Bermuda Rig Schooner Vs. Bermuda Sloop

Discussion in 'Boat Design' started by Bill PKS, Sep 28, 2009.

  1. PAR
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    PAR Yacht Designer/Builder

    Timo, yes, side by side masts have been tried. Another problem is the wind shadow of the mast on the extra stick doesn't help drag issues much. Even though the jib/main combination on a sloop does divide the rig into two luffs, the jib's luff hasn't the mast shadow the schooner has.
     
  2. Bill PKS
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    Bill PKS Junior Member

    I was always told that a sail went to windward because of an airfoil (airplane wing shape and old Bernoulli's effect) As wind flows faster around the forward leeward side, it creates negative pressure pulling on the sail. On reaches and down wind it’s more about wind pushing on surface area. More SA = More to push.
    So it would seem that if I maximize the Airfoil and reduce drag on trailing sail edges for pointing, with 2 masts, I can rig all kinds of Stay sails, Jibs, Chutes etc. for off the wind.
    But is there an inalterable turbulence between foresail and main that reduces pointing ability? If so, why doesn’t that occur between Jib and Main on a Bermuda Sloop? What about Jib, Foresail, ( Riggable ) Staysail, Main?
    Bill PKS
    While I still might not be able to beat a Sloop to win'ard, I'm trying to understand the aerodynamics, so I can get all I can get.
     
  3. Timothy
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    Timothy Senior Member

    Par I wonder if you would venture a guess as to why Nigel Irens would rig his latest design as a square topped cat schooner , with a boomless overlapping fore sail as apposed to a freestanding single masted sloop along the lines of say Eric Sponberg's SC 40( I realize That they are two completely different boats with completely different missions)? I cannot aside from ascetics see the purpose of the fore mast. I apologize if I gave you the impression that the M2 is a biplane rigged cat in my earlier post.
     

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  4. PAR
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    PAR Yacht Designer/Builder

    You've answered your own question Timothy with "( I realize That they are two completely different boats with completely different missions)".

    I could begin to guess at the design briefs in general, but what would be the point. Phil Bolger quite correctly described the schooner as a cutter with a mast in the middle of the fore triangle. It's helpful to think of a schooner this way.

    Bill the fastest sail powered craft in the world have just one sail.
     
  5. tkk
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    tkk Junior Member

    Bill PKS, this is the way I see it.

    Even if we forget the extra windage and masts shadowing the sail there is one principal difference between split sails vs one high sail. The split sails work like one wide (long cord) foil as opposed to the narrow cord high aspect ratio foil.

    The energy from the sail comes from the lift created by the foil when it bends the airflow, the larger the flow bent and the more it is bent, the more force you get.

    Only a certain depth of an airflow can be bent by a foil, so the size of the flow is dictated mostly by the span of the foil. The long cord helps bending the flow more and this is where schooners and other split rigs can compensate.

    But this bending amount has a limit: to create forward driving force the airflow at the trailing edge of the foil cannot be to weather of dead aft of the boat.

    When reaching a schooner can compensate the lower foil by sheeting the sails gradually tighter from jib to main and making the camber of the combined foil deeper. This will make the airflow to bend more. But when pointing (almost) to the wind this would just make the lifting force pointing straight abeam or even abaft of that. This would just heel (or capsize) the boat but not propel it anywhere.

    If the masts where side by side, like a biplane aircraft on its side, we could add the spans of the two foils because they work independent of each other. But then we would need to install the masts on a turntable to be able to have the masts always side by side relative to the apparent wind!

    Wing and wing with a kite between the masts would probably be the way downwind.
     
  6. Bill PKS
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    Bill PKS Junior Member

    PAR,
    But a windsurfer wouldn't be apples to apples,,, unless the guy has 4 hands??
    I think I'll do some drawings this weekend to try to explain my dilemma.
    Bill PKS
     
  7. Bill PKS
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    Bill PKS Junior Member

    Folks,

    Attached is an attempt at analysis of a Sloop vs. Schooner Rig, basically trying to figure out why a Sloop is faster to Windward than a Schooner.

    Doing this, I was reminded of the statistician with his feet in ice water and head on fire, commenting that on average he was doing fine.

    Thanks for any insightful comments.

    Bill PKS
     

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  8. tkk
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    tkk Junior Member

    My first comment is what I already said in my earlier post: the two sails don´t work as two independent foils. They are so close to each other that they work as one wider (low aspect ratio) foil.

    The more close hauled you are the closer the foils are. On a reach they are further apart and that is indeed when schooners shine.
     
  9. Bill PKS
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    Bill PKS Junior Member

    TKK,
    Why wouldn't a Jib cause the same problem with the Main of a Sloop, particularly as you consider the areas above any overlap ?
    Bill PKS
     
  10. tkk
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    tkk Junior Member

    I think it does, but in a schooner you have one more sail.

    I think it all boils down to "effective aspect ratio". If the masts and sails could be kept far enough from each other I would quess that what you are saying would be true.

    But they should be far enough on all points of sail, so the masts should be on a turntable and rotated as a whole, not just the sails around the masts. That way the apparent wind would see the masts and sails side by side and not one after another.
     
  11. Bill PKS
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    Bill PKS Junior Member

    TKK,

    Looking at the sketch above the Sail plan, it looks to me like when pointing up, the Sails are positioned to reduce "connectedness" as much as a jib would be .
    If anything there might be a venturi. I used to hear about the " Slot", but have heard the " Slot" theory was disproved ... I don't know why.
    If what you are saying is right, why should a boat have a jib? Just use bigger sails and masts ?

    Bill PKS
     
  12. tkk
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    tkk Junior Member

    I think the catboats are the best at pointing as high as possible, which is exactly what you are saying. Dividing the sail area is mostly done to reduce the size of an individual sail to a reasonable size for people to handle.

    Or to get mor strings to pull if you are just fooling around and not seriously going anyplace. For people like me, who want to build a schooner with 15´LWL just keep me and the kids occupied :D
     
  13. Bill PKS
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    Bill PKS Junior Member

    tkk,

    Good points .... I'm thinking .............I need to dig around for more information on the " slot".
    Schooner eh?
    Attached are sketches re Sails for a 14' skiff I'm heading toward building to test hull shapes preparatory to a larger schooner. ( 14' because there is no registration under 14' in NC.)
    No valid sketches on the hull yet.
    Welcome any comments.

    Bill PKS
     
  14. Bill PKS
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    Bill PKS Junior Member

    tkk,
    Sorry,, Sketches didn't attach .
    Bill PKS
     

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  15. Schoolbus
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    Schoolbus New Member

    About the idea that sloop single masts sails are faster than schooner double mast rigs, going to windward anyways, I want to point out that the International Fishermans Trophy was won for many years by a very fast schooner, the Bluenose, which could go 15-16 knots or so I read on the internet. Tea clippers could go even faster. It seems multimast ships could sail at speeds that would make some modern sailors envious? And they did it with heavy wooden decks, canvas sails, wooden blocks etc......this was a working fishing vessel...
    schoolbus
     
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