Shroud Angle

Discussion in 'Sailboats' started by SuperPiper, Oct 18, 2010.

  1. SuperPiper
    Joined: Jan 2003
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    SuperPiper Men With Little Boats . .

    Slow continuous rain and not a whisper of wind on Saturday.

    Snow flurries Sunday morning and a schedule commitment in the afternoon. The boat is on its trailer and there will probably not be another sail this season.

    I'd like to continue to research this through the winter and have a list of testing for spring. All suggestions are welcome.
     
  2. SuperPiper
    Joined: Jan 2003
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    Location: North Of Lake Ontario

    SuperPiper Men With Little Boats . .

    A Little More Detail

    The mast on this particular boat has been modified to rotate on its step. The prototype spreader is hinged to the forward edge of the mast and the new hounds fitting (not shown in the photo) leads all the shrouds to a point forward of the mast such that the axis of rotation is a straight line. As a consequence, this axis is angled forward relative to the mast. All of this is an attempt to add a stiff spreader that does not interfere with the rotation of the mast.

    I've talked myself into adding lower shrouds. But instead of attaching them to the mast, I think they should be connected to the non-pivoting spreader. So then, it occurred to me that the lowers could be attached with some offset from the pivot point of the spreader. Putting them forward would improve the supporting angle. Offsetting them to each side would counter the natural tendency of the spreader to rotate as the windward upper loaded up.

    The slab of plywood that is the prototype spreader will resemble a pin cushion before this experiment is completed. Maybe the attachment of the lower shroud to the spreader should be 'tunable' to allow adjustment on the water. Now that would be radical.
     

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  3. Perm Stress
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    Perm Stress Senior Member

    With anything rotating the point is that 2 points are always on a single straight line, while 3 points need careful adjustment and really stiff structure on at least one side. With your mast, most probably all three points will be NOT on the same straight line. Furthermore, as the mast is bendy, attachment on the spreaders will move in horizontal plane in all directions.
    Attaching shrouds to the spreader itself will create a structural challenge: a bolt/pin arrangement which hold spreader in place will be subject to downwards force, about 10 times larger than transverse force it do currently hold.
    As appear from the photo, cap shrouds make only a few degrees angle on the spreaders => horizontal force in spreader is small => lower shrouds will not make loads of difference in mast behavior and forestay tension.
    All the rotating masts I know about (yachting press and Internet) with more than one point for rigging attachment are rather stiff wing masts.
     
  4. SuperPiper
    Joined: Jan 2003
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    SuperPiper Men With Little Boats . .

    As you can imagine, any attempt to increase the amount of deflection of the upper shrouds at the spreader just seemed to increase the push on the mast at the spreader. More push mid-mast would cause the mast to bow forward and the headstay would go slack. I tried that last fall.

    By adding the lowers, I'm hoping to keep the mast bend in check. Then, I could extend the spreaders to get more support of the mast at the hounds and hopefully keep the forestay tight.

    Here's a question. Will adding more shrouds to the stick actually reduce the overall mast compression?
     
  5. Perm Stress
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    Perm Stress Senior Member

    "By adding the lowers, I'm hoping to keep the mast bend in check. Then, I could extend the spreaders to get more support of the mast at the hounds and hopefully keep the forestay tight. "

    Agree.

    "Here's a question. Will adding more shrouds to the stick actually reduce the overall mast compression?"

    Generally not. Not much anyway. compression in the mast is mainly determined by righting moment of boat, not by number of shrouds. Only pretension of shrouds will be more from two shrouds than from one. But this present only a fraction of working load and will largely disappear at the time, when full working load is reached (lee-side rigging almost slack when sailing).


    Once more about attachment of lowers to the same fitting (directly or via spreader) which hold spreaders attached to the mast. Most probably the fitting will be not strong enough to handle the load from shrouds. Serious modifications will be necessary.

    Why you do not want to make diamonds with large spreaders at the forestay level? Folkboats have this arrangement for many decades and do not have any problems with forestay tension.

    Regards
     

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    Last edited: Nov 16, 2010
  6. SuperPiper
    Joined: Jan 2003
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    SuperPiper Men With Little Boats . .


    That is interesting about the Folkboat. Thanks for the lead on that.

    Below are 2 diagrams. The 1st is the photo that Perm edited. The 2nd is my revision of Perm's idea. My thought is that the 2nd diagram improves the stiffness of the lower panel of the mast and that would be more likely to keep the forestay taut. It's kind of handy that the spreader is used to provide the offset for the 1/2-diamond. I think I saw something like this in a Moth photo.
     

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  7. Perm Stress
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    Perm Stress Senior Member

    The 2d diagram is a completely different idea from the 1st one. Please also note, that in 1st diagram, there are TWO spreaders, angled ~60 degrees from the Center Line; one of them (portside) would be seen almost exactly in-line with the mast, and therefore it is not drawn.

    Arrangement as in 1st diagram make the upper 2/3 of mast really stiff and so ensure, that when you tension mainsheet (or backstay, as in Folkboat), stiff mast efficiently tension the fore stay, while flattening the main all the way from boom to masthead in the same time.

    Arrangement as in 2d diagram make the mast stiff below the hounds, while topmast remain flexible (much more flexible as lower part). Here, when you tension the mainsheet or backstay, upper part of mast is bent much more as mast below the the foresatay. This way, tension in mainsheet primary make upper part of main flatter, with much less influence to lower main and to forestay tension. On the other side, this arrangement will make transfer of windward shroud tension to forestay tension more effective as it is now.

    in short:
    1st diagram -forestay tension is provided by tension in leech of mainsail
    2d diagram -forestay tension is provided by tension in the windward shroud

    "I think I saw something like this in a Moth photo"
    Most probably you did see an arrangement, which is commonly used on multihulls : the mast is supported by forestay and pair of shrouds, terminated on the same point on the mast; mast between hounds and deck is stiffened by some kind of diamonds. And no part of those diamonds is connected to hull, only to mast. This arrangement is very well suited to rotating masts. In 2d diagram arrangement is markedly different -only one diamond in CL of mast, with shrouds lead through short spreaders.
     
  8. SuperPiper
    Joined: Jan 2003
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    SuperPiper Men With Little Boats . .

    Pretty Darned Good

    The attached photo shows the new generation of (plywood) spreader spliced onto the 2009-2010 version. The new prototype was designed from info on this thread. The shroud meets the mast at 15 degrees. The angle in the longitudinal plane is 6-1/2 degrees and the spreaders are swept back at an angle of 24-1/2 degrees from the pivot point.

    You may be curious about the 24-1/2 degrees sweep, but the mast pivots inside the spreader so the spreader doesn't really interfere with the sail. That angle is also the angle of the shrouds in the plan view.

    The lower diagonals in the photo were just a test using dacron. The rig doesn't currently have D1s.

    I sailed with the prototype this weekend. It worked quite well. The forestay remained straight even in the gusts. I was expecting the mast to be bowed forward as the tension increased on the windward shroud. I was rather surprised when it didn't happen. My theory is that the tension along the luff of the mainsail more or less countered the push of the spreader.

    Any thoughts?
     

    Attached Files:


  9. Perm Stress
    Joined: Sep 2009
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    Perm Stress Senior Member

    Congratulations with results!

    Could you post a side view photo (when port and SB shrouds are seen as one)?
     
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