Spray Rails on Strip Plank Hulls

Discussion in 'Boatbuilding' started by Willallison, May 3, 2008.

  1. Willallison
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    Willallison Senior Member

    When building a strip planked planing hull, would one normally add the spray rails before of after FRP sheathing the external skin?
    From a fairing point of view it would seem easier to do it afterwards, but it seems somewhat unnecessary to have to glass over an already sheathed hull a second time if rails are added later....
     
  2. Meanz Beanz
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    Meanz Beanz Boom Doom Gloom Boom

    I would say after, that way you keep the properties of the hull skin uniform from an engineering perspective. It would be better if the rails where to be damaged that they where not apart of the structure, besides the weight a little extra glass will not hurt. I also think that it will be easier to lay the glass over the topsides without having to negotiate a lump, after all at this point you are dealing with large areas of glass, the last thing you need is extra bumps. Then glass the rail in with much smaller strips and fair it in with filler, that sounds much easier to me.

    2c

    MBz
     
  3. masalai
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    masalai masalai

    Just a thought from a "fool"? - - why not a slight step outwards for the upper section - facilitate more flair as well - built in - so to speak, as part of the design, for example, as attached. can send fef direct or other form of export from "FreeShip-Plus ver 2.90"...
    (I feel that, although this is an abandoned version, the chine should run straight from the present point at the bows to a point at the stern some 5 to 10 cm below the waterline for best effect, and be some 5cm in width especially near the forward third of the hull.) - - the upper sections could be flared a lot more. - still working on latest revision...

    Otherwise as Heinz suggested - look at how "X-IT" was done and that works nicely...
     

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  4. alan white
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    alan white Senior Member

    After, without question. The glass skin is of a strip build is far stronger if continuous, without stress-inducing interruptions like spray rails (or keels, rub rails, or anything else one might attach).
    The glass layer is what gives the much needed tensile strength (resistance to pulling apart) to the hull. This is the same strength imparted by crossing layers in cold-molding and plywood.
    Even with those other methods, unless you're faced with glass-sheathing someone else's already-built hull, it's much easier to sheathe without the part installed first.
    It's a good idea to over-drill your mounting holes, fill with thickened epoxy, and redrill to ensure the hull can't absorb moisture (which matters less on hulls that aren't sealed completely in epoxy--- they can "breathe" the water out naturally, but a stripper like yours will hold water til eternity.
    Then (I would) bolt the rails on with carriage bolts or flat-heads with the nut on the inside and bung the outside. It doesn't hurt to stiffen the mounting area with an additional layer of mat inside. 5200 is all you need to bed the rails.
    Another method you might not know about, Will, is how to traditionally prepare the rails (or any other longitudinal piece) for installation.
    This involves the back face of the piece, which is relieved 1/16" or so everywhere but the last 1/8" of the top and bottom edges. It can be done on the table saw or on a router table, or with a specialty (dado) plane.
    What this does is two things; it leaves more compound in (which compensates for movement far better at 1/16" than at 1/1000"), and you can micro-plane the faying outer edges much more easily when you fit the piece. Then the job looks really craftsman-like to the discerning eye.



    Alan
     
  5. Meanz Beanz
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    Meanz Beanz Boom Doom Gloom Boom

    Hey Alan,

    Assuming he is going to make up cedar rails wouldn't you epoxy them into place?


    Mbz
     
  6. Willallison
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    Willallison Senior Member

    Thanks Gents - confirmed my thoughts exactly...
     
  7. alan white
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    alan white Senior Member

    I wouldn't. I like to be able to take things apart. As long as I've got to bolt or screw the rails on anyway, substituting 5200 for epoxy allows removal later.
    Nor would I epoxy on a rub rail. One day I might replace it, and I'd rather not grind it off.
    I wouldn't use cedar for the spray rail any more than I'd core it with balsa. I would use something harder--- at least hard pine or fir or Spanish cedar.
    Things like rails get ground against docks.

    Alan
     

  8. Meanz Beanz
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    Meanz Beanz Boom Doom Gloom Boom

    Fair point, on my boat the gunwales are much further out than the rails would be and lower that most so they are not exposed but on most modern designs they'd be a target.
     
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