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Thread: Sheathed Strip Plank Backbone Methods

  1. #1
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    Question Sheathed Strip Plank Backbone Methods

    I'd like to hear opinions on methods to build the keel and stem backbone for a 30'-ish planing powerboat. The planking is to be cedar strip, sheathed inside and out, with widely spaced bulkheads to create a monocoque structure.


    One way to go is the traditional solid wood laminated keel and stem, which is heavy and time-consuming.


    The alternative is a laminated plywood keel and stem cut to the profile and sheathed is 'glass along with the strip-core structure. Lighter and less labor, though furring strips would need to be tacked onto the backbone to fasten the planking strips. This could be described as much like the Lord's Lightweight method.


    My concern with the latter method is the durability of the plywood backbone, even with epoxy/'glass sheathing, to bilge water. The other concern with water is the need for limber holes through a taller stringer-like keel and another source of water intrusion and decay.


    Again, keeping weight (and cost) down for this particular project is a big driver. I'll be interested to hear your experience with both methods.

    -G

  2. #2
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    Default Re: Sheathed Strip Plank Backbone Methods

    I would no use plywood. Half of the structure will be short grain, or with the grain running in its weakest direction.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Sheathed Strip Plank Backbone Methods

    No plywood if you are putting the laminations in the vertical direction. Nick's comment is the reason.
    But if you want light weight at the potential cost of durability, you could make the keel in the shape of an I beam.
    Solid wood laminations for the lower cap, plywood at a 45 degree ply orientation, and a solid wood cap.
    Then glass and somehow tie into the bulkheads, leaving limber holes on each side, but not thru the lower cap.
    A lot of pain for some weight reduction.

    First question is how much weight would you have in a solid wood laminated keel.
    Second would be how much in the I-beam, and how did you design it.
    Now is the "designed" weight reduction worth the complexity, difficulty of construction, and uncertainty in the structural integrity?

    Do you have a sketch of the use of "furring" strips - I don't understand.

    OBTW, its not really monocoque if there are bulkheads - but that is an English teacher like quibble.

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    Default Re: Sheathed Strip Plank Backbone Methods

    Why do you say laminating a keel is heavy and time consuming?
    Did you read that in a book or a magazine or ..... a forum?

  5. #5
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    Default Re: Sheathed Strip Plank Backbone Methods

    Perhaps a 30 foot lightweight strip built boat needs no keel or stem.

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    Default Re: Sheathed Strip Plank Backbone Methods

    Quote Originally Posted by wizbang 13 View Post
    Perhaps a 30 foot lightweight strip built boat needs no keel or stem.
    The keel on a 30-foot boat will be factored in as part of the hull girder structure bottom flange. leave the keel off and you will need to beef up the garboard area to compensate. Similarly, the panting of the bow planking in any sort of seaway requires a good strong landing for the hood ends to be connected to. leave that out and the hull could split up the stem.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Sheathed Strip Plank Backbone Methods

    It’s a monocoque strip built boat . Does a fiberglass boat have a keel and stem? Metal boat?
    Yes, the “garboard” , the lower structure needs to be stronger than the top, and yes to stringers , but I’m not sure about keel and stem.
    Assuming we are shooting to keep the weight down.

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    Default Re: Sheathed Strip Plank Backbone Methods

    Plywood is used as the stem and keel (or at least the central component of a plywood structure connected to the plywood frames) on a number of modern plywood sailboat designs. As in work by Paul Fisher and John Welsford. This makes sense for a plywood boat, as you have a big stack of the stuff to start cutting parts out of. But a strip-built 30’ planning powerboat. I say laminate up whatever you end up needing, make it as strong as you can per the weight to take the pounding. You will have lots of gluing ahead of you as it is.
    "Yeah, well, that's just, like your opinion man"
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    Default Re: Sheathed Strip Plank Backbone Methods

    Quote Originally Posted by wizbang 13 View Post
    It’s a monocoque strip built boat . Does a fiberglass boat have a keel and stem? Metal boat?
    Yes, the “garboard” , the lower structure needs to be stronger than the top, and yes to stringers , but I’m not sure about keel and stem.
    Assuming we are shooting to keep the weight down.
    The keel is a part of the structural girder, it is accounted for in the structural analysis which is why its scantlings are set out in the rules. Leave it out and you have a weaker structure so you have to put the cross-section area back into the hull's bottom.
    The scantlings of steel and fiberglass are also analyzed to ensure that they meet the strength requirement but they go straight to thicker as there is no structural keel involved.
    As to the lower structure being stronger than the top, true, there are different local stresses on the bottom - hydrostatic loading and on the deck - the impact of green water or falling crews feet, to be added on top of the global bending moments applied to the hull by weight distribution and wave bending.
    There are a lot of balls to be kept in the air.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Sheathed Strip Plank Backbone Methods

    All good points...


    What got me to thinking about this is that with inside and outside sheathing this is essentially a cored composite structure. The keel and stem are tied together at centerline with sheathing and the cedar strips provide massive longitudinal stiffness. I sketched up the keel and stem to help describe the problem.

    KEEL-STEM OPTIONS.jpg


    An internal keel can help with fastening the strips and tying the sides together along the centerline. It would also add depth and structure to the longitudinal axis of the boat but perhaps is not needed. Similarly, an internal stem helps fasten and tie the planking together, but the additional structural property of a laminated stem isn't needed with a heavy 'glass overlay. A stem cap is needed though to protect the strip hood ends and give protection from impact.


    An internal sawn wood stem would help in laying in a structural epoxy fillet and the 'glass overlays in that tight space and in spreading the loads. With that in mind, to avoid a discontinuity perhaps it's best to continue the internal stem down the centerline, diminishing in depth as it goes aft down the keel.


    In the end, it's a balance of the work needed and solving the required structural requirements for this type of boat.
    Attached Images Attached Images
    Greg S.


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    Default Re: Sheathed Strip Plank Backbone Methods

    If you are taking out an internal structural wall and replacing it with an RSJ, would you then saw off the lower flange of the RSJ, because you think that . . . .
    Don't mess with the structural design of a hull unless you are more qualified than the boats original designer.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Sheathed Strip Plank Backbone Methods

    Quote Originally Posted by Peerie Maa View Post
    If you are taking out an internal structural wall and replacing it with an RSJ, would you then saw off the lower flange of the RSJ, because you think that . . . .
    Don't mess with the structural design of a hull unless you are more qualified than the boats original designer.
    I omitted that the hull structure is an "egg-crate" system of longitudinal stringers and intermediate frames, connected to the deck and floor above.
    Effectively, a big beam.
    Greg S.


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    Default Re: Sheathed Strip Plank Backbone Methods

    ABS and DNV Rules specify the section modulus for keel structures. Calculate that value for a vessel the size and speed of your design, then calculate the Z-value for your structure and see how it compares.
    Hope for the best, but plan for the worst.

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    Default Re: Sheathed Strip Plank Backbone Methods

    Quote Originally Posted by siewertdesign View Post
    I omitted that the hull structure is an "egg-crate" system of longitudinal stringers and intermediate frames, connected to the deck and floor above.
    Effectively, a big beam.
    All hulls are big beams, which is why they do not bend when going over waves. Those with decks are box girders, open boats are channel sections laid open side up.
    Quote Originally Posted by mmd View Post
    ABS and DNV Rules specify the section modulus for keel structures. Calculate that value for a vessel the size and speed of your design, then calculate the Z-value for your structure and see how it compares.
    This^ if you are diverging from the designer's scantlings you have to do some serious sums to ensure that your boat will not break with you and yours onboard 4 miles out.
    It really is quite difficult to build an ugly wooden boat.

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