forum project: Develop Low Drag Device for Standing Rigging

Discussion in 'Boat Design' started by brian eiland, Jun 14, 2008.

  1. brian eiland
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    brian eiland Senior Member

    A recent posting on another forum brought this subject back to mind, and so I thought, what a great project it would be for the participates to this boating forum to collectively work on a method to reduce one the most comman drag factors effecting all sailing rigs, regardless of their type, shape, size, etc

    ...the Parasitic Drag of the 'round sectional shape' of virtually all standing rigging.

    Generally we all know that if we can provide some 'streamlining' to these round shapes we can reduce their drag by better than 1/2 or more. I can't believe that in this modern age of new materials, etc, that we can't develop a 'shroud cover' that might 'wrap around' a piece of standing rigging and provide a superior aero-shape, and rotate to the most advantageous wind direction.

    It would be a worthy project of this forum group, and we should set aside individual patent protection ideas as unproductive in this collective effort. Besides it might be difficult to find a manufacturer for this relativel small market of sailors, and in this current economy.

    Lets just invite all brainstorming ideas from which something might arise.

    :idea: :idea: :idea: :idea: :idea: :idea: :idea:
    __________________________________________
    When I first reconsider this old idea, I did a search on this forum for 'drag reduction'. I came up with this discussion submitted by a rigging guy Randy Hough who appears very concerned about drag issues. There is some great discussion there, and I considered added my 'project/contest' posting to his ongoing subject thread, BUT I was concerned that the discussions there had stopped, and that maybe a new title to the subject, and a more directed 'project' was in order.
    Rigs and Rigging Weight vs Drag
    http://www.boatdesign.net/forums/showthread.php?t=14881
     
  2. messabout
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    messabout Senior Member

    There are, or at least were, just such an item as you describe. A plastic extrusion with an aerofoil shape. The extrusion was not joined at the trailing edge and one merely snapped the shape around shouds, jumpers or whatever. The stuff worked after a fashion. I once had a small boat with that fairing plastic installed. When it succumbed to UV degradation, I took it off and did not try to replace it.

    I think that it would be interesting to quantify the drag of riging wire. Plenty of variables to deal with I'd suspect. Generally the drag is a function of the square of the wind velocity so perhaps the wire drag would be significant in a force eight but not worth much attention at force three.

    It seems to me that more favorable result could be found by cleaning up the hull and all the deck hardware, loose lines, and the not inconsiderable drag constituted by several crew members scrambling about the deck. Drag is also a function of frontal area. And then there is induced drag where an object protruding into an air stream may cause more than obvious drag because of its' proximity to another drag making body. All of this would matter on a windward leg in particular.

    Let's hear from those of you who know some of the facts about this subject.
     
  3. tspeer
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    tspeer Senior Member

    Hoerner's Fluid Dynamic Drag has some German research on cable fairings.

    Here are the problems that have traditionally stymied attempts at developing a decent cable fairing:

    - Sailboats experience a wide range of apparent wind angles, and have to sail on both tacks. A fairing with a fixed orientation (say, fore-aft for symmetry) is worse than no fairing at all because it will be stalled most of the time. So the fairing has to rotate freely about the stay.

    - It's necessary for the aerodynamic center to be behind the center of the cable, or else the fairing will not rotate. It's difficult to come up with a shape that has this property without having a substantial chord.

    - The lift on the fairing can lead to flutter, especially if the center of mass is behind the center of the cable. This leads to conflict with the need to have the aerodynamic center behind the center of the cable to make the fairing weathervane. The fairing has to be mass-balanced, which means a heavy leading edge and a light trailing edge, which is difficult to do if the whole fairing is made of the same material.

    - The flexibility of the cable can lead to binding if the fairing is all one piece. So the fairing has to be subdivided into many pieces. This isn't all bad, because it makes it easier to install and replace damaged sections. However, due to friction, the various fairing segments will not necessarily feather at the same angle, and will produce varying amounts of lift. The variation in lift along the stay leads to induced drag in addition to the profile drag of the fairing.

    - Added weight aloft

    - Dynamic effects of boat motion that can disturb the fairings from their feathered position. Whenever the fairing is not perfectly feathered, it is producing lift and additional drag.

    - At low Reynolds numbers, the flow separated more readily on thick sections. The fairing necessarily increases the frontal area of the cable somewhat, so unless the flow closes up adequately behind the "shoulder", the fairing may not provide a net reduction in drag.

    This isn't to say the concept won't work. Rather it's a list of challenges that have to be tackled to make a successful system.

    One thing you may want to consider is having a small tail boom on each segment. This will greatly increase the aerodynamic moment and make the segment feather itself more effectively. Of course, the tail adds weight and parasite drag. A small boom extending forward can achieve the necessary mass balance with less weight. The resultant fairing segment looks like a small airplane.

    There are things one can do to reduce windage short of adding fairings. The most important one is to eliminate any rigging that is not necessary. A somewhat larger diameter mast may be a better choice than a mast with lots of standing rigging to stiffen it.

    Instead of having multi-part tackles exposed to the wind, the purchase can be put under the deck (or in the boom) and only a single wire or high-tech fiber sheet exposed to the wind.

    If you recall the videos taken during the America's Cup, you saw that the running backstays were made in two pieces. On the windward legs, when they didn't need as much support from the backstays (because of mainsheet tension), they would disconnect one cable and bring it forward. They'd tuck it in behind the mast in the separation bubble that formed between the mast and mainsail where it could ride for free. Then they'd reconnect it before rounding the windward mark for the run.

    In all the discussions about synthetic vs wire/rod rigging, the windage of the rigging was rarely mentioned. As best I can figure, the added windage from the larger diameter of synthetic rigging may cancel out the benefits of its reduced weight. Rod rigging may be the best solution, but one would have to do a system-level trade study to see if this is the case.

    Two cables in tandem create less drag than one cable of the same total cross-sectional area. The problem here may be achieving adequate orientation of the two cables with respect to the apparent wind angle. However, it may be that two cables can be enclosed in a fairing that would be smaller in thickness than a single cable and the fairing could keep the two lined up with the airstream, at least toward the center of the cable. The ends may be a more difficult story, due to the difficulty in twisting the pair close to the attachment points.

    That should get you started.
     
  4. tspeer
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    tspeer Senior Member

    Rigging is just one place to look for drag reduction. Ever consider how much aerodynamic drag there is to rail meat?

    Sharp corners are very draggy. Hoerner has data on cylinders of various shapes. Starting with a rectangular cross-section and rounding the corners, a radius of 20% of the height of the cylinder can reduce the drag by 80%. There is not a lot of additional drag reduction from there to completely rounded. This means that 60% of the flat surfaces can be retained with little or no penalty. Even modest rounding of the edges of gunwales, cabin tops, etc. can provide definite benefits.

    A lot of sailors are fanatical about weight. They should be equally fanatical about frontal area. Take a picture of your boat looking at it from the direction of the apparent wind. Then add up the frontal area of everything that is visible in the picture. That's what you want to attack.
     
  5. rob denney
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    rob denney Senior Member

    G'day,

    An interesting question, but I think your answer is heading in the wrong direction. Instead of adding expense, complexity and weight of dubious fairings to draggy stays, why not remove the stays altogether and replace the stayed mast with a wing section unstayed one? Unstayed wing masts are stiffer for a given weight than unstayed round or oval masts and much less drag than a mast held up with wires, spreaders, mast and deck fittings.

    The weight of the unstayed rig is heavier, but it's centre of gravity is lower.

    The total weight (and cost) of the boat is considerably lower as the beefing up required is very localised, and is a relatively simple tension load in the horizontal plane, rather than compression and bending in all planes. The tension load is resisted by simple wraps of unidirectional fibre around the deck bearing, a ring frame to keep the hull in shape and if necessary, a fore and aft stringer along the keel. The compression and bending loads of the stayed rig require the entire boat to be strengthened, in some parts massively.

    A free standing rig has so many handling and maintenance benefits that for cruisers it is a no brainer.

    For racers, if the time and money put into stayed sloop rigs was put into unstayed rigs, I suspect it would also be a no brainer, and even if it wasn't the long term cost savings are more than enough to make up for any speed deficit for all but the absolute elite.

    regards,

    Rob
     
  6. Bruce Woods
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    Bruce Woods Senior Member

    Twenty odd years ago in another life we used snap on plastic foil mouldings to streamline the 3/4 inch diameter cable attached to a hydrocarbon sniffing torpedo contraption toed behind and below an exploration vessel. Most of the foil shaped mouldings each about 4 inches by 4 inches by 1 inch could actually be left on the cable and wrapped onto the last layer of the retrieval winch without damage.

    The C class Quest III had solid foil shaped standing rigging connected through a series of pulleys, to the mast and always remained parallel to the mast as it rotated. ( from catamaran sailing to win, by max press and chris wilson)
     
  7. brian eiland
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    brian eiland Senior Member

    Photos, Dwgs, Sketches, etc

    Any photos, dwgs, sketches etc??
     
  8. brian eiland
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    brian eiland Senior Member

    We know that 'no rigging' certainly cuts the aero-drag. But there are THOUSANDS of sailing craft out there now that might benefit from a simple plastic extrusion design that would cut down on the aero drag...even a little bit.
     
  9. Mild Bill
    Joined: Nov 2007
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    Mild Bill Well, not entirely mild.

    In aerodynamics, induced drag is drag due strictly to generating lift. Drag due to interaction between two bodies in close proximity is known as interference drag. If one body protrudes from another body (e.g., wings and tail surfaces sticking out from the fuselage), the interference drag may also be called intersection drag.
     
  10. Guest625101138

    Guest625101138 Previous Member

    What is the potential carrot.

    Lets take a light to moderate displacement cruiser (maybe a cruising cat) with 50m of standing rigging excluding the outer forestay that carries the jib. The rigging is 6mm wire.

    Lets say apparent wind at 45 degrees and boat is doing 8kts (nice hull/s) in 15kts apparent wind.

    At low Re# we take the stay Cd as 1.3. The drag on the standing rigging is then 12N.

    Now we design a nice NACA0025 fairing piece to fit over the stays. It ends up 7mm thick therefore 28mm long. Cd is only 0.05. The drag on the rigging reduces to 2N.

    Hence the reduction in drag is 10N. The component in the direction of the boat is 7N. At 8kts this is costing 24W. The sails would be generating something like 5kW under these conditions. I doubt the the 0.5% reduction in power on the hull would result in measurable speed increase.

    This is a first pass analysis. It does not consider added weight of fairing piece. It does not consider any benefit of slightly less disturbed flow getting to the sails. However I expect these would be second order effects of even less significance.

    I guess you would not be too devastated if you saw the fairing pieces gradually falling apart and falling off due to the ravages of sun, wind and water.

    For faster more easily driven hulls the drag on the standing rigging would certainly be of greater significance and worth reducing. It could easily be costing a percent or two in speed.

    Rick W.
     
  11. eponodyne
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    eponodyne Senior Member

    I don't think this idea would really shine outside of the "tall ships" arena: getting rid of the miles and miles of shrouds and ratlines and replacing them with rod rigging would certainly be worth a lot more than thinking about fractions of a percent on a square-headed Tornado catamaran.

    Just my tuppence ;)
     
  12. yipster
    Joined: Oct 2002
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    yipster designer

    my ladybird has the schrouds as sails, the set-up needs more study and more detailed drawings are planned
    now let me read what you all said... :cool:
     
  13. tom28571
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    tom28571 Senior Member

    Bruce Woods comments reminds me of devices used on instrument tow lines in deep water long ago. These were simply plastic ribbons that disrupted the formation of vortices. I think that the resulting reduction in drag was not due to a reduction in the absolute drag of the wire though. I think the idea is to break up harmonic resonance and thereby reduce the effective cross section compared to a vibrating wire. I tried this on the lifting cable on a swing keel boat and it did stop the vibration and I assumed that it also reduced the drag although I had no way to measure it.

    Perhaps there is more to gain from this method than streamlining of the stays. Tom Speers comments highlight the great complexity of any effective method of doing that. I prefer the unstayed mast route though and almost all my boats go this route.
     
  14. Nordic Cat
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    Nordic Cat Senior Member

    Here is some data on wire drag compared to rod rigging and a wire faired with an ellipse.

    Can't remember where i found this, but the message is clear.

    a 13 mm wire has 4.6 N/m drag at 20 m/s.

    Let's take a 40 ft cat with a 18 m mast.

    5 stays = 100 m
    Shrouds = 50 m

    At 20 m/s across the deck this gives us 690 N resistance.
    Add a bit from flag lines, halyard, topping lift, say this equates to another 30 m or 138 N.

    Total drag = 838 N or about 84 kgs/186 lbs of drag!!!

    This does not include lifelines, but with those we are getting close to the 200 lb mark.


    This is the reason I'm going for unstayed masts, and specifically wingmasts, that when trimmed and sized correctly have less than 10% of the drag of a conventional mast.

    Regards


    Alan
     

    Attached Files:


  15. Guest625101138

    Guest625101138 Previous Member

    In wind of 20m/s you would only need the rigging to make way down wind. This is getting into survival conditions and sure the rigging and anything else in the air presents a lot of drag.

    Have you done the numbers on the wind drag on the hull/cabin?
     
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