Square top mains?

Discussion in 'Sailboats' started by kenwstr, Dec 6, 2005.

  1. brian eiland
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    brian eiland Senior Member

    DynaRig camber & Twist

    The camber on the DynaRig can not be controlled, but was rather optimized at a 12 degree arc.

    And the twist one might wish to incorporate as the impending wind varies up the height of the rig was also disregared in favor of KISS.
     
  2. tspeer
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    tspeer Senior Member

    When you include either the gap between sail and surface or heeling moments, the optimum sail planform is not a conic section. So the debate of ellipse vs parabola is moot. What matters is how well you can approximate the optimum "downwash" distribution. The optimum planform is going to end up being more egg-shaped than either a parabola or a semi-ellipse.

    If you compare rigs on the basis of equal mast heights, then the fat-head main is going to come out on top. And when you include the planform contribution of a jib, the optimum main becomes more like a half-ellipse, with a lot of area cut away at the foot.

    But if you compare on the basis of maximum heeling moment, the traditional rigs don't look so bad. They allow a taller rig, which reduces induced drag.

    A flexible rig goes a long way toward having it both ways. A tall mast with a fathead sail planform for reduced induced drag in light winds, and increased twist as the wind picks up to reduce the height of the center of effort in heavy winds. When twisted off, the fathead main will have more parasite drag compared to a more tapered planform that effectively cuts out the unloaded sail area. But that may be a reasonable price to pay for the all-round performance of the rig.
     
  3. Richard Atkin
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    Richard Atkin atn_atkin@hotmail.com

    A layman's question for Tom or Brian: My catamaran design has a large gap between the sails and the deck (typical fathead bermudian rig) to allow a great view for all passengers. Is the gap under the sails a significant factor in sail efficiency, compared to all the other factors? In other words....is it really tragic to have a big gap there? I am not concerned about the increased heeling moment, as I am specialising my design for light air. I am more concerned about efficient air flow.

    If the gap proves to be a big problem (eg. it slows the boat down more than 1 knot)....I will think a little harder about the 'egg' shape.
     
  4. Richard Atkin
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    Richard Atkin atn_atkin@hotmail.com

    Going by the silence, I guess my question was a bit inappropriate/boring for this thread :D
    Well....the invitation is there for anyone else to give me just a crude perspective on the issue of gap between the foot of the sail and deck....but to be less of a highjacker, I'll post the question in another thread.

    Thanks for your previous replies regarding the dynarig, Brian. That rig has definitely got me thinking.
     
  5. brian eiland
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    brian eiland Senior Member

    Sorry, my silence is just because I have a lot of other things going on at the moment.

    Yes, that big gap at the foot of the sail can be quite detrimental.
    I'll just reference a few items that have been mentioned in "Sail Aerodynamics" thread.

    ...from posting #44
    "However, I think the important point is the huge vortex coming off the boom. It shows that the sail rig does not act like a a wing with double the
    geometric aspect ratio of the sail rig due to surface effects, and the optimum planform is not a semi-ellipse. The vortex is also strongly
    affecting the region of the mainsail where the chord is the greatest.

    People usually concentrate on the vortex at the top, but the vortex at the foot may be more significant. It's worth considering how to shape the mainsail so as to reduce the strength of the vortex and to move some of the sail area away from its influence. One answer is to use a wishbone boom and round the clew so the planform of the whole sail rig looks more like a sail-board rig. Tom Speer
    "

    ...from posting #49
    "On the other hand, there is another approach which we've used over the years which does work in some cases. This is to "endplate" or seal off the bottom of the boomed sails. If you can achieve this for even half of the foot length, the increase in efficiency is dramatic.

    On our 67' ketch, Sundeer, we were able to pick up five degrees in weatherliness--without losing boat speed, when we sealed the main and mizzen. We've just had seals made for Beowulf which we'll be testing in the near future, and will write up for SetSail.

    The area added is down low, where it is in turbulent air flow and where the breeze is much lighter. However, the seal effect is very powerful, and if you can make it work with your rig and deck structure, will generate a huge improvement. Note--the less efficient your keel, the more this will help as it reduces induced drag--which hit cruising keels harder than those found on racing boats. Steve Dashew
    "

    ....posting #55

    ....posting #93
    Discontinuities with the Fractional Rig


    All I have time for at the moment, visit Sail Aerodynamics
    Brian


    BTW, here's a good illustrative article on How Sails Work
     
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  6. Richard Atkin
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    Richard Atkin atn_atkin@hotmail.com

    Wow...you have filled my Christmas stocking! Thanks very much Brian. :)
     
  7. tspeer
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    tspeer Senior Member

    The gap has a significant effect on the performance. Unfortunately, unless you make the gap very small, you're not likely to improve things all that much.

    If you design a rig for minimum induced drag, considering the gap, and redesign again as the gap changes, this is the tradeoff between the induced drag and the height of the center of effort:
    [​IMG]

    This is not the same as the performance change due to lifting up the rig to change the gap - that will incur a greater performance change because the sail planform isn't being redesigned to help counter the change. So this may be taken as being somewhat optimistic.

    But it shows that changing from, say, a gap of 5% of the luff length to 10% of the luff length will increase the induced drag by on the order of about 6%. If you could reduce the gap to well under 5% of the luff length, then you could make big gains.

    "Gap" in this context means "carries no lift." If there's no lift provided by the hull, it counts as part of the gap, too. Chances are, the hull does carry some lift, but estimating how much is difficult. The hull is very long, so even if the lift coefficient is small, there's a significant amount of area and the lift could be more than you'd expect.

    This spreadsheetwas used to calculate the results above, and could be used to investigate the question if you include the jib planform as well as the mainsail (and maybe even some area for the hull).
     
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  8. Richard Atkin
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    Richard Atkin atn_atkin@hotmail.com

    Great stuff Tom. This helps me to get some perspective on whether I go for a windsurfer-style sail, or whether I try to 'seal' the foot. Plenty of food for thought. Thanks.
     
  9. tspeer
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    tspeer Senior Member

  10. Richard Atkin
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    Richard Atkin atn_atkin@hotmail.com

    aaaah....excellent

    Thanks again
     
  11. edvb
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    edvb Junior Member

    Squaretop sails

    Interesting discussion

    I am about to have a squaretop made for my Outrigger Sailing Canoe. Here is a quick video on how it sails with a 85 Sq Ft. main.

    http://www.youtube.com/watch?v=6I_7uDYg0QQ

    Tom after reading this do you think It will work Ok?

    I have been tweaking my Raptor for about three years and stage two is almost done. Instead of an 18' carbon mast I went to a 22' carbon mast. The sail is going to be a triradial design with a 21' 9" luff and a 8'4" foot. It will have an 15" squaretop and have 4 to 5 3/16" round fiberglass battens angled like the Hobie Bravo. The sailcloth will be a square Kevlar composite. It is 117.5 Sq Ft. versus 90 Sq Ft. on my old one. The Raptor in the video has 107 Sq Ft. But has a sail track and slab reefing. Mark also put stays on the mast. My Raptor will have Downhaul, Outhaul with Outhaul car on a 40" track and a traveler system where the traveler can be cleated at any position. I am also installing a new carbon boom the same size as the mast and it will have very little flex.

    Any thoughts before the sail is made?

    Before:
    [​IMG]
    After:
    [​IMG]

    Standard sail reefed. This is with a 24" track on the boom. The new one will be 40" At least you get an idea how leach tension and clew angle is maintained.

    [​IMG]

    [​IMG]

    [​IMG]
     
  12. tspeer
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    tspeer Senior Member

    I don't see why not. I'm a fan of square-tops and your planform looks reasonable to me. I was wondering how you were going to handle the battens, and now I see they are evidently flexible enough to wrap around the mast.
     
  13. brian eiland
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    brian eiland Senior Member

    Well thought out. I think she looks great. Hope you can handle the extra power
     
  14. edvb
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    edvb Junior Member

    Thanks Tom and Brian for helping me out on this.

    Just a few questions.

    I tried to space the battens about where they would be if they were horizontal.

    The top three measuring the leech are 3.25' apart. The two bottom ones going to the foot are 4.5' apart.

    The Sail has a luff sleeve that goes around the mast.

    These are the same size battens that are on the Hobie Bravo sail that has a 30" Squaretop verses the 15" that I have.

    The battens are very flexible until the wind fill the sail and they bow in one direction.

    The sail size, Foot and squaretop were determined by the CLR and CE of the old sail. The new one has the CE in just about the same place. The foot was reduced 6".

    Questions:

    Would you change the size of the Squaretop in relation to the foot to keep the CE in the right spot?

    Will the battens at that angle affect the airflow around the sail.

    Do you think the spacing is about right or do you feel it needs more or less battens?

    Do you think having the battens at an angle is a better idea than having them vertical?

    Thanks for any help on this before the panels are cut.
     

  15. edvb
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    edvb Junior Member

    Come on guys help me out here.

    One last question.

    With the angle of the battens are you better having webbing on the luff end so the end of the batten touches and are keyed off the mast through the webbing and then having the sleeve sewn an greater distance toward the leech to a have a better airfoil shape ?

    Or does it not matter that the batten ends at the end of the luff sleeve, a few inches from the mast?

    This is a new approach to a furling mainsail and I want to get it right the first time.

    I hope I said this right and you understand what I am saying.

    Any of the questions answered helps. Thanks
     
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