1959 27'constellation inner plywood replacment

Discussion in 'Boat Design' started by Twin Spirit, Jul 19, 2016.

  1. Twin Spirit
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    Twin Spirit Junior Member

    After reading thousands of posts about 5200, west system etc., I need to replace the inner plywood on the bottom hull and it seams everyone has their own way of doing it. If I choose not to do the 5200/epoxy etc. bottom what system should I use to bring it back to as close to original a it was constructed.
     
  2. Twin Spirit
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    Twin Spirit Junior Member

    Would love to hear what would be the best marine plywood to use and what kind of outer planking if planks need to be replaced.
     
  3. gonzo
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    gonzo Senior Member

    The planks are lauan or Filipino mahogany as they used to call it.
     
  4. Twin Spirit
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    Twin Spirit Junior Member

    Gonzo thank you for the info.
     
  5. PAR
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    PAR Yacht Designer/Builder

    Chris Craft used Douglas fir plywood, in fact custom ordered plywood (scarfed to length) stock. Unlike the MDO used on some of their other models (also custom stuff) the double and triple planked hulls were "naked" sheets (no paper).

    Replacement plywood should be BS-1088 meranti or okoume. If you know your supplier and can inspect the sheets, BS-6566 (meranti) could be used on the inner layers. In California there are lots of suppliers of real marine grade plywood. Insist on the BS stamp, not the APA 1-95 rate.

    As to the type of bottom restoration, all have good and bad things to consider. The traditional way (the way it was done originally) will be the fastest to leak and "work" it's fasteners, but retains originality. The reason 5200 and epoxy versions of this repair have come to be, is the need for a better seal between the layers.

    5200 was used first, as epoxy at the time wasn't in widespread use and no one really understood it. Ounce for ounce a 5200 bottom job costs more, than epoxy, but is slightly easier to apply. Some will tell you the flexibility of the 5200 is good for the boat, but in fact it isn't and those suggesting this, don't know much about cured 5200. Cured 3M-5200 is actually quite stiff, nothing like traditional caulking. Yes, it has a respectable modulus of elongation, but is more valued for its tenacious grip. In fact, the very last thing you want is an adhesive/sealant that offers movement between the layers. This just tests the pullout strength of every fastener (not good).

    Once epoxy gained a foothold, 5200 bottoms became the second choice, though (again) some diehards, just refuse to learn about the physical attributes of this goo and insist the 5200 bottom is better (wrongly so). Unlike a 5200 bottom, an epoxy job will marry the layers together, literally making them one single homogeneous skin of hull shell. In fact, you can skip the fasteners altogether, that's how good an epoxy job is.

    In the end, all three methods can get the job done, but anyone insisting the traditional or 5200 bottom is in anyway superior to an epoxy bottom, just hasn't been paying attention to the testing and industry advances, in the last several decades. Lastly, most of the manufactures (Chris Craft, Thompson, Owens, etc.) eventually moved to using an adhesive/sealant, but it wasn't 3M-5200 (polyurethane). All settled on a polysulfide (3M-101 is an example) which has less adhesive qualities, but much better elongation modulus. If someone was forcing me to do an adhesive/sealant bottom job (clients can be an uniformed pain in the butt sometimes), I'd use 3M-101, not 5200, though I'd try hard to convince them, that an epoxy bottom would be more bulletproof.
     
  6. Twin Spirit
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    Twin Spirit Junior Member

    PAR thank you for all the info I really appreciate it. When I use the 3M-101 should I apply it to the inside of the plywood at frame location only and then trowel it on the outside and attach the planks . Would it be a good idea to use penetrating epoxy first to seal the plywood and planks, or just the plywood.
     
  7. PAR
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    PAR Yacht Designer/Builder

    There's absolutely no need for penetrating epoxy on your boat. Penetrating epoxy isn't waterproof (not even remotely close in fact).

    You can use polysulfide at the faying points along the frames, but it's not as necessary as you might think, once the planking layers are all goo'd together. A 5200 or 101 bottom job relies on the fasteners to insure the structure remains tight. An epoxy bottom uses epoxy to insure the structure remains tight, which is why it's superior to the flexible sealant approuch.
     
  8. Twin Spirit
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    Twin Spirit Junior Member

    PAR I though a epoxy bottom on wood were not be a good idea since the wood will move over time and crack the fiberglass and allow water in between the layers. I have extensive composite background in aviation and I know how great the products today are, so if I understand you correctly I would bond the inner plywood to the frames with 3m-101 than epoxy bond the outer planks to the inner plywood and than apply fiberglass over the outer planking to seal the planks like using the West system? would I use a thixo like epoxy on the plywood to attach the outer planks.
     
  9. PAR
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    PAR Yacht Designer/Builder

    If this is the boat I think it is, a 'glass sheathing wouldn't be anything I would suggest. This is a double plank (my assumption) or a triple plank traditional build. Unlike solid timber, plywood doesn't move (okay not much) and it's relatively stable, which is why it's loved so much. I'm assuming the inner skin is plywood, with a solid wood external skin (planks).

    If it was me, I'd consider epoxy bonding the inner layer of plywood to the frame faying surfaces. This locks the structure down (frames, floors, stringers, etc.) helping other fasteners from getting yanked on (because of the stable plywood, bonded to them). I'd epoxy the next layer down as well, probably using staples or brads to hold this layer down, until the goo cured. If this was the outer layer and plywood, yes, I'd sheath this, for abrasion protection. If this was also an inner layer, no sheath, just more goo and the last layer of planking. Again, if the outer layer was plywood, I'd sheath it, but if it was solid wood, I wouldn't, unless it was a decorative external layer (fore and aft planking). If this is the case, I'd encapsulate the outer planking and consider a sheathing (abrasion protection). If the outer layer is more substantial (thicker than 1/4" - 3/8"), I wouldn't sheath it, unless a full encapsulation process could be established.

    This is the crux of the decision. Will the planking be encapsulated or not. If not, a 5200 or 101 bottom has some merit. If it is, there's no sense in the 5200/101 approuch, when epoxy will do the job better. Lastly, will this be a varnished baby or are you going to paint? If it's painted, I'd encapsulate and sheath the crap out of it (abrasion protection), knowing that under filler, primer and paint no one's going to know my sins. If this is going to be a bright mahogany beauty, epoxy is an option, but it's easier to do the 5200/101 bottom, knowing the planks are going to move around a bit.

    Just to toss more crap into the decision making process, you can get epoxy formulated to be considerably more flexible, than the typically more rigid/brittle versions. I did this on a traditional batten seam build recently. I knew the planks would move around and I could encapsulate them, but I couldn't fully encapsulate the battens, frames, etc., so I used an epoxy on their faying surfaces that offered some movement, without it breaking out. The hull is much more rigid than previously, but the structure can move a bit with moisture content changes.

    I've done a lot of restorations on traditional builds and the most important thing is, to understand what the approuch will be. You can't switch horses in midstream, so some serious answers to tough questions, need to be addressed up front, so you can make a reasonable plan of attack.
     
  10. Twin Spirit
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    Twin Spirit Junior Member

    PAR, yes this boat is a double planked hull with 1 layer of plywood inside and 1 layer of individual planking about 3/4" thick running fore and aft. Is it possible to epoxy bond the plywood to the frames and then epoxy the planking to the plywood then fill the planking seams with a flexible caulking, and then just paint the bottom since this will be no way a show boat just a family fun cruiser?
     
  11. gonzo
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    gonzo Senior Member

    If the planks are sealed with epoxy, they can be also bonded with epoxy. There will be no need to caulk seams, since they will be a single panel. Using epoxy it is faster and cheaper to do two layers of plywood.
     
  12. Twin Spirit
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    Twin Spirit Junior Member

    Gonzo I will need to fill the gaps between the planks since they have dried out. Would the outer 3/4" planks swell even when they are bonded with epoxy to the plywood and painted? Or will I run into problems if they do swell and break off the plywood.
     
  13. PAR
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    PAR Yacht Designer/Builder

    I wouldn't epoxy 3/4" solid wood planking to the plywood, unless you're willing to go the full encapsulation route on the solid wood planks. This isn't all that necessary. Glue the encapsulated plywood down to the repaired frames and bed the solid planking to the plywood with an adhesive/sealant approuch. You will caulk the outer planking and it'll look like a traditional double plank job.
     
  14. Twin Spirit
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    Twin Spirit Junior Member

    PAR so it is wise to use 3M-101 to bond planks to plywood and is it ok to use the 3M-101 to caulk the plank seams before painting the bottom?
     

  15. Twin Spirit
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    Twin Spirit Junior Member

    PAR just checking on you meaning of encapsulated, do you mean to just apply epoxy to all of the surfaces of the plywood or to attach the plywood to the frames with epoxy and then after plywood has cured to apply fiberglass over the plywood let it cure than attach planks wet with 3M-101 and screws?
     
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