Shape of a flexins mast, pros/cons?

Discussion in 'Boat Design' started by Gades, Oct 13, 2002.

  1. Gades
    Joined: Nov 2001
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    Gades Senior Member

    Shape of a flexing mast, pros/cons?

    Hi guys,

    I'd like to know the opinion of different sailors/designers on the shape of a mast. I'm talking about the transversal deflection. For example, looking to the drawing I attached, you can see different shapes. Obviously they're not real (not properly scaled), but it's easier to appreciate this way.

    So, playing with the spreaders (size and position) and the different diameters, I can get different bending shapes. Which one would you suggest, and why?
     
  2. Gades
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    Gades Senior Member

    The attachment didn't work properly, so here it's the image.
     

    Attached Files:

  3. Polarity
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    Polarity Senior Member

    A....

    lets the top fall off in the gusts and changes the sheeting angle according to the windsheer.

    Paul (Sailor!)
     
  4. Gades
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    Gades Senior Member

    Thanks Paul,
    I was waiting for your answer ;)
     
  5. DavidG
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    DavidG Junior Member

    I agree A,

    Main twists off at the head, allowing for wind sheer.

    Jib leach opens up in the gusts, giving a nice parallel slot.

    More practically, the spreaders will clear the jib leach.

    The rig will be easier to set up.

    The sailmaker will understand what you are talking about!

    David
     
  6. Gades
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    Gades Senior Member

    Thanks DavidG,

    I also have another question about the mast, actually about the spreader.

    I think I know how to calculate the horizontal angle (let's say the one you'd see in a plan view); but I do not know how to calculate the vertical angle (if I want/need it). Any information/formulas about this?

    Thanks again

    Fernando
     
  7. Polarity
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    Polarity Senior Member

    As far as I know they should bisect the angle of the shrouds so that they are purely in compression.

    Paul
     
  8. giramonti
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    giramonti Junior Member

    Spreaders

    Yes, the spreader should bisect the angle of the shrouds that cross its tip. This can be a bit difficult to determine, because theoretically, the two or three or four shrouds that cross a tip must be weighted then divided by the sum of the loads to determine the final angle (e.g. on a 1-spreader rig, at the bottom spreader, the cap shroud has a Br.str. of 5000kg, the bisect is 5.4°, V1D2 has a br.str. of 4200kg w/ bisect of 6.8° – to determine spreader angle: "(5000kg*5.4° + 4200kg*6.8°) / 5000kg+4200kg = 6.039°").

    Practically speaking, a sparmaker is not as precise as setting the spreader at small fractions of degrees. So, in this case the spreader will be set at 6°. At Carl Schumacher's office we set all of our spreaders at 6°, because experience told us that spreaders were always 6° +- 0.3°. I haven't analysed why this is the case, maybe its the way we designed our rigs, but it worked well.

    -Roberto
     
  9. Gades
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    Gades Senior Member

    Thanks again guys,

    I think I understand what you say/mean (I don't always do ;) )
    I'm going to post my load calculations and a drawing (with proper dimensions), with the spreaders as well (my first time).

    Cheers

    Fernando


    P.D.: Paul, I think there were some problems with your site today. I couldn't get to your links page (this morning).
     

  10. Polarity
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    Polarity Senior Member

    my personal site Polarity2 or work site Nonstopyacht Supply ? they both seem to work ok now, maybe just a glitch...

    But thanks for the heads up..


    Paul
     
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