A question about plywood/timber sandwich construction

Discussion in 'Wooden Boat Building and Restoration' started by Elmo, Sep 12, 2011.

  1. Elmo
    Joined: Dec 2009
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    Elmo Junior Member

    Hello

    I have a question about hull construction.The boat in question would be a scow type hull about 30 ft long x around 10 ft beam , similar in form to this model :[​IMG]

    While searching scows on Google I came across this note on the Selway Fisher site , regarding construction of a 45 ft wooden barge:

    What happens if the solid wood in the "sandwich" swells ?
    I imagine the (outer layer ) of plywood will experience significant tension within the plies of veneer , possibly leading to tearing ?

    Can the wood be laid in flexible caulk ,then fastened to allow for shrinkage / swelling or would you need to construct a plywood/ wood core / plywood sandwich with epoxy?

    How is this done correctly , is it even feasible ?
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Selway Fisher would obviously be the best place to start with your structural concerns, though knowing Paul's background in clever marine structural engineering, I'd suggest he's cover the bases he needs to on this particular design.

    I don't know if this is an encapsulated build method, but if it is, your concerns are unwarranted, as the wooden elements will be moisture proof.
     
  3. Elmo
    Joined: Dec 2009
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    Elmo Junior Member

    Hello PAR

    Thank you for the reply , and yes Paul did email me and cleared it up.

    Fully encapsulated plywood / wood / plywood sandwich .So ...that does not concern me one bit.

    The other reason I actually asked was because there is a group that is building a large scow here:

    http://www.google.com.au/url?sa=t&source=web&cd=3&sqi=2&ved=0CC0QFjAC&url=http%3A%2F%2Fwww.scowschooner.com%2Fconstruction.asp&rct=j&q=scow%20schooner%20project%20epoxy&ei=pRxwTqeQCqewiQfjkcG7CQ&usg=AFQjCNE1W0ZyzbOaJoZWQYvyd2NYMg3akg&cad=rja



    This quote from their website is what I was referring to :


    Much has been written lately about saving old, planked wooden boats by cold molding several wood/epoxy layers over the existing hull. Then Reuel Parker started building new boats in Florida laminating plywood and diagonal layers over longitudinal planking, using epoxy. We started experimenting with these methods on smaller boats.

    We settled on our own combination of these methods. We would build the schooner using the original cypress planking on yellow pine frames, but the cypress planking would be much thinner than the original. Outside of this first layer we would bond one or more layers of plywood with epoxy. One or more layers of polypropylene cloth set in epoxy would in-turn protect the plywood.

    The result will be that when visitors look inside the schooner they will be able to see how the original looked, cypress and pine. And, viewing the outside the boat will also look the same. You will see paint! The difference will be cost and ease of construction and vastly reduced maintenance schedule.

    Does not sound too clever to me ,
    Better them than me tho......

    Thanks for the reply.
     
  4. Wavewacker
    Joined: Aug 2010
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    Wavewacker Senior Member

    If I may, a question,

    While I have seen barges such as that I have never seen one sailed.

    How do they sail? Do they actually take those in blue water? Might have seen one in an old ancient movie going down the Nile....

    Seems that would really slam, but the PDRs seem to be popular, so what's the deal?

    Sure would be a simple build.......are they efficient with low power?
     
  5. PAR
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    PAR Yacht Designer/Builder

    PDR's pound just like anything of that shape. If kept flat they sail okay, but sailing enjoyment isn't why you make this hull choice.
     
  6. Steve W
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    Steve W Senior Member

    Wavewacker, some of the larger New Zealand scows used to cross the Tasman sea on a regular basis so i guess that would qualify as blue water. About a year ago i spent 3 weeks cruising the gulf coast of Florida on a 28ft Egret sharpie and much to my suprise it didnt pound at all under sail and even more suprisingly under power on the ICW when we were bolt upright and with an endless stream of powerboat wakes, i would say we probably only pounded a couple of times on particularly nasty wakes.
    Steve.
     
  7. PAR
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    PAR Yacht Designer/Builder

    If a powerboat wake caused this, just imagine what a nice short, steep, rolling swell would do in blue water, let alone a good fetch and some trades.

    There's a reason scows and this general hull form have the reputation they do. On an inshore lake with a bunch of guys trying to keep one on her lines, that's great, but other wise . . .
     
  8. Steve W
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    Steve W Senior Member

    Par, a trading scow such as were used around the coasts of the US, the Great lakes and NZ may well have pounded, i dont know, they are nothing like the recreational scows such as the A,C,E etc which we have in the midwest though. The Egret is not a scow but as a sharpie is a flat bottom boat which as i said, did not pound at all sailing in the gulf over a 3 week period, the powerboat wakes we experienced were way worse than anything the gulf kicked up as we were not heeling. This boat is my only experience with a flat bottomed hull and while it didnt really suprise me that it didnt pound under sail as when it was heeling its presenting a vee to the water but the mostly lack of pounding while motoring did.
    Steve.
     
  9. PAR
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    PAR Yacht Designer/Builder

    A working craft generally does best under a load, which buries the LWL considerably. This is why dories have their reputation, not some inherent design aspect. Working scows would have had lots of belly buried in the drink as well, so you don't pound when burdened, is my point, but you also don't sail well either.

    A light, flat bottom boat under sail doesn't have to pound if designed right. In this same vain, if designed to sail fast, it will pound. Everyone's heard the old saying about "sailing like a garbage scow", well again ,there's a reason it's a derogatory phrase. I have a fair bit of experiences with flat bottom craft and have been aboard both good and bad. I think it's more difficult to design a good sailing flat bottom then many other types, mostly because of the things you're "steering around" in the design development phase. You want this, but have to sacrifice that, etc. and in a flat bottoms, there seems to be more then the usual to satisfy, then other hull types. I'm not talking about a 12' punt now, but substantial craft of a few tons at least. Even the structure has to have special consideration, because of it's lack of depth.
     

  10. boat fan
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    boat fan Senior Member

    I suspect you are right about most of what you say about a scow PAR , but , they can sail quite well under the right conditions if they are loaded right.

    On a reach and down wind they can actually perform very well.
    Not so good close hauled.You cannot let them heel excessively of course , as they can then become quite dangerous.

    Yes , there is a good reason for that , agreed , but in all fairness , that reference in particular, has a lot to do with a rather heavily , ( almost over loaded ) hull.

    The sailing ability of scows often surprises many people.
     
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