Bowsprit Scantlings Calculations

Discussion in 'Sailboats' started by UNCIVILIZED, Nov 14, 2016.

    Joined: Jun 2014
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    UNCIVILIZED DIY Junkyard MadScientist

    I'm looking for where to find information from which the scantlings of bowsprits can be calculated. Meaning figuring out how big their cross section needs to. The primary reason being that I'm curious how a sprit's cross section increases if you (desire to) add length to a sprit. On keel boats, particularly. Specifically, on pilot cutter style designs, such as; Westsail's, Rafikis, Tayana's & similar boats. So ones stayed only in the vertical plane/orientation, mostly types built without strikers.

    Being able to figure out how much thicker a sprit would need to be based on increasing it's length by X% would be ideal. Though I'm guessing that to more accurately calculate such things one needs much of the boats numbers for righting moment, & similar, which are typically used to figure out rig scantlings.

    Does such lengthening perhaps have to do with the formula for stiffness, thus making it a cubed function? Or perhaps it's to the 4th, as are spar calc's? Though how these formulas translate into building a solid spar like a sprit I'm uncertain. Also, what about when sprits have dolphin strikers (AKA spreaders by another name)? Are then the spars & striker construction covered by the exact same formulas as are spars with spreaders? Particularly since from appearances, sprits & dolphin strikers would be subject to huge amounts of compression. Non? And as to how one figures out the strength of their attachments at deck level, I'm totally in the dark.

    Information & links on all of the above would be helpful, without doubt. Though at the moment, simpler formulas would be better. And if you've got any links to your favorite websites & reading on such things, they'd surely be appreciated as well. Especially as I'm not well versed in rigging more traditional boats, other than the basics in Brion Toss's The Rigger's Apprentice. And yes, I'm aware that the type of wood or metal used to build such things also plays a big role. So again, please help my DIY "rig studies program" :D

  2. messabout
    Joined: Jan 2006
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    messabout Senior Member

    To the best of my knowledge there are no "cookbook" ways to calculate sprit cross section.

    The load on an unstayed sprit would be calculated as if a cantilever beam. In any case the load is sure to be subject to a change of directions. That suggests that the section should be round so as to keep the same stiffness without regard to load direction. The moment of inertia calculation for a simple beam section looks like .... (.bd^3)/12 where b = width and d = depth. because b and d are equal for a round section, the fourth power is in effect.

    The whole deal gets more tricky when stays are added. Now the sprit is placed in compression mode as opposed to the bending mode in the wireless state. The rules for column load will apply with some healthy fudge factors I suspect. A building column just supports its constant load. A bow sprit is a different breed of cat because it is exposed to column loads that include impact loads as well. In a nasty seaway there may even be some beam loading caused by plunging.

    Losing the bow sprit in a rough sea would be very bad news. Maybe someone who knows more about this subject will reply.
  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    I think messabout has made a correct and very clear explanation of how to proceed. The only problem left (this is really the only problem) is to determine the force that the forespray transmits to the bowsprit. Although there are several procedures, the NBS has a procedure to calculate the forces on the stays quite simple and easy to understand

  4. JSL
    Joined: Nov 2012
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    JSL Senior Member

    On the few boats I have done with 'stayed' sprits (bobstay, shrouds, etc) I used the same formula as for the main mast to calculate the cross section (moment of inertia ). Mind you, these are a few small vessels (30' to 70') with sprits.
    One suggestion: For operation in short, steep, seas I try to limit the sprit length (overhang) to about the same as the freeboard: A long sprit with small freeboard could be vulnerable.
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