and battens via screws from the inside. He has layed the corecell in strips, then bogged the joins in the foam to seal them up before commencing the layup. The vac bag is sealed onto the prepped edges of the foam with tacky tape before starting also. Once this laminate is cured, the screws are pulled out from behind and he flips the whole thing over and can do the same to the other side.

You can bag onto anything, provided its not overly porous or you wont be able to get a decent vacuum.

 

Ah, I see. So what's on the other side of the foam - the side we can't see? Anything? Or is the foam itself airtight enough that he just tapes the top part of the bag directly to it?

 

http://www.boatdesign.net/forums/fiberglass-composite-boat-building/infusion-plan-43380.html

http://smalltridesign.com/Trimaran-Articles/Construction-Methods/Cold-Molded-Construction.html

http://www.nexusmarine.com/odysy_const.html