Vacuum Bagging On Plywood and Balsa Permeation?

I intend to build a strip-plank core boat over temporary male bulkheads, which will sit on a plywood platform base. The foam core strips will be edge glued with thickened epoxy to make an airtight shell. But I want to use the platform as the flange when vacuum bagging and need to seal the core at the gunwale to the plywood with something easily removed. See illustration, note that is not my actual boat shape or number of frames. just an illustration to explain my concerns.
1) What is a good, best practice, material to make a removable seal between the core and plywood base?
2) I assume I need to paint the plywood to get a good seal (I intend to use butyl tape to seal bag edges), but is some wood grain texture okay or do I need it pretty smooth? eg how many coats of latex paint?
3) I want to use 1/2" end-grain balsa in high load areas. Is the balsa porous enough that air will be sucked through it? I don't want air bubbles in my lamination. Should I seal it before laminating?
4) What should be used to 'tack' the foam to the frames that won't prevent removal of the temporary frames? Or do you remove these fasteners (eg fender washers and screws) before laminating & bagging?

 

I would guess it will depend on your vacuum system. While a few coats of paint would seal it (personally I'd epoxy seal it) that is not going to be your main issue. That base, and the core strips for that matter, are going to be subject to very large loads. Just think about it, you have made the entire inside of the mould a vacuum chamber. So at 5 in hg, that's ~ 2.45 psi/352 psf, so 11,289 lbs load on a 4x8 sheet. Without a lot of closely spaced frames to support the base and the core you could easily collapse the mould.

 

thanks for the reply. there will be man-hole access through the platform to the inside of the boat mold. so the inside of the core will be at atmospheric pressure, so no collapse issue.

 

So you're bagging the core also? Or just expecting it not to leak?

 

No, I'm not putting a bag on the inside. I'm expecting the core not to leak, hence the question about balsa core. This method has been used per the design of Ian Farrier. like the following video showing it.. vacuum bagging at 7min :

 

I am not real familiar with bagging the whole boat, but I can see some potential for troubles.

And, you probably want to test..

The only thing I can imagine working is butyl and plenty of it. The plywood base must be lifted with epoxy at least two lifts. Sand to 400 grit. All seams in the ply must be sealed with thickened epoxy, cured and sanded flat..

When you remove the boat; the big concern is loss of shape of the hull. I am not sure you have that planned, but foam is not likely going to support itself well on its length... you may need something to keep the keel straight, etc.

I would use hot glue to spot bond here n there the frames to the core, then when ready, heat a 4" trowel and break the bonds, but heat the trowel and not the foam! The foam will puff and distort easily. You can use screws and then crawl under and spot bond.

You will probably need to test to see if it holds vacuum, because it might not. Finding the leaks will be a task. There will be some; most likely. I'd lift the boat up so you can get under and listen for leaks. You can temp close the leaks back side with butyl which might help you find them all without too many iterations.

 

Thanks for the reply. The hot glue is a good idea especially since it can stay in place during lamination and bagging. Regarding the concern of core flex... I can increase framing to every 2.5 ft but perhaps that's not enough..? don't really want to do any more than that though... As you can see in the above video, the thickened epoxy that is used to glue the edges of the strips together provides a lot of rigidity to the core. In the video, the core takes a hard bend and he uses screws and wood slats to hold it down temporarily, but after he epoxies them together ( 3 or 4 courses at a time) he removes the screws and the core stays bent in place. Then he bags it from one side.

 

I think you will have to put one side of the vacuum bag over the bulkheads, then lay your foam strips over the inner layer and you have no need to make it air tight. Then you lay up your outer laminate and vacuum bag that, at which time you would simply seal between inner and outer layer of the bag. This way there will be no atmospheric pressure on the foam panels between the bulkheads, which would otherwise cave them in..literally.

 

I'll suggest you rethink your entire approach.
1. Don't strip longitudinally over frames, strip transversly over stringers. This enables you to use wide short foam pieces, depending on the hull shape up to full foam sheets. The closley spaced stringers will support the foam to hold shape under vacuum. Depending on shape you might need to thermoform the foam.
2. If possible, screw the foam to the stringers from behind, that way you don't have to fill the screwholes. This means either beeing able to get into the male mold, or using a female mold and fiberglassing the inside first. Only works with two people, one pressing the foam down, one screwing from the other side.
3. Epoxy the flange to the foam and cut it away later. You will have to dress the edges anyway.
4. There is infusion grade balsa, precoated by the manufacturer. You can also use a higher density foam instead.

 

well, I would still be concerned about the thing getting floppy some; might want to remove one frame at a time and replace with bulkheads

 

This is not a horrible idea; except he can't keep the frames glued if the bag is wrapped around. It would need to be done post gluing, because epoxy glues make razor sharp points that will damage the bag.

 

I have built two 40ft boats using similar technique.

Follow Rumors advise

Add a rabit around the perimeter for deck joining.
Seal the core with a coat of epoxy before laminating.
Roll the whole kit-n-cabootle over before removing plywood.
Use a flush cut saw to separate the flange. Then dress the edge with a belt sander. Add a bit of extra sides height to compensate for separation loss.

Edit.
We did cover the semi-plug with plastic film to prevent any out flow of epoxy from gluing the core to the plug. Too many screw punctures to be useful as a vacuum bag.

 

I think the main problem is to keep the whole thing from distorting.

 

Thanks Rumars. A few follow ups...
When you say screw from behind, will the foam (likely 1/2in thk, 4lb divinycell) actually hold a screw? I would think it would just pull out.
The reason why I think longitudinal strips will work is that I'm only running qty 5 per side of the boat, which will be fairly wide, and there's a bit of a chine between each edge. (Quick and ugly build method) The chine should add a bit of rigidity to the core, like creasing a piece of paper. I think I can plank the whole core in 1 day this way and avoid a lot of time with a bunch of stringers.
What advantage does epoxying the flange (plywood base) have over using a cheaper caulk or even bondo? ... ok 'pre-coat balsa' 10-4 will do! thanks

 

No mention of the overall size of the project?First thought is that you may struggle to get a leakproof base.If you manage that,the comment about distortion is very accurate as it may well distort to total collapse and destruction.You may be able to reduce the risk if you have a vacuum system with adjustable limit switches.Its also going to take a high capacity vacuum pump to remove that amount of air.You may be able to scavenge one from a scrapped CNC router and then may need to add some valves in places.