Vacuum Bagging On Plywood and Balsa Permeation?

The OP mentioned temporary bulkheads, thus I am sure if one could avoid bonding the foam to the bulkheads that would be best... If the foam was stapled to the bulkheads through the bag material, I dont know how badly it would leak ? Would probably limit the max vacuum. Would be near to impossible to seal those spots. I guess one could run a bead of sealer on each side of the bulkhead to the bag to try to limit air intrusion...

 

The LOA is 25ft and change. There will be man hole openings in the platform under the boat for access. The underside is not under vacuum. The only part of the platform that needs to be air tight is the 4 inches or so from the gunwale core/lamination to the bagging tape. I will seal the boat down to the platform.. one of my questions was what I should use to make that seal..

 

Best advice is an epoxy/fumed silica seal, but it will need to be recip sawed out, so you'll need to make an extension on the hull top edge for destruction as others mentioned.

And, a couple lifts of epoxy over the substrate will ensure the thing won't leak.

 

Sorry, but I think this is what will happen with your plan.....

 

You can not avoid the stringers, the foam is not rigid enough and will bend under vacuum. Stringers add little time, they are simple battens screwed to the mold with some distance between them. They lay naturally, no need to force them where they don't want to go, or glue them to each other. Then you lay a sheet of plastic over them, or tape them up and you are ready to go. If you can lay the foam in half a day, it takes even less to stringer the mold.
You can lay the foam however you like, the ideea is to minimize the needed seams. The less you have the better. Unlike wood, foam does not care wich way it is oriented for bending or strenght. Fixing the foam to the stringers can be done in multiple ways. What works really depends on the foam, the bend, and the amount of thermoforming you do. You can even staple the foam and seal the hole with thickened epoxy, after the lamination is done you simply rip the stringers out, the staple (or small headed screw) will rip trough it. It just means you have bigger holes to fill before laminating the other side. Sometimes sewing the foam on also works, you need to experiment, and keep in mind it's not mandatory to use the same method everywhere.
What is crucial in an open mold is sealing every single seam and hole in the foam. For example in the film you linked they grooved the foam seams after install and filled them with thickened epoxy. You can read about it on the dedicated site, www.fram.nl it's explained with pictures.
You don't need to seal the bag to a 90° flange, one inline with the planking works just fine and is simpler to install. In theory you can seal it to the foam with butyl tape, but I can no guarantee it works. Since you will need to trim the edge anyway there is no reason not to epoxy it on. Lightweight filler is enough for this. Just think of the 3-4" wide ply or plastic piece as another strip on the mold.

 

If he applies vacuum to the exterior skin only, it won't behave in that fashion; he wasn't explicitly clear on that at the beginning I thought

But, Rumars and I agree the foam is not very rigid along the keel line and this is why stringers are best

 

I'm curious how you accomplish that if there is a wet hand layup all over the surface you are trying to seal to ? I can see how one does that in a mold where the entire surface of the mold has been made impermeable and there is a flange surface provided that one can seal the sheet of the vacuum bag onto. But when the surface you have to work with is the part with the wet layup, then I guess I am confused. Especially in front at the bow. I have never seen a flat deck before, so maybe I am just confused ???

 

An easy and inexpensive way of vacuum bagging is to use a vacuum cleaner to evacuate most of the air first. The high vacuum pump then does not need to also be a high volume pump, which lowers the cost.

 

I've never achieved much of a vacuum on wood-it's porosity doesn't help and believing that each and every joint will be vacuum proof is a bit optimistic.With large holes in the base and a bag applied in each compartment-on the inside-it may just about be possible to achieve some level of vacuum.It won't be particularly easy and you still have to overcome leakage through any joints in the base.Is it a project that a first timer should attempt on such an important component?At the very least a dry run with a few vacuum gauges scattered around the surface would be cheap insurance.

 

It is possible to bag the mold/stations, then lay the foam and bag the outside. However, every hole made by a fastener needs to be sealed. I have done vacuum bagging mostly for teak decks, which is a lot more forgiving than a laminate. Even one missed screw hole is enough to reduce the vacuum considerably.

 

you have to seal the bag at the flange on the floor

once you have completed the wetout; you'd need to wipe the floor perfectly clean

I bagged the bottom of a Boston Whaler 13 and glassed it under vac by butyl taping on the rubrail. You still need a stack, I can't recall at the moment if I used peelply(most likely did), but for sure I had bleeder and breather. The breather offers protection for the bag somewhat. I had to tape my breather together at the seams for the job; his will be same, but larger. And this is all prepped on tubes for fast rolloffs. And here, he needs to strongly consider a tropical 2 hour epoxy as 60 minutes will vanish fast.

The main thing is you cannot have a single drip on the butyl and you cannot allow bag edges to get wet when pulling it over the part. It is not a one person job. Any epoxy on the butyl from a drip or bag wet spot will leak and takes minutes to fix.

Infusion would be far better, but not for amateurs.

 

I did half the hull at a time. This helps and he pught to consider same. The entire bag is readied on one side and laid over the hull, cuts way down on time. You cannot have one single strand of fiberglass under the butyl. I actually did the strakes separately and cut voids in the main layer; so some creativity might be needed at skeg or keel. This was 60 minute epoxy, but the scale of this job is much smaller and I was probably still stressed.

 

That is a much smaller boat,with a great deal less surface area to be covered.It also has a non porous substrate below the layer being applied and doesn't look like it has a seal onto a flat surface.So some of the information may be applicable to the OP's project.I repeat the point from my previous post:try to get a vacuum bag over the job and showing a healthy vacuum before you mix any resin.Yes you may have to throw away a chunk of bagging material,but the breather will be OK for re-use and it will save a good deal of money compared to throwing away that much glass and epoxy as well.During the process you will get to locate and eliminate leaks in stages instead of panic stations when the epoxy is beginning to harden.

 

Unless he uses closely space battens between the stations the hull will end up looking like a raisin. Foam is very soft and won't hold the shape well.

 

Hi, Thanks for the input. I have not said, nor decided, on wet lay-up or infusion at this point. One of my biggest concerns is a leak in the bag and infusion makes that a lot easier to deal with since you don't have a time limit... of course infusion has it's own issues and can ruin the whole boat. I am building a 1/4 scale model to test stability. I wasn't planing to vacuum bag that, but I may do an infusion test to see how my resin distribution idea works. assuming that resin flow out scales linearly.
If infusing, I will take Rumars advice and extend the core a few more inches and just use the core as the flange, which eliminates the boat to platform seal issue (though this does mean attaching a header to vertical core). I think it would be hard to keep that vertical area free of runs, etc. if doing a wet layup. So in the case of a wet layup I would likely still use the platform or other horizontal surface as a flange.
I will also run a wood stringer under each of the core planks to ensure they hold their shape. Other than the weight of the laminate, and bagging stuff, the actual vacuum suction should not affect the loads on the core. So I can pile cloth/weights on the core ahead of time to test deformation under the weight of the layup materials.
I'm not too concerned with filling temporary screw holes for attaching the core to the frame. I've done a lot of drywall, so well practiced in covering screws holes and seems with putty. That said, hot-melt glue seems like a good suggestion to attach the core to frames without holes, assuming the core itself doesn't melt.. easy to test. IMO any internal bag is made moot due to frame attachment holes.
Note: I have done a lot of work with glass and epoxy (mostly repair work) including vacuuming some small areas. But this is my first large project with a foam core.