Rotating Wing Mast – theoretical discussion

Discussion in 'Sailboats' started by Man Overboard, Nov 15, 2006.

  1. groper
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    groper Senior Member

    The engineered CF mast manufacturing specs ive seen, only use hoop ply fibres where there is heavy localized loads which would instigate "lozenging" such as at the freestanding rotating mast bearings, boom connection, or stay connections... bulkheads inside at these locations also... the rest of the mast has only 0 and -+45deg fibres...

    ive often read Erics remarks about mast size and he always prefers to go for the minimum section size for efficient engineering. This sounds intuitive, but its at odds with things ive read from tom speer etc in that the drag from a rotating mast, is largly offset by the sail attached to it, so i dont beleive you can look at the mast in isolation in this regard. If a thicker wing section is less finiky, then its likely going to be a better performer as you cannot always keep the wingmast in perfect trim, all the time.
     
  2. petereng
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    petereng Senior Member

    Erwan - You are better off buying a tube from CST composites in sydney. They make very good tube at a good price. www.cstcomposites.com but there must be a local winder near you? Cheers Peter s
     
  3. Panos_na
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    Panos_na Junior Member

    Very interesting topic.

    My thesis was on CFD Analysis of 2-D Rotating Masts.
     
  4. Erwan
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    Erwan Senior Member

    Peter,
    Yes CSComposite would be perfect if not sor far. There is a local winder, but I must provide the detailled winding, not only the EI like for CS probably, or similar industry player.

    With your different comments, I have already a clearer picture, thanks to Groper too.

    Cheers

    E
     
  5. petereng
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    petereng Senior Member

    Erwaqn - Well a std CST tube would be 70% UD and 30%DB so just get the mandrel size look at the CST sizing for thickness and the winder should have enough info or just tell me what they can do and I'll confirm good/bad cheers Peter s E=90Gpa
     
  6. sigurd
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    sigurd Pompuous Pangolin

    Hi Peter, that would be a generous offer, and quite interesting, though I feel like there is a bit more aerodynamic work and stuff before I can set it more in stone.
    Bearing centers could be about 1m, boom height over top bearing about 1m (wishbone on bearing around mast). I'm thinking that the bottom section has shear web and possibly is coreless (carbon), whereas the top has obviously no shear web but has corecell.
    So 12 m total, 3t.m absolute max RM, what is a good starting point for bottom section thickness, structurally - 15cm? 20? What info would you need?
    Here is a top section I am investigating, it is an xflr5 scaled tspeer P30212 to 33% and 6%. I think bottom section has to be a fair bit shorter, to make the top section less prone to lozenging (by substituting the leading and trailing edges for shear webs). The sailtrack is in the middle of the windward side.
     

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  7. petereng
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    petereng Senior Member

    Sigurd - The next step is to decide whether your mast is a monoque design (ie the shape is the structure) or you are going to get a round tube and the wing shape is a fairing. I usually run a round mast first and optimise it to establish a benchmark. Then move to the next step. Cheers Peter S
     
  8. Erwan
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    Erwan Senior Member

    Thanks Peter.

    Good idea.

    Cheers

    EK
     
  9. gggGuest
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    gggGuest ...

    Doesn't a lot depend on whether you are planning a flexible mast or not? In dinghy mast terms I've never been able to figure out how you could get reasonable mast bend characteristics on a long thin wing mast section.

    Bethwaite got round this on his wooden wing masts by having a roughly square and highly tapered structural component with a balsa wood fairing on the front.
     
  10. petereng
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    petereng Senior Member

    Yes - Its good if you know the flexure character first. But in this case for this size we are designing a mast that will have about 15% tip deflection at full load. ie the mast is 11m long, 1m buty so 11/100*15=1.65m max. So I'll design the section to support the agreed Rm but not fdeflect more than 1.65m Cheers Peter S So the next question is what safety factor would you be comfortable with sigurd? Or to the general listeners. I've designed with a SF=1.1 up to 5. LR uses 3 minimum. I usually get around this problem by providing the Sf=1.5 SF=2.0 and Sf=3.0 answer so the client can pick. Sometimes the difference between 1.5 and 2 is quite close so they pick 2 vs 1.5 etc
     
  11. sigurd
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    sigurd Pompuous Pangolin

    Hi petereng, don't see why I would use a tube. Maybe for the part that goes through the bearings. Plain wing with shear web lower section. Without shear web but with foam in the skins for the upper section.

    If I go with that sort of shunting mast as shown, flex would tension the halyard, so not desirable really. Not sure about how this type of rig behaves, are there any others?
     
  12. sigurd
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    sigurd Pompuous Pangolin

    As long as it does not break below the bearings it should be reasonably safe. It would fall to lee of any people, unless it broke the wrong way (taken aback).

    SF would depend on weight and cost, I really don't have the experience to tell what is a "safe safety factor", but the 3t.m RM is with a lot of big people onboard, more than the boat likes, probably SF 1.5 is enough...
     
  13. petereng
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    petereng Senior Member

    I consul against using a foam sandwich for a mast. Sandwich construction is great for a boat hull but wrong for this application. A sandwich is locally stiff but not globally stiff. The foam can do a thing called asymetric bending and it will "roll shear" It also runs the risk of only one side of the sandwich transferring load and failing again. Sandwich is not good for tubes especiallu masts. Plus the shear web may not be needed. I have not yet designed a mast that needs one. When using a stressed skin type of construction you need one tho help form the shape. But if the shape is moulded I don;t think you will need a shear web, unless the mast is very very thin at which point I don;t think it would be locally stable enough.Cheers Peter S
     
  14. petereng
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    petereng Senior Member

    Sigurd - re your flow analysis. You need to consider your software does not do a aero-elastic analysis? ie your sail shape is a fixed shape? If so you run the risk of making judgments on the wrong flying shape? You have a gap between the mast and sail so you are using bat cars? Just a note - all the free standing wing masts that I have done in which the wing transitions to a round for the bearings have had problems acheiving a good buckling factor in the transition area. So the base area ends up being heavier then expected. havn't done one in years so maybe I'm clevererer now so will be interesting. Peter
     

  15. sigurd
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    sigurd Pompuous Pangolin

    Ok. I think Rob Denney used foam in one of his masts? Rob, you there?
     
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