rotating freestanding wing mast section and mast bend

Discussion in 'Sailboats' started by Timothy, Jun 22, 2010.

  1. Timothy
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    Timothy Senior Member

    I am trying to determine how I should construct a rotating freestanding wing mast. I want to make the mast as light as possible and still maintain appropriate bend characteristics for gust response . My feeling is, that bearing in mind that this mast is for a lead mine, and that the boat is going to heel in the gusts rather than accelerate like a multihull, in order to automatically twist , flatten and loosen the leach of the sail it would be better for the mast to bend along the long axis of the mast section in line with the pull of the sail ,rather than in the direction of the short axis. I have decided that a 2 to 1 cord ratio is appropriate for what I want. I could build a tube or a box of carbon fiber and add nose and tail fairings of lighter and more flexible materials, or I could build the mast as a wing section in two cored halves split along the long axis. Alternatively I could build it in two haves split along the short axis and use the join overlaps to provide thickness and extra stiffness across the short axis. I am not an engineer but I think that it would be easier to design a laminate schedule for the tube or box with fairings than the other options. My computer tells me that providing I can find appropriate materials for the fairings this could also be the lightest option. Am I overly concerned about the direction of the mast bend? Rob Denney is building a rotating free standing mast made from two resin infused flat panels. He indicated that this was the fastest and easiest way to build a mast that he has used. The mast is for a multihull so mast bend direction is not a concern.
    On the other hand I know that the masts on Team Phillips were made from tapered tubes with staggered fairings with membranes, and that Eric Sponberg has used both tube and box structural elements with fairings and has also designed an elliptical section with the wall thickness varied to control mast bend direction. Any thoughts on the matter would be appreciated.
     
  2. rob denney
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    rob denney Senior Member

    At 2:1 I am pretty sure you will be able to engineer fore and aft bend into a tube without fairings. Fairings need to be pretty solid to handle the compression from full length battens and big roaches. The sail track also complicates the stiffness.

    Mast bend requirements are the same for any boat. You want it to depower at a given wind strength. With a small chord mast, you also need shape in the sail, so bend will take it out. With a large chord mast (mine is 20% of the sail chord), the sail is virtually flat. Any shape will show up as a bulge behind the wing. These sails depower best by bending sideways or by under rotating the mast.

    Not sure what your mast specs are, but we are currently building an assortment of 17m long freestanding wing and tube masts for 4 x 12m biplane cats. Some of them are also folding and some are mounted on stub masts. As it is a bulk order, the prices are pretty good. eg $16,000 for a stub mast mounted folding round tube mast $Aus16,000 If you want a price, please let me know your specs. The fold means it will fit in a container.

    rob
     
  3. Timothy
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    Timothy Senior Member

    Rob . Is that 17 meters from the deck or mast stub and bury? I would think the RM of a 12 meter cat has to be greater than a 12 meter lead mine. Could you tell me the estimated weight of the wing mast ? Is the wing tapered? Is the round tube mast that much lighter? What are the bearings made from and are they included?
     
  4. rob denney
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    rob denney Senior Member

    17m overall, including bury. RM is about 18 tonne meters (6 tonne boat, 6m wide). Yours will be a fair bit less, I think. Suspect you will err on the safe side if you work on the weight of your keel times the freeboard plus draft.

    Not sure of the weight due to the folding echanism and the stub mast, but the one piece mast on a similar rm boat weighed 120 kgs.

    Wing weight is very section dependant. If it needs a sheer web, or core in the sides, it is a little heavier and more expensive. These ones do and are $22,000. If they did not, the wing would be lighter. The stub mast and the folding mechanism make up a significant portion of both these prices.

    Wing is tapered in both section and laminate.

    Bearings are acetal or UHMWPE. Costs a couple of hundred dollars to get them machined.

    rob
     
  5. idkfa
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    idkfa Senior Member

    "a sheer web" could you explain a bit? 45deg fibers? tks.
     
  6. rob denney
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    rob denney Senior Member

    a web up the middle of a wing mast to keep the flat sides apart. Usually thin foam, full of holes and with +/-45 fabric each side. A 2:1 mast won't need one.

    rob
     

  7. Eric Sponberg
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    Eric Sponberg Senior Member

    Mast advice

    Timothy,

    Much of how you build the mast depends on what skills and equipment you have at your disposal. Many of my mast designs are built on male mandrels suspended at the ends. The mandrel position circumferentially is rotated and set by hand, and then 0/90 cloth, +/-45 cloth, and unidirectional tapes are laid up onto the mandrel in a prescribed sequence so that the maximum strength and stiffness are obtained for the least amount of weight. The mandrel can be a whole section, or a half section (either port-starboard halves or LE/TE halves). If using LE/TE halves, then they can mate at the shear web in the middle. I really like shear webs because they hold the section in shape. Without the shear web, extreme bend can cause the section to collapse and therefore lead to premature buckling of the mast.

    In general, you want the smallest size of mast section that is consistent with strength and stiffness. That, coupled with a minimum wall thickness to prevent premature buckling (generally, 3% of mast diameter, minor diameter in the case of a non-round section) you end up with an ideal mast section and weight for any given boat design. I prefer an entasis taper which has slightly larger mast sections up high than a straight taper mast. This affects strength and bending, of course, and tends to keep the mast weight lighter rather than heavier.

    How much bend is appropriate? You want enough that will allow the wind to spill out of the sails but not so much that the sail inverts. A small percentage of bend is good (tip deflection/mast height above deck). If the top of the mast bends as far as the width of the deck edge in way of the mast, that is way too much. A non-round mast will actually twist slightly as it bends to the side and pulls in line with the mainsail fabric, so there is always a certain degree of side bend as well as fore-aft bend. On a rotating mast, it is not too important to calculate ahead of time what that twist will be. If you design the mast as if it is bending only sideways, and that it is strong enough and stiff enough to handle that load--and that is the worst case real-life scenario--the mast will behave acceptably under load.

    As for bearings, I like ball and/or roller bearings, and I prefer metal bearings over plastic bearings. On my plastic bearing designs, where the bodies of the bearings are aluminum and the balls are Torlon, I have found that the balls squeeze out of round and bind a lot under load. Stainless steel bearing balls or rollers are what I prefer. Journal bearings (sleeve bearings with no balls or rollers) I have found to bind way too much except on the smallest of boat designs. On a huge wingmast for an 80' sloop that I have recently finished, we are going to machine the bearing bodies out of aluminum ingots and fit them with stainless steel rollers. This is due to the preference of the bearing builder. Aluminum and carbon fiber don't mix well (aluminum corrodes really quickly) so there has to be generous insulation between the dissimilar materials. If I had my drothers, I would design the bodies out of Silicon Bronze which is easy to cast and machine, it has very favorable strength and stiffness properties, and it is high on the galvanic scale so that corrosion would be much less of a problem.

    Finally, as I am sure you are finding, you have to be very careful with the lay-up sequence of the carbon fiber laminate to make sure that you have enough fiber in the right direction and in the right place within the laminate so that the layers and UDR strips are properly "balanced", meaning that they are placed for the most efficient load carrying ability. You also want a laminate schedule that minimizes the amount of cutting that you have to do. In my laminates, UDR is never trimmed except for length, only the 0/90 and +/-45 layers need trimming and placing. Also, fairly precise metering of the epoxy resin to the laminate is required for proper fiber content (directly proportional to strength and stiffness) and that the laminate has to be applied in stages to allow vacuum bag consolidation of the layup to compact the fibers. Resin infusion for carbon fiber laminates so far has proven very difficult to accomplish successfully. The only other good method of layup is with prepregs. Either way, the laminate requires periodic consolidation at various stages during layup. Finally, the mast really should be post cured according to the resin manufacturers recommendations to bring strength and stiffness up to the required design properties. To confirm those properties, you should do sample layups ahead of time, incorporating the proper mix of fibers and fabrics that you will have in the laminate, and have them tested in a laboratory for compression strength and modulus, tension strength and modulus, and shear strength and modulus so that you know what properties to design to.

    Those are some general guidelines for mast design. I hope that helps.

    Eric
     
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