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Thread: Vacuum bagging final layer of thin sheathing

  1. #1
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    Default Vacuum bagging final layer of thin sheathing

    I am considering building a cold molded hull. Thin strips will be stapled together over rolled on thickened epoxy to the previous layer. When the final layer is applied, would it be advisable to vacuum bag the entire hull to ensure consistent clamp pressure throughout? I plan to use 105, 206 (slow hardener) West System epoxy. In lieu of traditional plank on frame, the thought of not having to bevel planks, caulk the seams and deal with a leaky boat each spring while she swells up after being stored all winter seemed to be appealing.

    The boat is 20ft with a 6ft beam with a 3/4" hull. The plan is to use (4) 3/16" layers of sheathing. The first running fore and aft, the next two running at 45 degrees to each other and the final layer running fore and aft. The final layer, an exact copy of the first layer, will be spilled similar to plank on frame, so they could easily be numbered and set aside while the epoxy is rolled on and then laid up, stapled down and then vacuum bagged. There would be about 10 strips to lay up on each side.
    The build has 1" square oak frames and with the first layer stapled and glued to the frames, the completed hull will be rivet and roved to the frames to achieve a traditional appearance. A layer of glass will complete the hull, then sanded faired, primed and painted.

    Sound like a plan? Shoot holes through it or praise it, either way feedback is welcomed.

  2. #2
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    Default Re: Vacuum bagging final layer of thin sheathing

    Quote Originally Posted by glmeadows View Post
    I am considering building a cold molded hull. Thin strips will be stapled together over rolled on thickened epoxy to the previous layer. When the final layer is applied, would it be advisable to vacuum bag the entire hull to ensure consistent clamp pressure throughout? I plan to use 105, 206 (slow hardener) West System epoxy. In lieu of traditional plank on frame, the thought of not having to bevel planks, caulk the seams and deal with a leaky boat each spring while she swells up after being stored all winter seemed to be appealing.

    The boat is 20ft with a 6ft beam with a 3/4" hull. The plan is to use (4) 3/16" layers of sheathing. The first running fore and aft, the next two running at 45 degrees to each other and the final layer running fore and aft. The final layer, an exact copy of the first layer, will be spilled similar to plank on frame, so they could easily be numbered and set aside while the epoxy is rolled on and then laid up, stapled down and then vacuum bagged. There would be about 10 strips to lay up on each side.
    The build has 1" square oak frames and with the first layer stapled and glued to the frames, the completed hull will be rivet and roved to the frames to achieve a traditional appearance. A layer of glass will complete the hull, then sanded faired, primed and painted.

    Sound like a plan? Shoot holes through it or praise it, either way feedback is welcomed.
    I'm responding as somebody who replaced the entire keel and hull (up to the water line) of a 47' glass boat with encapsulated lead ballast using about 49 giant layups of 1808 and 2415 and a vacuum bag system.

    I don't know if the pressure of bagging would squeeze the veneers any closer to the hull than they already would be with stapling. Obviously, the best you're gonna get is about 15lbs/square foot (atmospheric pressure). Since you're not bagging biaxial cloth, there's not much air to be sucked out of the layup, either, which is part of the beauty of bagging. You would have to work like a demon and use REALLY slow hardener to get all those planks in place, then the bleeder cloth to soak up excess resin (you wouldn't use peel ply because that's for glass laminates, and if you use thickened resin, the bleeder cloth probably wouldn't soak that up anyway). So you might decide to go with straight plastic. But then you'll have epoxy patches along all the seams that will make fairing a lot more burdensome. (I bagged a J30 with a 14' hole in the side once and only used only plastic to vacuum the balsa core into place). Setting up the bag is a fairly clumsy process, especially one to cover the size of an entire side of a boat. You have to work very clean so no epoxy goes beyond the laminate because you have to surround the surface to be bagged with butyl tape, then position the other stuff if you're using it (bleeder cloth, peel ply), and then get the bag on without getting epoxy on it to make sure it sticks to the tap, and of course, the bag has to be lying out flat with the hose taped to it in some clean place.

    so my main concerns, to sum up, would be: is there any advantage? and Two, how much time is the epoxy going to give you? Finally--have you got a HUGE compressor (for the venturi type of vacuum pump) or else a real electric vacuum pump and all the fixins?

    Obviously there are people who have cold molding down, and you should definitely consult with them. This is just my take on it.

  3. #3
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    Default Re: Vacuum bagging final layer of thin sheathing

    I am a fan of diagonal cold molding, and of vacuum-bagging, so you got my vote. I do wonder, however, about the riveting phase - how do you intend to put the 'glass over the rivets on the exterior surface without voids/blisters? (Personally, I would skip the rivet step.) Have help applying the exterior 'planks', and do a couple of dry runs to practice before committing to epoxy - speed in application is paramount, and practice makes perfect, especially with help unfamiliar with the process. Finally, what kind of boat? The skin thickness seems to be rather heavy for a 20-ft boat, unless it is a work boat or high-speed planning hull.
    Hope for the best, but plan for the worst.

  4. #4
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    Default Re: Vacuum bagging final layer of thin sheathing

    At least to me, the idea of drilling a lot of holes in a cold molded hull for rivets (which should be totally unnecessary structurally, with each hole a potential rot pocket on a cold molded hull) seems like a very bad idea. The glass may help resist abrasion a little bit if you're beaching, but not much unless you want to build up some serious, multi-layered grunch patches in specific areas (and if you do, there are better fibers to do it with) so glassing maybe not add enough to bother with. The idea of glassing over copper rivet heads also sounds like asking for trouble if you ask me. The typical methods of cold molded construction (the mold method, the stringer frame method, etc.) have been figured out and proven for better than forty years and there doesn't seem to be a lot of reason to reinvent the wheel, especially when making rather dramatic compromises which are basically nothing more than cosmetic add-ons.

  5. #5
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    Default Re: Vacuum bagging final layer of thin sheathing

    Maybe practice the bagging on the inner layers where looks don't matter as you go to determine whether you want to do the outer layer. That would help you acquire the proficiency you'll need in order to do a good looking job too.

    Assuming that you'd be applying the outer layer fore and aft for the traditional look with a bright finish I question the value of glass above the waterline. You'll need to maintain a lot of varnish in order to protect the epoxy from UV light. That won't be quite so critical with varnish over bare wood.

    If you do glass over fasteners you'll want to countersink the heads just a little and fill with thickened epoxy or very thin wood plugs first.

    Avoiding holes all the way through the oak framing will be avoiding potential rot spots.

  6. #6
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    Default Re: Vacuum bagging final layer of thin sheathing

    Thanks for the feedback.

    I reckon I could lay up the final layer by rolling on the glue for each 2 or 3 planks at a time to keep me from racing. Also, the rivets were considered to provide a means to mechanically fasten the hull to the frames in lieu of just relying on the glue alone between the inner most plank and the frame. I would be using copper nails and roves with the head countersunk a bit to later be covered with thickened epoxy. The fastening would be done when the hull is complete with two people. The boat would be upside down with all the molds removed and would be resting on its sheer clamps. One person inside to drill the hole, place the rove on the nail and push it onto the nail and nip it off while the person outside would countersink the hole, put the nail in the hole and hold a backing iron while the person inside peens the rivet. Rivet and rove is preferred as 1" square frames are a bit small for screws. Also, using this build approach, you'd never know where the frames were from outside. The staples are monel applied with an air staple gun and compressor. The last layer would have the staples removed with a tape strip after the glue is has hardened. I think a final layer of glass and epoxy would be a good idea. I'd put down my butyl tape around the perimeter, have the plastic film all ready to go and roll on the epoxy, then the film and suck the air. Yes, no???

    The boat is a Dark Harbor 12 with a 20ft LOA. There would be a layered transom to hide the ends of the strips. This whole process is a bit of a hybrid I guess. I've seen where folks have edge glued full thickness planks only to have the seams buckle when she swells and then planks that crack when she drys and it don't seem to matter how dry the planks are when you put them on.
    Last edited by glmeadows; 02-28-2019 at 12:40 PM.

  7. #7
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    Default Re: Vacuum bagging final layer of thin sheathing

    The advantage of the vacuum bagging will be the clamping force, not the surface quality. Pockets of epoxy, glue lines and wrinkles left by the bag will still need considerable fairing. One challenge will be getting a good seal without leaks from the perimeter of the bag or through the inner layers. Sheathing a half hull at a time is doable with help. Pre measure your epoxy and mix/apply pint at a time. I'd suggest setting up a dry run with the bag and pump first.

  8. #8
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    Default Re: Vacuum bagging final layer of thin sheathing

    I see now that you're going to paint. I missed that the first time around. That being the case I would just use more staples, leave them in (especially if they are monel), and forget about bagging. That will eliminate a lot of stress and some expense.

    You'd do well to search for staples the correct length for the job before committing to any strategy. I found that there are plenty of short staples available for manual staplers but not much for use in pneumatics.

    You're going to find that until you've built up enough thickness to allow the use of longer staples it will be a challenge to get the 3/16 veneers to lay flat, especially on the inner layer. Starting with 3/8 bead and cove strips for the first layer will allow you to use longer staples for subsequent layers, and even then staples that aren't so long that they protrude on the inside may not be able to hold 3/16 down tight. Bagging each layer would help with that, or using thinner veneers.

    By the time you've laid up 3/4 inch I think you'll find that most of the frames are redundant and the planking being just glued to them will be more than sufficient, but if you want you can just leave whatever fasteners are already in there in place.

  9. #9
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    Default Re: Vacuum bagging final layer of thin sheathing

    Hi,

    I'm building a 16' cold molded hull in my basement. I am vacuuming bagging the strips as I go. I'm about an hour from you. Feel free to stop by and I will show you the process - and you can even help!

    Regards,

    Dean
    Bolton, MA

  10. #10
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    Default Re: Vacuum bagging final layer of thin sheathing

    And you will still be asking for bilge water leakage and moisture penetration into and around every rivet inside of the boat. You are trying to mix two very different technologies, which can each work well in their proper context, but which don't go together well. Not to mention that trying to clip and peen rivets above your head while working inside an upside down hull would be a really horrible experience. Who decided that 1" square frames were actually large enough to bother with and would benefit the project, and/or were adequate if the hull skin does need that type of extra reinforcement?

    Same basic deal with the fiberglass. I really doubt that you will be beaching a Dark Harbor (at least not intentionally, and if you do, abrasion resistance is going to be well down the list of fix it stuff that you'll need to worry about). A thin layer of fiberglass on the outside of a 3/4" thick cold molded hull adds virtually no strength. If you hit something hard, it's not going to save your bacon and if it's just instances like occasionally being up against trailer bunks, rubbing against a dock or similar, the epoxy coating itself will be what takes the hit.

  11. #11
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    Default Re: Vacuum bagging final layer of thin sheathing

    To the best of my knowledge, commercially available veneers are 1/8" thick. I used 3/16" veneers on our cold molded boat and had to produce them in my shop. A 3/16" strip in WRC is quite stiff and you have some serious curves to negotiate with the Dark Harbor 12.

  12. #12
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    Default Re: Vacuum bagging final layer of thin sheathing

    If you want to retain the ribs and rivets then have them actually do something and spare yourself a lot of work. Double plank longitudinally with 3/8". First layer riveted to the frames, second layer epoxy glued to the first. No staples or vacuum needed, just a lot of clamps. Build right side up, you can rivet without help.

  13. #13
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    Default Re: Vacuum bagging final layer of thin sheathing

    A few random thoughts:
    The scantlings seem heavy for such a dainty boat as a Dark Harbor 12. The Luders 16, a similar if slightly larger boat had hot molded hulls of remarkably light scantlings.

    I'd talk with Alec Brainerd at Artisan Boatworks. He builds Dark Harbor 12's in plank-on-frame, tight seamed. IIRC, there was a WB article in the last year or so.

    Particularly for the first two layers, getting a vacuum-tight bag will be a real challenge. I'd think of plastic staples (Raptor) as an alternative.
    Many years ago, There was much discussion here about Art Read's construction of a lovely Dark Harbor 12. I fear that all the photos have drained into the great bit bucket.

  14. #14
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    Default Re: Vacuum bagging final layer of thin sheathing

    Quote Originally Posted by JimConlin View Post

    Particularly for the first two layers, getting a vacuum-tight bag will be a real challenge. I'd think of plastic staples (Raptor) as an alternative.
    Many years ago, There was much discussion here about Art Read's construction of a lovely Dark Harbor 12. I fear that all the photos have drained into the great bit bucket.
    Vacuum bagging is pretty straight forward. I typically get 20-23 inches of Hg with my bags. Sometimes as low as 18". I use Raptor staples exclusively to hold the strips in place until the bagging is complete. The 1/4" staples don't have enough holding power to keep the 1/8" veneers tight to the 1/4" bead and cover underlayment if there is much curvature. (I can't use longer staples for fear of penetrating through the bead and cove into the cockpit.) The veneers for the tightest bends have to be steamed follow the hull lines. My boat was not designed to be cold molded so some aspects have been more challenging than expected.

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    Default Re: Vacuum bagging final layer of thin sheathing

    Quote Originally Posted by Rumars View Post
    If you want to retain the ribs and rivets then have them actually do something and spare yourself a lot of work. Double plank longitudinally with 3/8". First layer riveted to the frames, second layer epoxy glued to the first. No staples or vacuum needed, just a lot of clamps. Build right side up, you can rivet without help.
    Now that's a great idea worthy of some thought. THANKS

  16. #16
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    Default Re: Vacuum bagging final layer of thin sheathing

    I did not invent this, it's old news. Paul Gartside uses it on his boats. His website has photos. http://www.gartsideboats.com/custom-...gn-ila-98.html and http://www.gartsideboats.com/custom-...design-95.html
    He builds upside down and uses a thinner inner course and a thicker outer layer since he fastens with screws from the outside and needs the thickness for bungs. That's how it was done traditionally, without epoxy. How you do it is up to you. I think the fasteners should be on the inner planks to avoid bungs, you don't need the potential leaks. The planking is glued to the frames and rabets anyway, it's not like you can take out the fasteners and replace a plank. Of course without screws or rivets from the outside you need to double up on the clamps when glueing.
    The method is a lot faster than cold molding and simpler to use when converting smaller boats from carvel since it retains all framing and scantlings.

  17. #17
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    Default Re: Vacuum bagging final layer of thin sheathing

    Ok folks. Thanks for all the feedback. For better or worse, I'm gonna use first and last layer to be fore-and-aft and the two interior layers to be diagonal. I will not be using any mechanical fasteners but simply rely on the glue between the innermost Plank and frames.

  18. #18
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    Default Re: Vacuum bagging final layer of thin sheathing

    I think a test of staple holding power into 3/16 stock is in order.

  19. #19
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    Default Re: Vacuum bagging final layer of thin sheathing

    If your frames are on close enough centers you may be able to get away with always stapling thru to the frames, especially if the veneers are quite narrow.

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    Default Re: Vacuum bagging final layer of thin sheathing

    Do the NAs know what the primary stress direction is in the inner and outer layer of a hull?
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

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    Default Re: Vacuum bagging final layer of thin sheathing

    All that work to make your boat stable and leak proof?
    I bet I could build two stripped planked boats by the time you get the first lam on.

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    Default Re: Vacuum bagging final layer of thin sheathing

    Quote Originally Posted by wizbang 13 View Post
    All that work to make your boat stable and leak proof?
    I bet I could build two stripped planked boats by the time you get the first lam on.
    You're slowing down Bruce, wasn't it more like 4 to one when you were younger?
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

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