Forces on mast foot

Discussion in 'Hydrodynamics and Aerodynamics' started by aiprt, Nov 11, 2024.

  1. aiprt
    Joined: Jan 2014
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    aiprt Junior Member

    Hi all,
    I'm having difficulties finding a clear explanation on how the sail forces are transmitted to the hull.
    Mainly through the rigging?
    What forces act on the mast foot, except what we know about the mast compression.
    What with the mast foot on the deck/cabin top?
    Any contribution appreciated.
    Thanks
    Gilles
     
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  2. skaraborgcraft
    Joined: Dec 2020
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    skaraborgcraft Senior Member

    It would depend on the mast, and if it even had rigging. Free standing masts usually have more reinforcement at the deck area if keel mounted, but some masts can be free standing and in a tabernacle which also (usually) extend down to a keel member.
    Mast size and rigging is usually sized through working the numbers for Righting Moment, stiffer boats and multi-hulls usually require a stronger mast section and rigging diameter. Ring frames and bulkheads and compression posts as well as shroud plates help dissipate loads over a greater area of structure.

    Some one with more knowledge will be along shortly.........
     
  3. aiprt
    Joined: Jan 2014
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    aiprt Junior Member

    Thank you for your quick answer.
    I have been asked about a stayed mast on the cabin top.
    rgds
    Gilles
     
  4. skaraborgcraft
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    skaraborgcraft Senior Member

    The Righting Moment has to be figured to give working load to start with, then the underlying support structure of the deck, be it bulkheads, deck beams/partners or compression post, can then be calculated.

    With a stayed mast, there is always loaded compression even with no sail set. What size boat?
     
  5. aiprt
    Joined: Jan 2014
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    aiprt Junior Member

    The discussion was about a Suprise 25' sailboat.

    Is there only compression on the mast foot, or is the mast foot also a path for the sail driving forces to the hull?
    Most studies on wind/sail forces don't say how those forces are transmitted to the hull, not the magnitude of those forces on which part of the mast.
    Thanks for your attention,
    Gilles
     
  6. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    The mission of the shrouds and stays is to absorb the forces that could cause the mast to bend. In a normal boat (not a racing boat) it is usually enough to study the compression in the mast and in the floor/beam/pillar that supports it. Shrouds and stays, on the other hand, do transmit various forces to the hull.
    Perhaps the attached file will help clarify your doubts.
     

    Attached Files:

  7. DCockey
    Joined: Oct 2009
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    DCockey Participant

    What do you mean by "a path for the sail driving forces to the hull"?

    Consider a rig with an unstayed mast (or rig with only a forestay) stepped through a thwart or dack the mast foot in a mast step. For such a rig on a reach or a run the longitudinal component of the force of the mast on the mast step will aft be towards the stern of the boat, not forward towards the bow. (The direction of the force closehauled depends on the details of the rig, sail and sail trim.) So it is tempting to say the force at the mast step is holding the boat back and not a path for the force from the sail to drive the hull. But if the foot of the mast can freely slide fore-aft in the mast step the rig will collapse and there will be zero sail driving force. The aft component of the force on the mast step is essential to counteract the foreward component of the forces on the thwart/deck and of the sheet on the hull.

    The forces between the mast and hull are a balanced system. Removing one component of that system will either cause the system to collapse or to re-balance with a different set of force components. This is as true for any sail rig.
     
  8. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    Aiprt,

    Good questions especially given the 10 year "lurking" component.
    Your first post/thread since you joined 10 years ago, wow.
    Welcome.

    I couldn't find a good forces diagram.
    But realized you don't need one.
    Look at what the hull is being forced to do in strong winds.
    Imagine the forces involved in a pitchpole.
    Where do these forces come from, the sails.
    How are they transferred, the mast and rigging
    and ultimately the hull.

    Hulls have been known to split wide open athwartship
    directly under the mast, sinking in seconds.

    I like your website.

    Cheers
     
  9. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    Don't discount the the forces transferred from sail to hull through the sheet. In fact, many rigs rely upon the sheet to stay the mast from pitching forward under load. Think of running back stays. Without a sheet to trim the sail, you couldn't release the backstay.
     
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  10. aiprt
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    aiprt Junior Member

    Thanks to all who answered.
    Obviously the case depends on the geometry of the rigging, but according to TANSL's document, there is indeed a definite horizontal force component on the mast foot (Rx).
    The idea was getting a ball park order of magnitude for Rx as compared to the well-known compression Ry.

    Calculating mast and rigging.pdf.png
    Regards,
    Gilles
     
  11. aiprt
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    aiprt Junior Member

    Thans BlueBell for your answer and your kind words
    Regards,
    Gilles
     
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  12. tlouth7
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    tlouth7 Senior Member

    There will always be horizontal forces at the mast step, but the magnitude of them depends on the geometry of the rig, in particular the shrouds.

    - For an unstayed rig we see large horizontal forces at the upper and lower step. Mostly these resist the heeling and pitching moment of the rig, but the difference between force at the upper and lower will give a net horizontal force into the hull.
    - For a stayed rig where the foresail meets the mast at the same height as the backstay (i.e. a masthead rig or fractional with runners) the horizontal force is transfered directly into the stays and so this sail generates no horizontal force at the step.
    - For a stayed rig with no spreaders the mast is only constrained at two points, so unless the forces from the mainsails and its sheet balance perfectly to act through the hounds there must be a horizontal force at the mast step. To first order this acts perpendicular to the boom, though there is also typically compression in the boom due to sail geometry and mainsheet angle and this is also passed into the step.
    - For a stayed rig with spreaders, and especially with swept spreaders and lowers the picture is more complicated. Any forward and sideways forces acting above the step will to some extent be transferred into the stays.

    Here is a fun thought exercise:
    Sail fabric can only carry tension loads tangent to the surface of the material. On say a close reach the fabric of the genoa might arrive at the forsetay from slightly behind (such that no fabric is forward of the stay). This means that the genoa is actually pulling the stay backwards slightly. So where is the forwards force being transferred from the sail to the rest of the boat? It must be via the genoa sheet, and that effectively transfers all its force to the hull through the winch that holds it (and possibly any turning blocks depending on geometry). In other words when sailing under genoa alone the entire boat is being towed along by one winch!
     
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  13. Sailor Al
    Joined: Feb 2021
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    Sailor Al Senior Member

    @tlouth7 Is that also true for the main sheet when close reaching or working with a main as well?
     
    Last edited: Jan 9, 2025
  14. Will Gilmore
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    Will Gilmore Senior Member

    In part, it would be. The main sheet is under constant tension and much of that tension has a foreward component to it. However, there is likely plenty of forward force along the luff of the main, as well. I would think that Force would be particularly strong at the tack, clew, and peak.

    -Will
     

  15. Sailor Al
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    Sailor Al Senior Member

    But, just as @tlouth7 said of the tension in the jib providing a force on the forestay pulling backwards, wouldn't the tension force in the mainsail also pull backwards at the luff track on the mast?
     
    Last edited: Jan 10, 2025
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