Opinions on alternate wood for daysailer keel

Discussion in 'Sailboats' started by longfellow, Sep 18, 2009.

  1. longfellow
    Joined: Apr 2009
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    Location: upstate NY

    longfellow Junior Member

    Eighteen foot open daysailer, centerboard, bent oak frames (.75in x .75in), carvel planked (.625 in thick) hull, knockabout rig of approx 200 square feet. Diaplacement will be close to 1,200 lbs. (A very close copy of an alden O-Boat but of a different architect's design). I would like to use longleaf pine in place of the white oak keel. The plank's finished cross-section is 1.625 in by 5.50 inches. It has a fair amount of camber and needs to be sixteen feet long. It widens naturally to accept the slot for the centerboard and tapers to about two inches at both ends.
    I am proposing the swap in YP but intend to duplicate the strength in pine by bumping up the moment of inertia of the existing cross section so that bending stress, once calculated, ends up being about equal to that with oak. I am assuming that using bending stresses to find the new cross section in yellow pine, is better than using either sheer or torsional.
    Comments are very much appreciated.
     
  2. Petros
    Joined: Oct 2007
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    Petros Senior Member

    That is a reasonable approach, probably more than most would do. Also watch the cross gain compression strength at places like the mast step, you might want to spread the load out to prevent crushing the grain in high compression areas.

    What are the strength properties of the pine vs. oak (do you have them handy?).

    Also consider that white oak has much higher density than pine, so your empty weight might be less, which might affect stablity unless you compensate with ballast.

    I make substitutions of materials all the time on the smaller boats I build, and if it worries me I do what you are proposing, match size to the critical strength using the new material specs. As an engineer I think most stock plans overkill the strength (making them heavy), that is they are very conservative to accommodate a lot of abuse and poor workmanship. I suspect you will be fine, I personally would not hesitate to do just what you are doing.

    Have a good time building it and do not fret over your materials substitutions.

    good luck
     
  3. longfellow
    Joined: Apr 2009
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    Location: upstate NY

    longfellow Junior Member

    Thanks very much Petros.
    I was surprised to find that longleaf actually has greater bending strength than white oak. Its modulus (E) is also greater. It does however have less compressive strength normal to the grain so your suggestion to beef up the mast step and distribute the load more, is dead on. I can take care of that. I was nearly ready to break down the entire lofting and molds which are all complete and ready for the keel, stem, transom and then ribbanding but your input has lifted quite a burden and renewed my enthusiasm for continuing.
    Thanks again,
    Ed
     
  4. Petros
    Joined: Oct 2007
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    Petros Senior Member

    Another consideration occurred to me, you should also check fastener holding strength (usually listed as dowel bearing strength). For most fastener locations there should not be an issue because there are not loaded to their capacity. Bbut in critically loaded areas you might want to add a few more fasteners, or go up in sized, if there is much difference in the dowel bearing strength.

    If the difference is less than 10-15% you are probably fine, but if over that better up the number of fasteners to compensate. Critical locations would be chain plates, fore stay, rudder mounts, keel attachment, rowlocks (if so equipped). and any and all fasteners in the prow area and keel.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Most of the highly loaded fasteners will likely be through bolted and a non-issue.
     

  6. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    If it is true YP then it should be OK. They sell a lot of pines as YP that are not the same quality
     
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