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Thread: Ply or Coosa?

  1. #286
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    Ghost - Yes, the bulkhead mounts in the middle of the built up strip which is more like 4-5" wide.

    George - Your reasoning makes sence on the use of pine.. On my hull the plywood is not covered in glass except for the 4-5" border where it is tabbed into the floor and hull. The joint along the deck is epoxied and bolted. If Pete glasses the entire bulkhead, then I'm sure the pine is more than adequate.

    Pete - Do you have any pics of your forward bulheads where they join the hull?
    1978 Magnum Starfire "MAYHEM"
    1994 Donzi 22 Classic 540/Blackhawk - Sold
    1982 Midnight Express 32 SS

  2. #287
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    Marshall....I totally missed what you were talking about in the first picture, then the repost by Ghost and it popped out.
    Gotcha!......the core strip expands the area of the load (and grip) to the hull.......Now I can see how a hard spot would
    occur if I glassed the bulkhead "to tight" in that area.......thanks.

    George......I remember your analogy in suitable materials used in bulkheads & stringers. I remember you piecing together
    pine one by's (for a bulkhead or something like that on the TR) as a better alternative to using plywood....penetrate with
    epoxy to seal the deal.
    1983 Cigarette 35' Mistress
    1976 Magnum 27' Sport
    1986 Donzi 18' Classic
    NEVER FORGET 9/11

  3. #288
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    As a precaution to lowering the cockpit sole I'm adding 2 more stringers (L & R of the Keel)
    There sized and fitted & will be glassed in later. All the flat oak rails are in and ready to recieve
    a level & lowered plywood floor.

    A question for the Magnum guys:
    I believe (but not sure) magnum made 2 cutouts in the sport cockpit floor for what I believe to
    be access to the fuel lines mounted on the keel stringer just below the floor...does anybody have
    a picture of there's to post?.....I'd like to add these, but need some mearsurements.
    Attached Images Attached Images
    1983 Cigarette 35' Mistress
    1976 Magnum 27' Sport
    1986 Donzi 18' Classic
    NEVER FORGET 9/11

  4. #289
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    On my recent vacation to the Abaco's I visited the Albury Boat Works, they install a fully glassed tube to route the fuel line through. A bit more work initially, but fuel line replacement is much simpler. Pull the old line out, slide the new through . . .

    Need to ask, white pine for the additional stringers? Even made of just 3/4" thick marine plywood it would be stronger, and should you ever have a bit of water get inside the plywood will last much longer. We all know that pine is how Baja built their boats . . .

    can't wait to see the new floor, and more images. Lowering the floor is a future plan for me . .

    George, I need to disaggree with your theory on solid wood vs. plywood , especially a marine grade plywood. The strength is in the lamination of multiple layers. I learned this years ago when working for what was at the time the worlds largest crutch manufacturer *. We switched from solid birch to a thinner laminate with light plastic filler. The torsional and compressive strength far exceeded the solid hardwood. Another example is contemporary woodframe construction where the floor joists are now done with strand board I beams. The strength comes from the muliple layers bonded together, much like multiple layers of fiberglass.

    * This company was Cove Craft Inc. They also developed the first laminated slalom water ski.


    Mario

  5. #290
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    Mario, notice I didn't say that engineered laminate couldn't be the product of choice...if it's engineered to do so.
    The problem w/boats is people replace stringers w/plywood, marine or otherwise, and don't change the layup schedule to compensate, it'll be weaker because the longitudinal fibers (the wood fibers that resist bending) are cut in half. An example: take a 1" X 4" X 6' long piece of clear pine or birch (or any other good construction wood), also cut a 6' long piece of 3-1/2" wide X 3/4" plywood (which is the actual size of the 1 X 4). Rig them so they are anchored at each end but unsupported in the middle, and the 4" dimension is vertical. Start loading the middle of each "beam", and see which one breaks first.
    As far as floor joists go, the laminate web carries no real load and doesn't add any particular strength to the joist, they merely keep the top and bottom flanges separated. The majority of the strength is the top and bottom flanges. The laminated web makes up the neutral axis of the joist. That's why you can cut holes in the web and run AC ducts and wiring through them.
    Now, if a person builds a laminated stringer, and considers the inside of the hull as a "bottom flange", and adds a top flange to the assembly, I'd have no problem w/it.
    George Carter
    Central Florida
    gcarter763@aol.com
    http://kineticocentralfl.com/


    “If you have to argue your science by using fraud, your science is not valid"
    Professor Ian Plimer, Adilaide and Melbourne Universities

  6. #291
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    Pete,

    The underfloor stringers look like they will be nice and strong, overkill if anything imo. Will you be doing the same but shorter in the engine bay?

    - Marshall
    1978 Magnum Starfire "MAYHEM"
    1994 Donzi 22 Classic 540/Blackhawk - Sold
    1982 Midnight Express 32 SS

  7. #292
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    Phantom post appeared twice for some reason!
    Last edited by scippy; 03-03-2013 at 12:12 AM. Reason: phantom post
    1983 Cigarette 35' Mistress
    1976 Magnum 27' Sport
    1986 Donzi 18' Classic
    NEVER FORGET 9/11

  8. #293
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    Mario,
    Coverage and saturation w/epoxy to fully penetrate the (pine) stringers will most likely exclude (I hope) water
    intrusion.........Don't know much about the Baja boats, but if they practiced a redundancy of good care and good
    glass work maybe pine would not be the weak point.

    George,
    Take away the top & bottom flange of an engineered ply I beam and there goes the mechanical advantage.....
    I felt comfortable enough to use 1" X 6" pine board, certainly after I laminated 2 pieces together it felt more
    stout than the ply and was so much easier of a wood to work with .i.e, routing, shaping...I figure if I saturate
    well (penetrating epoxy) and take care to fully glass in ...it would be a solid core material.

    Marshall,

    I have the bilge stringers already made. Looking at yours, they seem to lay more towards the keel than dead
    dead center of the bilge........maybe off center by 2"?
    1983 Cigarette 35' Mistress
    1976 Magnum 27' Sport
    1986 Donzi 18' Classic
    NEVER FORGET 9/11

  9. #294
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    I'll measure and let you know Pete.
    1978 Magnum Starfire "MAYHEM"
    1994 Donzi 22 Classic 540/Blackhawk - Sold
    1982 Midnight Express 32 SS

  10. #295
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    Quote Originally Posted by gcarter View Post
    Mario, notice I didn't say that engineered laminate couldn't be the product of choice...if it's engineered to do so.
    The problem w/boats is people replace stringers w/plywood, marine or otherwise, and don't change the layup schedule to compensate, it'll be weaker because the longitudinal fibers (the wood fibers that resist bending) are cut in half. An example: take a 1" X 4" X 6' long piece of clear pine or birch (or any other good construction wood), also cut a 6' long piece of 3-1/2" wide X 3/4" plywood (which is the actual size of the 1 X 4). Rig them so they are anchored at each end but unsupported in the middle, and the 4" dimension is vertical. Start loading the middle of each "beam", and see which one breaks first.
    As far as floor joists go, the laminate web carries no real load and doesn't add any particular strength to the joist, they merely keep the top and bottom flanges separated. The majority of the strength is the top and bottom flanges. The laminated web makes up the neutral axis of the joist. That's why you can cut holes in the web and run AC ducts and wiring through them.
    Now, if a person builds a laminated stringer, and considers the inside of the hull as a "bottom flange", and adds a top flange to the assembly, I'd have no problem w/it.
    George, the dimensional lumber makes sense. Now, what species is best? Kind of like engine oil.

    Jeff
    " Just piercing the surface"

  11. #296
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    Pete,
    Stringer to keel roughly 17" , from outer stringer 16".
    - M
    1978 Magnum Starfire "MAYHEM"
    1994 Donzi 22 Classic 540/Blackhawk - Sold
    1982 Midnight Express 32 SS

  12. #297
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    Thanks Marshall ...much apreciated!
    1983 Cigarette 35' Mistress
    1976 Magnum 27' Sport
    1986 Donzi 18' Classic
    NEVER FORGET 9/11

  13. #298
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    Jim,
    Here's the little construction project I just got finished with "cover the assets..............................Now there's no excuses to get her done!
    You can see the damage from Sandy when a tree limb crushed the fence in the yard........Glad it happened before the (mag) port went up.

    IMG_8240.JPGIMG_8243.JPG
    1983 Cigarette 35' Mistress
    1976 Magnum 27' Sport
    1986 Donzi 18' Classic
    NEVER FORGET 9/11

  14. #299
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    Pete, I like your dollies!

    Well done!
    George Carter
    Central Florida
    gcarter763@aol.com
    http://kineticocentralfl.com/


    “If you have to argue your science by using fraud, your science is not valid"
    Professor Ian Plimer, Adilaide and Melbourne Universities

  15. #300
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    NICE!! I want one of those.
    "I don't have time to get into it, but he went through a lot." -Pulp Fiction

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