Building a 21' classic runabout

Discussion in 'Wooden Boat Building and Restoration' started by TBH736, Apr 20, 2016.

  1. TBH736
    Joined: Apr 2016
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    TBH736 Junior Member

    The project: Nelson Zimmer's 21' Palm Beach
    Boat building experience: none, this is my first build :)
    What most say about my endeavor: Your building a what!? :p

    Hello all, this is my first post even though i have been working on the boat for almost a year now. I had wood shop in high school and have stuck with it as a hobby whenever my real job allows me to have a shop set up. I wanted a big project and i think this is about as big as they come for one person.

    If all goes well, i will be bolting on the chines and keel in the next few weeks and as i get closer to planking the bottom i find myself with a few questions. My original plan was to build the bottom like the boats from Hacker Craft, 2 layers of 1/4" mahogany on opposite diagonals and an outer layer of 1/2" mahogany planks all with West System. Now i am thinking i want to use 3M 5200 for the 1/2" planks. looking for any insight on mixing West System layers with a 5200 final layer.

    I have seen bottoms with 1/4" plywood and 1/2" planks on top of that with 5200. My thought is that my 2 layers of 1/4" in West System will act similar to the plywood and the bottom would be good.

    Thanks for any help!
     

    Attached Files:

  2. tom28571
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    tom28571 Senior Member

    I admire your ambition and your goal. I've seen some bottom boards glued and caulked with 5200 but never plywood bottoms sandwiched with it. Several similar materials might work fine but why go away from the general practice that has gained acceptance all round the boatbuilding industry? Epoxy glued plywood lamination works and is well proven. I don't see any limber holes in the bottom frames.
     
  3. gonzo
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    gonzo Senior Member

    Also, 5200 will cost several times more than epoxy. Further, considering your boatbuilding experience being none, modifying a design is a really bad idea. Build it the way the plans indicate.
     
  4. TBH736
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    TBH736 Junior Member

    Tom, thanks for the reply

    Limber holes are coming, wanted to do some ruff fairing on the frames before cutting them in.

    A few years back i had the opportunity to tour the Hacker Craft factory and talk with some of the builders there. They have been building boats close to 100 years and their construction methods are well proven in the industry. At the time (about 2009) they were building bottoms as i described. 2 layers of 1/4" mahogany on opposite diagonals with an outer layer of 1/2" mahogany, all applied with West System epoxy.

    I have read about the disadvantages of West System for this application, i.e. micro-cracks due to the movement of the wood and the rigidness of the West System. I was thinking that using 5200 on the outer planks may make for a more durable bottom.
     
  5. TBH736
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    TBH736 Junior Member

    Gonzo,

    The big guessing game with modifications to proven plans......

    The plans are for a 'wet hull' that is not conducive to trailering in and out of the water multiple times each season, something which i will be facing. I fell in love with the lines on this one and it was about the size i wanted to tackle.

    I guess cold molding best describes the method I am using which will definitely add more weight to the finished product but it will be evenly distributed and have little effect on the center of gravity.

    As for the cost of the 5200, I am willing to pay it if it will produce a superior product.
     
  6. Canracer
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    Canracer Senior Member

    I have used 5200 a few times for various projects. I just can't envision how it could be useful for cold molding a hull. It's extremely gooey and it's not strong when spread very thin. Micro cracks be damned, epoxy is by far the best stuff for your hull.
     
  7. gonzo
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    gonzo Senior Member

    5200 is too flexible. If you use fasteners, they will have to take all the stress because the bottom won't be a single panel, but a bunch of wooden strips moving between each other.
     
  8. TBH736
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    TBH736 Junior Member

    Great point, I had not thought about it in that respect.
     
  9. PAR
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    PAR Yacht Designer/Builder

    There's only three ways to build this hull shell, traditionally, which leaks terribly in fairly short order, a combo of traditional and flexible goo (3M-5200 or similar) which is typically a repair, not new construction and of course epoxy.

    If this was a restoration, a strong argument for a traditional method could be made, though insurance companies would prefer an augmented approuch. If this was a repair (new bottom planking for example), some insist on 5200, but this is based on an old school believe (from the 60's and 70's) that it works better than anything else, which isn't supported by testing and the facts, not to mention insurance company opinions (and their imposed rates as a result). Lastly is epoxy, which is the bullet proof method and no it doesn't suffer from the things you've heard (cracking, etc.) unless it's done improperly. Insurance companies will cut you a break with this technique and it's the bee's knees in the industry, by those that have done the testing and repairs.

    You also will save a crap load of money, if you don't use West System resin systems. Ditto System Three. These two are industry leaders and though their products are good and reliable, discount formulations are all over, so look around. You can get epoxy at 1/2 to 1/3 the price you'll pay with the big boys, all with the same physical properties and performance.
     
  10. TBH736
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    TBH736 Junior Member

    PAR,

    Great information, thanks for the insight on the use of 5200. The more i think about it, the right answer seems to be sticking with the original plan of using epoxy.

    As for the epoxy, i have never looked for anything other than West System. If i go with another system, are there any i should stay away from?
     
  11. TBH736
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    TBH736 Junior Member

    All frames are permanently fastened to the girders and after a lot of sanding and painting, there are a few coats of penetrating epoxy and bilge paint on most of it. I hope to get the keel installed this weekend.
     

    Attached Files:

  12. PAR
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    PAR Yacht Designer/Builder

    I suppose there are some bad ones, though the most common are just fine. Check with epoxyproducts.com, bateau.com and raka.com for the lowest priced epoxies available.
     
  13. SukiSolo
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    SukiSolo Senior Member

    As far as micro cracks are concerned, IF the timber is nice and fully dry it should be fine. Inspect and flex each piece well before using, as any flaws should show up. Sometimes uneven bending might show a 'hard' point which you might wish to avoid, but generally decent stuff does not suddenly develop micro cracks. Avoid any edge splitting tendency pieces too, but if you can control the saw cutting of the pieces, generally these are minimal.

    I'd suggest that you read PAR's thoughts on 'penetrating' epoxy before using it though.....;) ie complete waste of time...
     
  14. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    There are some areas penetrating epoxy offers some benefits, but nothing compaired to the wide spread BS being played out by the major formulators and some old school die hards, that just can't believe the industry testing that's been preformed, which suggests they need to "catch up" with the times.

    Epoxy doesn't "micro crack" though it can transmit a crack from the substrate. There are some formulations (brands) that seem more brittle than others, but in all cases (of the commonly used marine versions) the modulus of elongation is quite high, well past what the usual wood species can present. If a crack does develop, it's in the substrate and commuting into and possibly through the epoxy. Douglas fir is infamous for doing this, but other species can as well.

    Depending on the mahogany you use (there's about 40 different sub genus of mahogany), some are much better at being bent over a jig than others. As a rule, mahogany doesn't like to be bent very hard (quick curves), but will accept easy, sweeping bends which are typical of molded builds. With the thin inner veneers, you can use a heat gun to convince the more difficult areas of the hull to lay down (forefoot). Steam isn't necessary on that hull, besides mahogany doesn't steam very well anyway. Fortunately mahogany is pretty porous, so it'll take heat and epoxy well.
     

  15. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    Epoxy is the main glue and coating of both backyard and commercial builders of paneled plywood or cold molded boats. Nothing else is so universally accepted by a majority of builders. As in everything about any craft project, the builder needs to learn how to get the best use of his material whether its lumber, plywood or synthetics. Here on the NC coast, very large and very expensive boats are built and the overwhelming majority of one-off sportfishing vessels are built by laminating layers of plywood with epoxy. Work boats are now mostly done in steel.

    Some people do have a negative opinion about epoxy though. Some people also don't allow their kids to be vacinated. Both groups are simply ill informed.

    There are many many formulations of epoxy intended for an extremely wide variety of applications from daily home use to the aviation industry. Requirements of our marine applications for glue and coatings are not very demanding compared to other industries and all of the available epoxy suppliers are easily able to meet our needs.

    Epoxy should not generally be used to bond large timbers or anything that lacks dimensional stability although some flexible brands can serve there as well.
     
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