How big a boat can built with glued lap construction ?

Discussion in 'Boat Design' started by PeterSibley, Feb 10, 2011.

  1. BATAAN
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    BATAAN Senior Member

    Here are the MM scans so we can see again the lovely boat proposed from lapstrake plywood.
    A note on the story of the lapstrake CC breaking up. The sea will smash even the stoutest craft to toothpicks if you do something stupid, so this has little to do with construction details, other than have very strong ones.
    The Potato Patch, or "four fathom bank" where this occurred is a truly vicious, extensive shallows exposed to 5000 miles of fetch all the way to Japan and the breakers here on a stormy day can be truly unbelievable. The beaches downwind are littered with tiny bits of smashed up boats and ships going back to the Gold Rush with even the stem iron of a Clipper ship still on the beach in Bonita Cove.
    The dead guys we searched for that day either lost power and drifted into the wrong place or went there seeking fish.
    One is bad luck coming from poor preparation, the other is ignorance mixed with arrogance, which ought to be on many tombstones.
    A note on isolating plywood from moisture. Where I worked we used to cut out a part and fit it, then lay it flat on the bench and heat it very warm with a heat gun and squeegee hard in a mix of catalyzed WEST resin and 10-15% stove alcohol. The combo of hot wood, thinned resin made thinner when it hits the hot wood and squeegee pressure forces the sealer deep into the ply, which sucks it even harder when cooling. Let harden, flip the piece over and repeat. Go over with a sharp cabinet scraper. Re-coat with no heat and full strength resin but use the squeegee.
    This technique works very well on plywood under Polyurethane paints or epoxy sheathing. The reason for a keel made from thin laminates of Douglas Fir is that there is no large piece of timber to soak up water and the thin lams soaked in resin and coated with sheathing are more stable than any large timber when immersed. Sheathing does not stay stuck to large timbers when they go through a wet/dry cycle too many times. But small lams have much less shrink and swell so everything stays more stable.
     

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  2. PeterSibley
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    PeterSibley Junior Member

    Bataan ,

    thanks very much for remembering me ! and also for the tip on preservation .I'm still hoping to find a source of pressure treated plywood of good enough standard to use on the hull .
     
  3. BATAAN
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    BATAAN Senior Member

    Skip the PT of cheap ply and use good marine ply treated as above. You won't get poisoned so much and it'll last as long or longer and the coatings will stick better.
     
  4. PeterSibley
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    PeterSibley Junior Member

    As I said Bataan ,it's not about price ,it's about the stuff not rotting .
     
  5. Perm Stress
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    Perm Stress Senior Member

    Completely agree on this.
    Leaving obviously weak parts and structural discontinuities in boat fit the definition of stupid too... (No matter the reason -neglect, lack of experience with new build methods, etc. .)
     
  6. PAR
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    PAR Yacht Designer/Builder

    Diluting epoxy is the work of novices and folks that haven't a clue of the chemistry they are screwing with, particularly in light of all the recent testing and documentation available on the subject. God, I can't belief people still do this. It's not a linear molecule that can be cut or stretched, like polyurethanes and vinylester. We've only known about for about 20 years now.

    Simply put, the amount of penetration is meaningless in a waterproof coating system. What testing and restorers have been saying for many years is, it's the quality of the coating that counts, not how deep it's soaked in. 100% solids products (only) that can resist moisture vapor penetration (the real key) are the only methods that truly work in the long term. Dilution just reduces the solids content and dramatically affects the physical attributes of the cured matrix.

    By the way, epoxy isn't "catalyzed" folks (regardless of brand), it's activated, a huge difference and distinction, also why epoxy is so much more waterproof and has such a better set of physical properties.
     
  7. ancient kayaker
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    ancient kayaker aka Terry Haines

    Par, in that case the top coat of epoxy need be (should be?) only a film. Any waterproof coating will resist rot until compromized by substrate cracks, abrasion, age or UV and requires maintenance. The epoxy will require UV protection with paint or varnish which can be abraded so that system requires maintenance too.

    Given that consideration, does the epoxy "undercoat" do anything better than several coats of paint or varnish - other than offer more resistance to certain insects?

    For my own boats that are dry-moored I just use house paint, on the basis that some houses I have occupied seemed perfectly fine after 100+ years. Of course, that may not be the case in other parts of the World ...
     
  8. BATAAN
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    BATAAN Senior Member

    Of course. In the case of your proposed build every one of the laps inside the boat is a water and dirt pocket like any lapped boat. I've had bad experiences with pressure treated things in the past and just urge caution and exploring all ways to solve the problem of rotting ply.
    Oh no. This turned into another ramble so be warned.
    If for one possibility:
    Set up the timber backbone and 4 or 5 molds
    Line out the boat to suit and batten off one side.
    Spend a long time on the above.
    Now build a timber foundation for the ply shell.
    Make the first two strakes thicker, garboard and first broad, of something real like Huon pine or Kauri or Port Orford or Alaska cedar (NOT Douglas Fir here), scarfed, riveted, the lot.
    On this normal construction, unsheathed, now T-shaped backbone apply the thinner, non pressure treated plywood strakes of the shell, each one scarfed full length and sheathed with the thin cloth Ruell recommends on the bench, both sides, the first 'ply shell' plank or transition plank (second broad) bedded in 5200 in the lap and rabbets and riveted to the timber first broad.
    From there on up it's sheathed ply, each one epoxy saturated and sheathed after fitting and before hanging and good thick 5200 seams at the stem and stern. These planks if carefully done will not get wet internally until a rude neighbor's rusty steel grinds at it, and then is easily repaired due to the epoxy composite approach to the shell. Easier to repair than regular lapped. Here's a couple pics of a repair I did to a broken lapped boat with very thin cedar planks using stitch and glue and then band aids on the breaks. Three years on it's holding up perfectly.
    Don't try to fit the ply shell tight to an unsheathed timber stem but leave room to fill with enough rubber to allow movement. The timber parts get caulked as usual, cotton and oakum in the garboard/timber keel seam and up the stem and stern to where the ply starts topped with plain portland cement then painted with red lead and then bottom paint. This is easily tightened like any wood boat if that chunk of metal on the bottom works things loose.
    In the 'old days' cement was required by insurance companies because the surveyor could instantly see if a seam required attention. BERTIE my big yawl has had cement as underwater seam compound since 1984 and works as above.
    Better than a ply garboard down the line by a long shot.
    Thickened epoxy in the laps and buried S/S screws. I like the good stainless square drive self tapping finish screws available here in the pacific NW. These can be driven well below flush and angled in the lap and totally isolated from getting wet by the epoxy and paint until you grind the lap against rocks and expose them, which is bad. On my neglected skiff this construction has lasted 17 years so far and shows little sign of quitting or rotting as nothing holds moisture. The surface is all slick and drains and has a really bulletproof paint on a stabilized surface.
    At the sheer the ply sheer strake needs good sheathing and guards.
    The ply deck glues to the ply sheer strake with a 1/8" 5200 seam to allow for movement but fastens to a sheer 'chine' common to strip built boats. This gets the fastenings farther from the edge of the ply and makes everything work better.
    Frames could be grown crooks, bronze castings, or epoxy laminations with 1/8" lams or a combo but the lams would be best where the ply plank job is, and also the deck beams. This thin lam makes the epoxy go pretty much all the way through the pieces and things don't move if you keep the surface finish up.
    The lower part could be riveted to real wood and glued/screwed to the ply planks. Or grown or sawn or bronze lower floors that lap the ply shell and laminated frames that don't reach to the keel to keep the lower ends out of bilge water, which you shouldn't have any of we hope.
    The low spots in the interior that pocket water in the laps can be found and if epoxy/ply build are quite amenable to being filled with the right goop to fair them out as it is only needed in a small midships area if the laps drain behind the frames and there are no joggles. Other wise it's a problem at every frame.
    Just building something and soaking it all in CCA or whatever the preservative of the moment is does not make a nice place where you want to hang out with kids, family and or girls etc. It smells and the splinters give you nasty skin reactions. Working it with it is not good for your own health.
    An epoxied shell and deck painted with white Awlgrip inside and out would be clean, smooth and easy to live with, wouldn't smell after the first week and you could keep up with little time and effort.
    This could go fine on a conventional backbone but in my thinking a timber keel means timber garboards and broads and then ply from there on up.
    A little off topic but concerns epoxy and lapstrake and longevity.
    This little relic was stored outside under a shed, which collapsed on her in a snow storm, and then was left there awhile and then moved around and neglected for 10 years before I got to work on her.
    Didn't want to replace planks as the idea was a restoration instead of a repair so managed to combine epoxy and trad techniques successfully here again.
    The frames were removed in the broken area, then the individual planks were repaired in place and re-riveted to their neighbors, then the pulled-together and repaired shell had some new frames slid in under the keelson. Only had to scarf one frame repair.
    Center photo is finished product at boat show.
     

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  9. BATAAN
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    BATAAN Senior Member

    The alcohol was recommended to me by the WEST salesman.
     
  10. PAR
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    PAR Yacht Designer/Builder

    The use of epoxy generally falls into two categories, adhesive and coating. Leaving adhesive aside, epoxy coatings on wood require encapsulation to be truly effective, though a lot of people think all they need a is a few coats, painted on and this gets them a moisture vapor barrier. With other materials that aren't as affected with environment variables and the physical changes as a result, this can be true and epoxy, used as a straight coating is effective, assuming the film is thick enough.

    When I say epoxy, I mean epoxy, as most paints aren't epoxy, though do have enough of the diglycidol ether molecule to keep the advertising police off their backs. Most paints have a very poor quality epoxy and it's often very diluted.

    For a coating to be effective it must resist moisture vapor penetration, regardless of molecular base. For example shellac is better then traditional varnish, unless you introduce alcohols or some other solvents into the equation. When you go down the list of prospective coating materials, epoxy and polyurethanes stand on top. Most of the thin film polyurethanes are lower then epoxy in moisture vapor resistance, though if you increase film thickness polyurethanes start to really shine. All the other "usual suspects" such as oils, waxes, polyesters, thin polyurethanes are substantially down the scale in comparison.

    What epoxy does more then any other coating is effectively seal off the substrate so much, that environmental changes have no effect. This isn't true of say a varnish, even if you used full encapsulation techniques or the other coating "choices"

    Houses don't have the dynamic loading boats experience, nor are they immersed under a few atmospheres either. One of the additional costs absorbed by the marine paint formulations, is peel strength and percentage of elongation before yield. As a rule, house paints can cope with a clap board that moves around during the spring rains, but would suffer terribly on a traditional carvel or lapstrake that see's repeated wet/dry cycling. Marine paints do a good job of keeping the color in place (gloss retention, elongation, tensile strength, etc.), but they generally let in a lot of moisture too. Flat paints actually absorb moisture, marine grade or not! This moisture vapor transmission is what lets trouble start on wooden structures.

    Polyester (with gel coat) on first glance seems to stop enough moisture vapor to be called waterproof. In the long term and has many runabout transoms, soles and boat blisters can attest, this isn't the case. Polyester lets just enough moisture vapor through to soften cores, delaminate built up assemblies, etc. Vinylester is a little better and if thick enough, could be considered waterproof (on wood) but it's cost and application rival that of epoxy, so why chance it, unless you're a solvent based manufacture and don't want to tool up for an activated resin system like epoxy.

    Epoxy primers have the benefit of making most substrates accept any top type of top coat. This isn't the case with other types of primers.

    In the end, testing has repeatedly proven that substrate penetration doesn't mean a thing. Even a coating that just lays on top of the substrate, with minimum of peel strength, but with high enough moisture vapor resistance can do a much better job at water proofing, then a well penetrated coating that has less the 97% moisture vapor penetration potential. The bottom line is how well does the coating keep water out, not how far into a substrate it can soak in.
     
  11. BATAAN
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    BATAAN Senior Member

    Three True Nautical survival rules learned from river rafters:
    1. Don't be stupid.
    Now this starts with the boat you pick, the care and attention you give it and the humility you need to learn from your boat, because she will teach you lessons especially on those days when rule #2 below kicks in.
    It continues by not engaging rule #3 below until after safely moored, not carrying too much sail for conditions, not crossing a nasty bar on a hard ebb, rising wind and coming darkness.
    It continues further in always making sure your fuel is clean, the filters new, the crew sober and in life jackets, the VHF batteries charged and someone knows where you are and when you're expected back.
    All these things make rule #2 below less likely, but never prevent it.
    2. **** happens.
    Always, and at the worst time, but especially if you violate rule #1 above.
    3. Bring beer.
    See rule #1 above.
     
  12. BATAAN
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    BATAAN Senior Member

    We always did it as only the first coat, following ones were full strength and applied according to industry specs.
     
  13. PAR
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    PAR Yacht Designer/Builder

    West doesn't recommend dilution, though if you are not going follow their recommendation, they prefer a solvent with a similar molecular pressure, like isopropyl and also those solvents that are hydrophobic rather than hydrophilic, for obvious reasons.

    The first coat is the most important. It forms the base of the coating and if weakened by dilution, then the subsequent coats will have the same weakened "grip" on the substrate. How much weaker could it be, with just a 15% dilution? Well try 50%! If this is acceptable then go for it.

    We've long been past the days of home brewed concoctions, that out preformed the commercial offerings.
     
  14. BATAAN
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    BATAAN Senior Member

    This was in the early 90s and seemed to work at the time and the heat (very hot plywood) was the more important ingredient it was thought. As with all technology, practice reveals better methods. Thanks for the update on how it's done today.
     

  15. PAR
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    PAR Yacht Designer/Builder

    Heat is still employed and is the only recommended method to lower viscosity, unless you are self formulating and employing non-reactive modifiers to lower viscosity (like Raka epoxy does). The heated work method has been refined and now is called the hot on hot method, where both the work and epoxy are heated.
     
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