Newbi project

Discussion in 'Materials' started by JaredT, Aug 2, 2008.

  1. JaredT
    Joined: Aug 2008
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    Location: Mobile AL

    JaredT Junior Member

    OK I am new to this and have only had limited experence with composites. I am fooling around with the idea of building a lightweight 24' monohull, but the more research I do the more confused I become. There are several good cores on the market that are strong, elastic, and light. But which is the best for my applacation I don't know. I am leaning toward core cell for the hull (jaguar marine has had good luck with it in very high stress racing hulls) and Coosa bluewater for the transom and stringers, This might not be the best choice but is seems to be a direct repalcement for wood, making it almost idiot proof. (which I need lol) I thought about using kevlar, but the strength to weight of s-glass with its lower cost make it attractive. This is my first project so I am very open minded to any sugestions.
    I also have a question. Most boats seem to have two (or more) longitudinal stringers, could a boat be lighter if it's beam strength came from one central beam that runs along the keel? Much like a backbone chassis in a car is much more rigid and lighter than a ladder bar frame?
    Anyway my goals are as follows
    1 a dry hull weight of 1000lbs (rolled gunnel)
    2 24' length w/8.5' beam
    3 capability to run 30+mph on 115hp loaded. (not a high speed boat)
     
  2. alan white
    Joined: Mar 2007
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    alan white Senior Member

    It is true that a single member would most effectively connect the bow and the stern, but unfortunately the panels suspended between also need to be stiffened. Hence, a comprimise structure is designed, one that takes into account localized stiffness, whole panel stiffness, and weight.
    For this reason, too many small stiffening members are no better than a large single one.
    It turns out, however, that you can have your cake and eat it too by having lots of very light but dimensionally tall stringers. The price you pay is a lot of extra (and more sophisticated) labor.
    The strongest and lightest hull would be made up of many transverse and longitudinal stringers forming a tall egg-crate. Then the stringers could be relatively thin plywood panels because they would be supported by their right-angle neighbors.
    The entire grid would be further stiffened by having a top bonded to it.
    There are penalties, as always. Such structures will not flex much, so point loading is much increased (much like a nail attempting to enter an egg).
    My suggestion would be to use fairly conventional methods first time around.
    a boat should be reasonably stiff but also reasonably resiliant, unless it is built for one specific purpose such as racing, in which case lightness and stiffness are favored over longevity.

    Alan
     
  3. JaredT
    Joined: Aug 2008
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    Location: Mobile AL

    JaredT Junior Member

    Any opinion on coring materials? And do you know the ideal amount of S-glass for different thicknesses of core cell. Nida-core is offered as a pre-made sandwich with 18oz woven but I don't know if that is anywhere near Ideal, especaly since 18oz is standard on all thicknesses.
    I know what you are saying about smaller stringers. The car indestury tried that in the 1960s making "bird cage" chassies; very stiff, but very prone to localized failure. As opposed to the "backbone" cars that are to this day thing of legend. (Silent Sam and Lotus for example) I am thinking a combo of a relitivaly thick 1" lightweight core, and a tall and light backbone with the fuel tank built in. The thick core serves two purposes 1. you have to have your flotation foam anyway and core cell is a closed cell, coast guard approved foam. 2. It eliminates the need for bunches of smaller reinforcements. Besides core cell and Nida-core are both only 5lbs cu ft. Can the lighter cores be used in structural members (stringer bulkheads) or do they give too much under load? In a tall stringer is the coring material even that important, does the fiberglass around the core absorb the impact load?
     

  4. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    As you might guess, cored panels don't do well with point loading. They are wonderously stiff however. All depends on your expectations of use.
    Here's an example of two distictly different methods of building a boat hull:
    One is a light, fiberglass/cored hull. The other is plain old steel.
    The former won't survive a direct hit by the corner of a shipping container. The latter not only would survive. It is capable of bashing against that container for hours. Compare a beer can and an egg. Attack each with a spoon. What kind of strength do you want?
    But you want a light hull. Hopefully, you've got experienced help. While I know little of core materials, I do know that solid wood cores can be quite light and they are far more suited to a novice's first build. In particular, strip-building, which is more resiliant than cored. It's also easier to do. I'm sure you've seen how this process is done.
    Chris White, a marine architect who designs livaboard performance multihulls, has specified 1/2" red cedar coring and (I'm pretty sure) single layer glass inside and out. The word "coring" is a misnomer here, since much longitudinal tensile strength comes from the cedar. The glass provides cross-grain tensile strength, abrasion resistance, and waterproofing.
    Strong enough to sail across oceans but light enough to do 25 knots at times.
    To answer your question about impact absorbtion, cored panels react very differently depending on the exact type of impact. For example, a 1 lb ice pick could be driven through an otherwise very strong cored hull. Yet a flat 50 lb object could give it quite a hard knock without any problem.
    Interestingly, many rather heavy boats have been built using, for example, balsa cored underbodies, but those boats had heavy enough outer layups to afford some real protection.
    No so for lightweight boats, which have only enough outer skin to provide a tensile element to the sandwich. You could say that the heavier boats could afford to be cored but the lighter boats could barely do so.
    In the end, it's up to you, but I would personally choose another method that gave more bang for the buck.

    Alan
     
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