How do I find out the force on a mast?

Discussion in 'Boat Design' started by Bookmaker, Jan 26, 2010.

  1. Bookmaker
    Joined: Jan 2010
    Posts: 16
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Los Angeles

    Bookmaker Junior Member

    Hello.

    I need to calculate the approximate rotational force on a mast and have no idea what formula I should use.

    Based on a mast height and boom length giving me an amount of sail square footage, and a given amount of wind in knots, how much (in foot pounds) of force will be applied to the mast trying to 'rotate' it around. So I'm looking for some kind of formula. Any help out there? Thank you in advance.
     
  2. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 208, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    Not entirely sure what you want to calculate, but the heeling moment from the rig is in equilibrium with the righting moment from the hull, usually taken at 30 degrees heel.

    If it's stayed then the mast is in compression and the stays are in tension. If not stayed then its the standard beam with bending moment and shear.

    You might want to read "Principles of Yacht design" or something similar for a primer.

    If you actually want to calculate torsional forces, they are the least of your worries, if the mast doesn't buckle it will be torsionally stiff enough.
     
    Last edited: Jan 28, 2010
  3. haru
    Joined: Jan 2010
    Posts: 50
    Likes: 1, Points: 0, Legacy Rep: 21
    Location: Terra

    haru Junior Member

    Please provide a picture explaining the setup. Also provide any info, and then elaborate your question as detailed as possible.
    This may seem exaggerated but in this kind of setup one different info can mean lots of work. Actually, for each connection point a formula need to be created plus some more work depending on your questions.
     
  4. Paul Kotzebue

    Paul Kotzebue Previous Member

    I'm assuming you mean the mast rotating about a vertical axis.

    If the mainsail and boom are free to rotate about the axis of the mainsail luff (which most are), there will be no rotational force on the mast. It's a hinged connection that has no moments.
     
  5. gonzo
    Joined: Aug 2002
    Posts: 15,500
    Likes: 1,040, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    There is going to be a negligible force from the slides or boltrope.
     
  6. Paul Kotzebue

    Paul Kotzebue Previous Member

    Good point Gonzo. It is the relatively small rotational force at the boltrope that allows a freely rotating mast to rotate.
     
  7. gonzo
    Joined: Aug 2002
    Posts: 15,500
    Likes: 1,040, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    It is too small to worry about. I don't think anyone calculates masts taking that into consideration.
     
  8. messabout
    Joined: Jan 2006
    Posts: 3,168
    Likes: 333, Points: 83, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    Rotation,if any, depends on the positition of the shroud attachments. If the tangs are set rearward on the mast, then rotation moment is largely opposed. Whereas, if tangs are attached to the front of the mast, then rotation force is likely. In addition, the location of the gooseneck pin with respect to the mast center line is quite influential.

    I suspect that the calculation of rotational moment could become very involved. Sail area, sheeting force, trim angle, even the L/D ratio of the sail, along with the hardware positioning variables.
     
  9. haru
    Joined: Jan 2010
    Posts: 50
    Likes: 1, Points: 0, Legacy Rep: 21
    Location: Terra

    haru Junior Member

    To simplify:

    The sail calculations only need wind force and lift force on it.

    At the boom base there is always a torque force down due to weight. When turning centrifugal force is added. With wind an additional torque force is added radially.

    These same forces are transmitted to the connection point of the mast.

    Then there are torque forces with or with out wind along the sail edges or stays. This is also transmitted to the mast at the connection point.

    Then again there are torque forces along the inner sail edge and where it's connected.

    Then there is the wind resistance force on the mast itself.

    Add all up and you got the forces acting at the base of the mast.

    That's about it. Note these do not include forces introduce by a boat or waves etc.
     
  10. Bookmaker
    Joined: Jan 2010
    Posts: 16
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Los Angeles

    Bookmaker Junior Member

    OK. Though I appreciate all the things everyone has talked about when it comes to booms and pins and goosenecks, etc. this isn't what I need. Imagine that the boom was hard mounted to the mast and cannot rotate separate from the mast. Now, I want to rotate the entire mast and boom as one object. I need to calculate the amount of pressure in foot pounds that will attempt to rotate the mast. The formula would need to have a variable for the length of the boom, height of the mast and force of the wind and it would output an approximate amount of force in foot pounds that was trying to make that rotating mast... rotate. Any ideas?
     
  11. SamSam
    Joined: Feb 2005
    Posts: 3,900
    Likes: 198, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    I would guess If the boom was mounted to the mast as one object there wouldn't be any rotational force as long as the main sheet was doing it's job and holding the tail of the boom in place.

    On thinking about it more, the boom wouldn't exert any rotational force but the sail in it's track would. It wouldn't have leverage like a solidly mounted boom without the main sheets, that is x amount of force applied to a lever several feet long, but x amount of force applied to a lever the length of the distance from the track to the center of the mast, a lever a few inches long. But then again, the boom would resist any rotation from the sail on the track.

    I know it doesn't answer your question on how to find the amount of force, but is just my guess on how the force might be applied. Does it make any sense?
     
  12. Bookmaker
    Joined: Jan 2010
    Posts: 16
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Los Angeles

    Bookmaker Junior Member

    OK, I'll try again... Take away the sheet and let the wind move the mast freely. I still need to calculate this force.
     
  13. Bookmaker
    Joined: Jan 2010
    Posts: 16
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Los Angeles

    Bookmaker Junior Member

    I understand that what I'm asking isn't normal for a typical rig. But go with me. If the full force of the wind on the sail was attempting to rotate the mast with nothing else to help, how much rotational force is applied to the mast?
     
  14. SamSam
    Joined: Feb 2005
    Posts: 3,900
    Likes: 198, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    Sorry, I understand what you are asking but I'm no good with numbers and formulas.
     

  15. Grant Nelson
    Joined: Feb 2005
    Posts: 210
    Likes: 12, Points: 0, Legacy Rep: 163
    Location: Netherlands

    Grant Nelson Senior Member

    what you want is simply the wind pressure on the sail... you can assume its located at the geometric center of the sail which will take into account all the variables you mention, and that will give you the arm - the distance to the mast, which which will give you your pounds per foot, or what ever related measure you need.

    Someone else is going to have to tell you how to find that presure per sq foot (probably a search in the forum will find it)

    But I wonder why you want this? Are you going to try to rotate the mast via some gears or something at the base of the mast? If you are going to use the traditional main sheet, then the rotational force is not that important, but rather the vertical righting arm and the stresses it puts on your mast (downward) and rig (pull upward)...
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.