Mast building.

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

  1. Eric Sponberg
    Joined: Dec 2001
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    Location: On board Corroboree

    Eric Sponberg Senior Member

    In my wingmast designs, I don't even pay attention to the ratio of size between the mast chord and the sail chord, and that is because, it would probably make the mast way too big. It would have too much sail area, too much weight, and be overly stiff. Let me qualify that by saying I have not been involved that much is racing boats, so craft like the C-class catamarans are a different sort all together, and the following does not apply.

    I design for strength and stiffness. That necessarily makes my mast sections small in relation to the chord of the soft sail. It also keeps weight and cost down to fairly reasonable levels. That is to say, a big mast section is heavy and expensive, two things that are anathema to virtually all sailors.

    The other thing that I do, which I have reported before, is I use elliptical sections for my wingmast sections. I attach a picture of a few typical sections from my design for the GT80 wingmast, taken at the top of the mast. I use an ellipse because the leading edge shape is very forgiving to airflow--the flow stays attached easily to the nose and surface of the mast. There is a small separation bubble that usually occurs right behind the mast, but the flow quickly reattaches to the sail. this bubble is trapped by the boundary layer, and so the rig is "aerodynamically smooth", as aerodynamicists have been known to say.

    The other reason that I use the ellipse is because it is very easy to engineer. The cross-sectional area, the moment of inertia, and the section modulus can all be calculated each by simple equations. This way, I can easily vary required wall thickness in a spreadsheet at each section going up the mast with minimal and quick keystrokes. If the mast section were a truer aerofoil, then each section becomes a more laborious calculation using another integration spreadsheet, and you have to check that at regular intervals going up the mast. For the GT80 mast, I did that at 500 mm intervals. For my Globetrotter 66, and typical of most of my mast designs, I do that at every foot (12", 300 mm) along the mast. Doing this for a true wingmast section would just take a lot longer for what I see as little perceived additional benefit, at least aerodynamically. Both sections work about the same--maybe there is less separation bubble behind a truer wing, but then again, there is the greater chance of a knuckle edge on the leeward side, and flow separation, if the mast is over-rotated, or a bigger separation bubble on the windward side. I see my mast sections as quite forgiving in use.

    I tend to go for 2/1 ellipses, with the chord length 2x the thickness. Some people go narrower; the Gougeon Brothers' Stresform masts, for example, were 3/1 wing sections, and even they were not true aerofoils shapes, just simply tortured plywood, more lenticular in shape than a true aerofoil. That's all fine and dandy--the narrower the mast, the more flexible it will be, which can be a concern if it bends too much, and the harder it will be to keep flow attached. That is, like a rudder or a keel, narrower cross-sections have narrower windows of acceptable angle of attack where flow stays attached. Thicker sections have a wider window of angle of attack for attached flow. Over the years, I have settled on the 2/1 ratio.

    Eric
     

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  2. frenette
    Joined: May 2011
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    Location: Southern California

    frenette Junior Member

    I don't claim to be in this league but I start out with the loads, and mast/sail shapes I want and work back from there. You can build wing masts that bend fore/aft to be flattened with the main sheet for example.

    Based on trial and error racing and F27 to F28s the thicker mast idea appears to be bunk in real life. My mast is what about 25% thicker than the factory f28 mast. The only point of sail this should hurt at all is going to weather but I can't tell the difference boat for boat. Going down wind thicker is a plus as you can bend the air more. The sail area in the mast vs the rest of the rig is so small is again in the scale of doesn't mater.

    What maters in this order seems to be:

    1) The mast is a drive shaft so if it can't transfer the load effectively to the boat it's going to be slow. There is a concept of mast bending to enhance sail plan shapes but this needs to start with the concept of flexing is a loss of energy so the shape change must pull more energy than it looses.

    2) Weight of the best shaped mast in the world won't even come close to compensation for pitching motion. Drop a couple of pounds out of the top of the mast and loose 1 bob on every wave. This adds up.

    3) Shape should be soft enough to flow even when the mast is off wind direction. All these nice papers show mast in best flow vs mast in best flow for the total rig. Going to weather you need the mast straighter in the air flow than you would expect because you can't close off the slot without loosing power. An extra 1/2" on a 38' mast hasn't made enough difference to make the larger mast slower. I don't do ellipses because my designs have been built on a triangle model where the shear web has the bulk of the uni build out. The air flow on ellipse is however pretty forgiving. Think of mast rotation as just another trimming variable where you can power up or depower the rig based on what's needed. The rule of thumb is point the mast into the air flow. It's not quite that simple if you have a well arched aft shape as you can bend the air more going down wind pulling the rig foreword balancing the rudder gaining load as the boat speeds up.

    4) I have the fullness pulled foreword so the mast will feather into the wind without sails. At some point larger (read furniture) boats need to sit at the dock etc. So this is why I've been using NACA in the 35 to 45% cord to thickness range. The thickest part of the mast in these shapes is about the first 25% of the mast section. This is a very forgiving shape.

    5) Where possible taper the strength as you go down the mast. Tapering the shape will to some extent taper the strength but masts fail in the lower 1/3 of it's length. If you build from the inside out you can put this extra in without effecting the shape. This is last on the list because the bigger the square top main the more you need strength to keep the mast in column. A little twist is a good thing but unless you racing class or near class boats it's rather a large amount of R&D to get this even close to right.

    I really enjoyed reading this thread.
     
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