Mast building.

Discussion in 'Multihulls' started by redreuben, Aug 18, 2012.

  1. redreuben
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    redreuben redreuben

    Hi people, I'am just a bit curious;
    When building anything tubular, like hulls, floats and beams bulkheads are used to keep the surfaces in column and greatly increase stiffness.

    Why then is this not done with masts ?
     
  2. cavalier mk2
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    cavalier mk2 Senior Member

    It actually is on wooden masts in the areas that take the spreader and stay loads.. On a aluminum stick there is usually a spacer around the bolts going through the mast for the same reason.
     
  3. TeddyDiver
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    TeddyDiver Gollywobbler

    Depends of the material thickness and inner diameter.. You could use them in mast and spars and I have seen some spars with them. The main reason is to prevent buckling..
     
  4. PAR
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    PAR Yacht Designer/Builder

    If "on masts" you're referring to aluminum extrusions with standing rigging holding it up, the wires serve to prevent buckling, by keeping the mast in column. Stayed wooden masts also rely on this, but have localized reinforcements (swallowtails) to prevent localized crushing and compression loads, plus eliminate stress risers within the mast walls, as a result of attachment and reinforcement stiffeners.
     
  5. redreuben
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    redreuben redreuben

    I guess I was thinking aloud, I was mostly thinking of fabricated masts like carbon tubes. Very difficult to do with an extrusion !
    Many wingmast builds seem to use frames and skin.
    I understand the use of compression posts and blocks in timber masts, just thought it would be useful in general to keep wall thicknesses and weight down as well as reducing the weight/complexity of stays by stiffening the tube in the first place.
     
  6. Eric Sponberg
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    Eric Sponberg Senior Member

    The others above have answered the question adequately--extrusions, wall th./dia ratio. I have done wingmasts both ways, with and without bulkheads. On Saint Barbara (http://www.sponbergyachtdesign.com/SaintBarbara.htm), the wingmast is a hollow tube, but the side walls are thicker, with more unidirectional fiber, than the end walls. This compensates somewhat for the differences in the ExI factor sideways compared to fore-aft. This mast, according to the owner whose idea this was for this type of laminate, reports that the mast is amazingly stiff, and the boat beats all comers. He built the mandrel himself (he owns a huge sign building business) and sent it to Composite Engineering in Concord, MA, for laminating.

    On other designs, such as for Copernicus, the GT80, and now my Globetrotter 66,
    http://www.sponbergyachtdesign.com/Copernicus.htm
    http://www.sponbergyachtdesign.com/GT80Mast.htm
    http://www.sponbergyachtdesign.com/Globetrotter66.htm

    I am using small web frames spaced as intervals which assist in keeping the mast from buckling. I have never really cut back on the laminate weight or thickness--I engineer that first as if the webs were not there. So my internal structure is added weight. The stations are spaced far enough apart where there still could be some local buckling in the mast wall if I were to cut back the wall thickness. As many of these as I have designed, I have not found that optimal design yet where I can really pare the laminate weight down to make up for the added weight of the internal structure. The more bulkheads, of course, the more laminating, fitting, and gluing, and so the more labor is involved in building the mast. Labor is time and money, so there comes a point of diminishing returns cost-wise as to how many bulkheads you put in. I have not found that point yet on my designs. Mostly, the bulkheads, shear web, and leading and trailing edge structures greatly facilitate building the mast, particularly for non-professional mast builders. You build the mast from inside out with these structures, putting in all the internal stuff like sheave boxes and conduits for electrical, lightning, and halyards, etc. You can add local reinforcement as needed. Then the whole thing gets skinned over with the carbon fiber laminate.

    On Copernicus, the internal structure is light plywood and veneer--it turned out pretty heavy, the whole mast ending up about the same weight as the original stayed mast. The boat performs much better than before, according to the owner who really loves the rig; he sails faster and points higher, mostly due to the lack of rigging drag and the new mainsail shape which has a lot of roach in it that he could not have before with the stayed rig. On the GT80, I cut back on the plywood, and also substituted foam and carbon for the shear web and stations. I still used wood-epoxy veneer on the inside side skins. On the G66, I am getting rid of as much wood and wood-epoxy as I can to keep the weight as low as possible, going all foam and carbon. Replacing the side veneers will be a one-ply thickness of carbon double-bias cloth, prelaminated, then bent and glued into place.

    I developed this technique for wingmasts back in the '80s when I wanted to show how to build a sophisticated wingmast in your own back yard. I used this technique for the mast on my own stock plans for the Delft 25 (http://www.sponbergyachtdesign.com/DelftPlan.htm). Details were written about this mast design in Professional Boatbuilder Magazine, Issue #14, Dec/Jan 1992, in an article, "WIng Masts", by Ted Hugger. That article also described the Gougeon Brothers tortured plywood "Stresform" method of constructing wingmasts. GB used plywood transverse stations in their designs to act as formers--they were removed after the side walls were glued on. These masts were plywood only, they had no carbon fiber on them.

    Eric
     
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  7. cavalier mk2
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    cavalier mk2 Senior Member

    Eric shouldn't you be able to go with lighter walls/skin using the web frames? The engineering would suggest it.
    I like the stressform masts but plans seem impossible to come by, does anyone have any scantlings for them?
     
  8. DCockey
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    DCockey Senior Member

    Keep in mind there are several different failure modes for a column. The one covered in into level structural design is overall column buckling which depends on the section modulus. One way to try to take mass out of a column is make the skin thinner but increase the overall section size so that the section modulus remains sufficiently large.

    But there is another failure mode which can occur as the skin becomes thinner even when the section modulus is large enough to resist overall buckling. It is localized buckling of the skin due to the axial stresses in the skin and is not directly related to overall section modulus. An example of this mode is an empty aluminum beverage can. Load it axially and it won't globally buckle. Rather the skin will locally buckle. One way to prevent localized buckling is to add bulkheads or stiffners on the skin. Of course these add mass which reduce or eliminate the mass savings of the thinner skin.

    And then there are the loads on a mast which are not "in column" such as those from the sail luff, stays, boom/gooseneck, etc. These can also cause failure, again both global deformation and local deformation.
     
  9. Eric Sponberg
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    Eric Sponberg Senior Member

    Theoretically, yes, I should be able to do that, but I have not had the money to do the necessary testing that would allow me to test for the optimum design specifics--wall thickness vs. station spacing vs. section width and length, for example. Carbon fiber masts are big expensive structures, and such testing would cost A LOT of money. I have not had a client for a mast that would consider spending the funds to do such testing, and I could not afford to do it myself out of my own pocket. Well, I could, but I would have a hard time justifying that to my wife and family.

    Based on the research and testing that I have done (quite a lot for a client who is a fiberglass flagpole manufacturer, and we test his poles regularly) I do know that for composite tubes, the wall thickness to inside diameter ratio, t/ID, must be at least 0.03 (3%) in order to avoid premature local buckling under a bending load. I use this threshold value in my designs. It actually helps to define an optimal design for any given mast. You always want the mast to have the right moment of inertia and section modulus, and you can always get that with increasing section size. But you also want the smallest size of mast possible to reduce drag. So the t/ID ratio helps to determine exactly the right size for the mast structurally. Fortunately, this usually comes out to reasonable weights and costs when specifying carbon fiber.

    And there are other factors that feed into preventing buckling, such as the ratio between on-axis fiber to off-axis fiber, the fiber content by weight, the resin used, etc., etc. But I have always steered away from using cored skins in my masts because it adds complication and weight--a cored laminate would not reduce the required total carbon fiber skin thickness and weight. The core only adds weight and building complication. Therefore, solid skin masts are the lightest, cheapest, and easiest to build.

    Eric
     
  10. cavalier mk2
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    cavalier mk2 Senior Member

    Thank you Eric, very educational. I remember reading about how Nathanial Herreshoff tested mast models to destruction then scaled them up when working out ratios. Still that was for racing where people want to spend the money to optimize and are willing to take on the risks of development.
     
  11. Eric Sponberg
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    Eric Sponberg Senior Member

    I am sure that Nat Herreshoff also had an R&D budget, good companies do, and a company usually has a lot more discretionary R&D money than an individual has.
     
  12. Emerson White
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    Emerson White Junior Member

    Bulkheads within a mast I think would reduce the flexure of the mast by reducing it's ability to compress and distort along an axis perpendicular to the major axis. However, is flexure necessarily a bad thing? The round shape of the hollow mast distributes stresses evenly. If you were to put bulkheads in the shaft I think you might just be concentrating your stresses at one point and contributing to a failure, which can then cascade to greater damage.

    I think a wonderful biological example of a tube with bulkheads is bamboo. Bamboo needs the bulkheads for biological functions that a mast simply does not have to deal with, but in order to deal with the 'bulkheads' (called a diaphragm) the bamboo thickens the area incredibly, contributing to a higher weight and a higher material cost (carbon isn't free). Now I don't work with bamboo and so I don't know if the failure point is at the nodes, in the inter-nodal spaces, or if they are evenly distributed, but I would think that we could follow bamboos lead and view the bulkhead as an expensive addition to a hollow pole.
     
  13. Silver Raven
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    Silver Raven Senior Member

    Eric - I'm sorry about the above 'error in position'. However I do hope you can be bothered to sort it out. Here's with great hope. Ciao, james
     
  14. Eric Sponberg
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    Eric Sponberg Senior Member

    Emerson, it is precisely that ability to "distort" that is the problem with composite tubes in bending. You don't want that distortion to happen. Which is not to say that you don't want the mast to bend, you do a certain amount, but the distortion causes it to bend too much. Here is what happens when the mast wall is too thin, and/or there is not enough fiber in the circumferential direction: As the mast bends, the the section shape begins to collapse. That is, the tension side and the compression side physically move closer together. This causes the moment of inertia (I) and section modulus (SM) of the section to decrease. This reduction in I makes the mast less stiff--it bends even more, and the reduction in SM causes the stress to rise too much too soon. So the behavior of the mast is not as predicted. Ideally, you would like I and SM to remain the same during the full range of loading. This is less of a problem on round or nearly round cross-section masts. The more a mast section looks like a wing shape, the more it requires a shear web which holds the sides in position and takes a lot of the shear load. This shear web is oriented transversely going up the mast.

    Alternatively, or in addition, you can have horizontal webs inside the mast which also do the same thing as the shear web--they help the mast keep its geometric shape under load, but unfortunately, bulkheads do not carry any shear. So in that regard, they are less useful than a shear web. If you are building a mast on a mandrel, which involves laying it up with no internal structure, putting in bulkheads or a shear web is nigh on impossible to do. But mast building by using a male mandrel is relatively easy and ideally suited for production masts when you are building a lot of them.

    In my one-off wingmast designs, I have both a shear web and horizontal stations (bulkheads). See the picture below which is an excerpt from the laminate schedule drawings for the mast on one of my a recent designs. I do this because in one-off construction, we don't have the benefit of a mandrel to create the shape of the mast--that has to be built first, kind of like building a plug for a boat hull, but in this case, the plug stays inside the mast. This internal structure is relatively light--in my current design that I am doing now for my Globetrotter 66, all this structure is two skins of +/-45 carbon cloth on 1/4" (6mm) thick Core-Cell foam core. The shear web and bulkheads are connected along the trailing edge and leading edge with carbon-foam nosings, so the whole looks like an egg crate, more or less. All this structure, captive in the mast, allows for building and positioning of all the other stuff inside the mast at the same time, for example sheave boxes and conduits for electrical wiring, lightning protection, and the halyards--you put all that stuff together first, closing off the shape with a one-ply layer of +/-45 carbon fiber cloth. In the picture below, I used plywood for the nosings and thin hardwood plywood for the side walls, but I've improved this with all-carbon this time. So now, you have all the structure set up and put together--it looks like a mast, but it does not have all the carbon fiber wall thickness put on yet. That goes next, building up strip by strip of UDR and layer by layer of +/-45 and 0/90 cloth in a very precise layup schedule. Once built, it is very easy to rig because all your internal stuff is in there and held well in place so it doesn't go banging around inside or getting tangled.

    All this internal structure does add a bit of weight to the mast but it makes it relatively easy to build the mast when you don't want to invest in a mandrel for a one-off build. It adds to the structural rigidity of the mast under load so that the walls won't collapse. I dare say that if the mast bent to a really great extreme, you might be able to see slight buckling between every station, but in normal sailing conditions, the mast never bends that much so you don't see it. It also bends predictably--I calculate what the bend will be during design, and I know it will be pretty close to that because the section shape is always staying the same during bend--the sides are not collapsing.

    Eric
     

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  15. Eric Sponberg
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    Eric Sponberg Senior Member



    Silver, I am less familiar with the A-class cats and all that might be entailed in their behavior. Why is are the smaller size mast thought to be too small--do they bend too much, or do they seem limber in any way? If so, then making the mast bigger in section would solve this. However, it is generally considered that in any rig, the mast makes up a sizeable proportion of the total drag of the rig. In his 1978 paper from SNAME, "Effects of Masts on the Aerodynamics of Sail Sections," author Dr. Jerry Milgram's concluding remark is: "Although the form and friction drag of sails without masts is small in comparison with the induced drag of most three-dimensional rig geometries, the addition of a mast to a sail raises the sum of its form and friction drag to the same order of magnitude as the induced drag of that sail."

    That is, the mast is a significant producer of drag, and any effort to minimize that effect is effort well spent. Drag causes the resultant force of the rig to point a bit sideways, contributing to more side force in the rig at the expense of less driving force. Reduce drag, and you reduce side force, and put more of the energy into driving force which makes the boat go faster. Making the mast smaller, consistent wtih strength and stiffness, is beneficial.

    So, that is why I ask, what is going on with the behavior of the A-class cat rigs that would demand a larger mast? Are they not strong or stiff enough, or is something else going on there?

    Eric
     
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