I call your bluff. The naked mast drag. A myth?

Discussion in 'Sailboats' started by pbmaise, Nov 17, 2013.

  1. pbmaise
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    pbmaise Senior Member

    I have heard this claim frequently.

    Now does anyone who actually knows about aerodynamics want to chime in here.

    I think the claim is false false false and there is no evidence to back it up.

    I have limited aerodynamics knowledge however, I do know that drag around a fixed object can be reduced by shaping it like a tear drop. The tear drop itself need not extend to the full tip of the tear drop. Rather the form need only be introduced and the wind will naturally follow the tear drop shape.

    Wind tunnel tests showed that a true tear-drop shaped body offered only a slight improvement in efficiency to the Chrysler Airflow design. In 1935, Georg Hans Madelung, a German engineer, professor, and aircraft designer, showed that a vehicle does not need a long tapered tail at high speeds.[3]

    I firmly believe that any sail behind a mast will increase drag on the mast. There is no such thing as extra drag when naked. It is a lower drag when naked.

    Before making this claim again, please read.

    http://en.wikipedia.org/wiki/Kammback
     
  2. gggGuest
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    gggGuest ...

  3. jonr
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    jonr Senior Member

    I have found that the drag of the mast is more than offset by the sail. So much so that the boat will go upwind with the sail up and it won't with the sail down :).

    > showed that a vehicle does not need a long tapered tail at high speeds.

    "Does not need" and "doesn't not benefit from" are very different statements. See the wikipedia statement with "he could gain most of the benefit..."
     
  4. daiquiri
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    daiquiri Engineering and Design

    Actually, it might not be so for "any sail". For some sails yes, for some no, for some yes and no - depending on which part of the mast you're considering. :)

    Take a look at this picture taken from Hoerner's "Fluid Dynamic Drag":
    Splitter plate.gif

    What you see are several 2-D shapes in an airflow with Reynolds number between 10000 and 100000. For a mast with a diameter of, say, 250 mm (around 10 in.) it corresponds to an airspeed of up to around 12 kts.

    The red rectangle shows a case which might be representative of a circular mast with a sail (in tension) behind it. The only thing that lacks is the sail camber, or curvature. As the caption below the pic explains, if the cylinder were naked, it's drag coefficient would be 1.03. With the so-called "splitter plate" (which is the sail in our case) the drag coefficient drops to just 0.59.

    So the effect of the splitter plate is to reduce the drag of the cylinder, respect to the naked one - by 43% in the case of a straight plate in the picture, at that particular wind speed.

    What the splitter plate does is to impede the formation of vortex shedding behind the cylinder. The vortices left in the wake of a naked cylinder at this Reynolds number contain energy, which has to be provided from some external source. Hence, by canceling the vortex shedding one also cancels the relative portion of the drag.

    Now, it is clear that the effect of the splitter plate will strongly depend on its length (relative to the diameter of the mast). The plate itself creates the friction drag, which will be proportional to the length of the plate. So it is reasonable to expect that there is a certain maximum length of the plate which will still give the net decrease of the drag of the system cylinder+plate. Below that length, the drag is lower because the friction drag increase of the plate is less than the vortex drag decrease of the cylinder. Above that critical length, the net drag will be higher than the drag of the naked cylinder. What is the value of that critical length, I don't know. :)

    So, to use your poker terminology: you win this hand if the sail chord behind the mast is sufficiently long. You lose if it is too short.
    Now, the bermuda sail is triangular - very long chord at the boom and zero-length at the top. But the mast diameter is never zero, along the whole length... So, the conclusion is...? :)

    Cheers
     
  5. DCockey
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    DCockey Senior Member

    Vortex shedding by a cylinder is inherently a two-dimensional process. Disrupt the two-dimensionality such as with a spiral wrap and the vortex shedding decreases substantially or stops. Near the top of Bermuda sail the flow will not be close to two dimensional so my guess is there will not be vortex shedding with the sail up.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Since one needs a mast to sail, it makes a difference when your calculating the drag based upon with or without the mast. Including the mast in the total drag calc's adds around 11% as noted in this excellent paper, but of course depends upon many other factors too, as Daiquri has pointed out.

    With and without mast - sails.jpg
     
  7. daiquiri
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    daiquiri Engineering and Design

    Yes, that is another thing to consider. However, near the top of the bermuda sail the flow is so messed up and dependent on the type of rig that frankly I wouldn't dare to guess anything without knowing the details of the sail plan.
    For example, in case of a fractional rig, apart the area behind the head of the jib the flow on the mainsail is not so remarkably 3-dimensional - as this CFD simulation by Mikko Brummer shows: http://www.boatdesign.net/forums/at...50-sail-aerodynamics-classic6mawa25.jpg-2.jpg (taken from the post #370 of this thread: http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-25.html#post233257)
    Cheers
     
  8. pbmaise
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    pbmaise Senior Member

    Daiquiri - Your information is for circular masts not shaped masts

    I certainly agree that if you were looking at drag around a circular mast, that something behind would decrease drag. However, very few us us have circular masts. Most typical yachts have shaped masts.

    Valid research regarding drag of a big object, like a car, applies to drag of an object of the same shape.

    Notice the how closely a shaped mast looks to the latest and greatest fuel efficient cars like this VW concept car.

    Shaped masts, and shaped cars have been around a long time. Shaped cars that end in a flat back are called kammbacks. Ever see a circular car? Of course not they are all shaped. The more fuel efficient they are the closer they are to being true kammback. The first one I remember was the AMC Gremlin. I also recall the Citroen.

    So I guess I have to specify what a naked mast is. A naked mast is shaped. Or perhaps I should say shapely. Better to look at something shapely that is naked.

    The cord length of a kammback is zero.

    This discussion stems from is an off-shoot of the aft-mast argument. Some claim the mast at the very back of the boat with no sail behind it causes more drag then a mast in the middle with a sail behind.

    Thanks
     

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  9. michael pierzga
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    michael pierzga Senior Member

    I dont understand your scenario.


    Aft mast with no mainsail the mast is parasitic and behind the foresail accelerated floe...So yes. Naked .aft mast in accelerated flow is a windage problem

    Any mast is windage
    A mast without a sail is not worth having
     
  10. daiquiri
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    daiquiri Engineering and Design

    It is not sufficient to specify that a naked mast is "shaped" or "shapely". Those words mean nothing in a technical discussion. Everything that has a shape is shaped. :p

    I disagree that mast sections have much in common with "kammback" car body style. Typical mast sections have shapes which do not show the main feature of a "kammback" - namely, the relatively sharp cutting line of the rear part. That is the characteristics which allows a neat separation of the airflow at the desired point of a car body. Masts are generally rounded at the rear, most of them are more akin to an ellipse than to a kammback - and hence will be subject to a degree of vortex shedding. And hence, imo, the splitter plate effect applies.

    Typical mast sections:
    Mast sect.gif

    Besides that, in majority of situations mast sections are not exposed to an airflow aligned with their axis of symmetry. That's another important departure from the Kammback example. But, on the other side, it is also a big departure from the splitter-plate example in my previous post...

    The main reason for giving masts a cross-section shape which tends towards a rectangle is structural. For the same external dimensions and wall thickness, a section tending towards a rectangle gives around 15% more load-carrying area and 30% bigger second moment of inertia. Or around 15% less weight for the same second moment of inertia. These numbers mean that a section shaped in that way can carry a bigger load and be less subject to the buckling instability (masts are subject to high levels of compression) than a circular or an elliptic one. The aerodynamics has little to do with that.

    Cheers
     
  11. jonr
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    jonr Senior Member

    > A mast without a sail is not worth having

    Exactly. So what's the point? Are we talking about an anchored sailboat and whether a 1 foot wide sail should be put up to reduce load on the anchor?
     
  12. pbmaise
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    pbmaise Senior Member

    Okay let me try again

    1. There are claims by some that a naked mast...i.e. a mast with no sail behind it produces more drag than a mast with a sail behind it.

    2. It is my contention that many masts, including medium 62 foot high masts like mine, are shaped tear drop.

    3. Further, that it has been well established that there isn't a need to extend the tear drop shape to the full tip. Vortex drag behind the mast is still reduced if the full tear drop shape isn't present.

    4. In the automotive world, this is called a car with a kammback. Kammback cars have been around over 80 years. Manufacturers that try and minimize drag to improve fuel economy or speed return again and again to the kammback design.

    5. These cars are essentially like 1/2 of a mast section. Air flow is up and over and backdown. The cars in the photos below sure look to me like 1/2 of the mast sections posted.

    6. Now picture a car with kammback with a completed tear drop shape. In reality this is already the case with many mast sections. Specifically my mast has a near perfect tear drop shape.

    7. Adding a sail behind my mast would do nothing to reduce the drag of my mast. I will agree that in the case of round masts, that having something behind the mast that helps form the tear drop shape is good. However, a flat thin sail behind a round mast isn't going to help form that shape very much. Besides round masts is not what is in contention.

    8. Yes any mast creates windage. If I could afford a sky hook, I would not need a mast. :)

    9. I contend that air flow across my mast is mainly horizontal.

    10. I also contend it is the overall drag of the entire mast and sail that matters. Take your pick.

    A. A mast in front of the sail that creates drag and disturbs the first 1 to 2 feet of sail rendering that sail area in effective.

    B. A bolt rope supporting the sail with a far cleaner entry into the oncoming wind, and far far shorter disturbed air section.

    In the first case you need more sail area for the same net drive. In the second case there is a contention that the mast will experience higher speed air that comes off the tail end of the sail. Perhaps that might be true if the tail end of the sail was perfectly in the middle of the sail boat. However, my experience sailing has seen that trailing edge of the sail is far from the center line.
     

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  13. daiquiri
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    daiquiri Engineering and Design

    You contend a lots of things there, but with very few factual arguments. Just generic conclusions based on how some masts (like yours) and some cars are made.

    What else can be added to what has been said in the previous posts?

    Perhaps this: a mast is not a car. ;) :p
     
  14. philSweet
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    philSweet Senior Member

    Bare masts usually fall into two categories, boat at anchor and boat reefed down in a blow. Anyone who's lived aboard knows that most masts pump readily enough, so vortices happen. Preventing them from happening in general is all but impossible. It is easy to deal with a small range of conditions, but in general, it is very hard to eliminate. One thing I look at on rigs is if the mast panels will vibrate in sympathy.

    When reefed, the masthead is really corkscrewing around, so it probably can't form a steady oscillation that can be felt as mast pumping, but that hardly means that the mast isn't casting off vortices.

    The vast majority of masts are fixed. The wind is never directly on the bow. My boat does much better in a blow (35+ knots, biggish seas) with a considerably scandalized main twisted off, boom in hard, even overcenter, and all the jib I can carry. This is basically a heaved-to condition, making 3-4 knots at 65 degrees to the wind. Some of the benefit is due to roll damping and weight aloft, but some is due to reduced drag. There is a lot of difference between taking in the second reef and powering the sail up. I lose about a knot or 5 degrees of pointing doing that. 38' boat, 56' airdraft for mast. Big, fat, oval, old IOR stick. OM4C section, 9.5" x 5", see below-

    http://www.lefiell.com/cruisemast.html


    I disagree with your contention that the conventional mast is bad for the sail behind it. The reason my boat has such a large section is that the mast area was considered to be under-penalized in the IOR rule, so bigger masts added some free area and a lower suction peak, which apparently yielded a net gain. I've seen boats the size of mine with masts the same size as my spreaders.
     

  15. DCockey
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    DCockey Senior Member

    Comparing automobile aerodynamics and mast aerodynamics are fundamentally flawed for two reasons. First the Reynolds number for the flow around the car is much larger than for flow around a mast - several orders of magnitude or so,

    But more importantly the flow around an automobile is inherently 3D. Experiments have shown that the flow is inherently different than the flow around a 2D shape with the same profile. 3D shapes like automobiles generally do no have periodic vortex shedding.
     
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