Aero benefits of a straight forestay

Discussion in 'Hydrodynamics and Aerodynamics' started by Vincent DePilli, Apr 24, 2013.

  1. Vincent DePilli
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    Vincent DePilli Junior Member

    I sail a Corsair F31R trimaran (three wire rotating rig). I have been trying to figure out how much benefit in terms of upwind VMG might be available from modifications that make it possible to have a really straight forestay. Whey I raise this issue on Sailing Anarchy, most posters simply say—“cut your sails to accommodate the forestay sag, and quit worrying”.

    Which is excellent practical advice, no doubt. But If I was a practical person, I would not be a sailor, would I?

    So my question is whether in theory at least, a straight forestay is better than a saggy one (ignoring special cases such as need to power up in light air etc.), assuming in each case that the sail is properly cut for the amount of sag.

    Obviously there are other questions—like whether any gain from a straight forestay is worth the weight, windage and complexity that might result from adding running back stays, whther the vessel or the mast is able to handle the stresses attendant on getting a straight forestay, etc., etc.

    But it is not even worth addressing those questions if there is no potential payoff at all!

    If you think that a straight forestay is in fact an aero benefit, can you describe why? What happens to the airflow that degrades the lift/drag ratio?
     
  2. michael pierzga
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    michael pierzga Senior Member

    A forestay that allows you to go from saggy to bar tight gives your sailmaker the most versatility in sail shape.

    Is this needed on a cruiser ?

    Good question
     
  3. tom28571
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    tom28571 Senior Member

    I'm not a sailmaker but I would think that a sailmaker could make a superior jib if they knew that the forestay would never sag. As it is and assuming that only one jib is used, they have to design the sail to accommodate sag of varying wind pressure.
     
  4. Vincent DePilli
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    Vincent DePilli Junior Member

    But that is precisely not the question-- we know the sailmakers can compensate to some extent for sag.

    The question on the table is whether a perfectly cut jib on a straight forestay, will necessarily have a better lift drag ratio than a perfectly cut jib on a saggier forestay.
     
  5. Petros
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    Petros Senior Member

    Thinking out loud; I suspect, as someone that once spent years doing computational fluid mechanics, that a good design can be optimized for either condition. There would be a slight theoretical advantage with a straight fore-stay since all of your lift will be in one direction rather than having them at slight angles that vary with the curve of the fore-stay.

    The big compromise however comes for the varying sailing loads and angle of attack. In other words, the airflow is optimized for certain design conditions to create the best L/D, but any time you are off that design condition the L/D will be compromised.

    I do not know a lot about sail making, but I would guess a tight fore-stay would be easier to design an optimized sail. However this will put much higher loads on the rigging than one that is slack, risking failure or making it necessary for much heavier (and costlier) rigging. So the question is would the extra weight and cost be worth any hypothetical drive increase.

    A tight fore-stay might have less tendency to flog or whip in rough conditions, which may have a significant advantage since the sail maintains it best shape for more of the time when pitching and rolling.
     
  6. viking north
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    viking north VINLAND

    As a cruiser and not a racer,I've always been lead to believe the ideal situation is a tight straight forestay with a jib cut to match. Logic says the sailmaker doesn't have to play a guessing game to compensate for sag thus has a better chance of ending up with a correctly cut sail. I'll stick my neck out further and say without sag, aerodynamically the airflow makes contact with the leading edge and flows along the sails surface in a more uniform parallel pattern. I visualize it like the surface of an aircrafts wing where hollows mid span would most likely cause turbulance. I have no formal training in sailmaking or rigging but right or wrong i have always looked at sag as a no no and set up my rigs to avoid it like the plague, always using double backstays for strength and ease of a simple tightening system. Ok the fox hole has been dug :D
     
  7. Vincent DePilli
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    Vincent DePilli Junior Member

    Well Viking, that is an on point answer, and consistent with my equally uninformed intuitiion.

    I am hoping that one of the CFD guys will respond with twenty seven eight-by-ten colour glossy photographs with circles and arrows and a paragraph on the back of each one explaining how it all works.
     
  8. viking north
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    viking north VINLAND

    Well Petros and I have stirred the pot I sustect the boys will add spice :p
     
  9. sean9c
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    sean9c Senior Member

    There is no such thing as a straight forestay. There is always some sag. Dork around with it if you like, add the complexity to running backstays, a few trips to your sailmaker or a new sail to fit your new configuration. Is it going to make you any faster? Maybe. On a monohull I'd say you'd likely be able to point higher. On a tri are you looking to be able to point a little better? Don't know, I wouldn't think pointing is apriority. What's your mast rotation going to think of your running backs? They will try to make your mast rotate, it will take some force to counteract this. You're going to add compression load to the mast. What's the mast ball and structure going to think of that?
     
  10. viking north
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    viking north VINLAND

    Not literally a perfectly straigh forestay -- but one within reason . Also not running backstays but double backstays. Back down in the foxhole and off to sleep for today. :D
     
  11. PAR
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    PAR Yacht Designer/Builder

    Rod rigging is about as close as you can get and it'll sag a bit. Simply put, if a sailmaker is given enough information (not usually the case) he can cut a jib that will work well across the wind strengths you ask him to sew up a jib for, regardless of stay material and rig type.

    From a physics view point, you'll never get a straight forestay, just because of the lousy "purchases" you've got to tolerate in the rest of the rig. Yacht design is accepting the convolutions and entanglements of a SOR. If the only item in the SOR is a straight forestay, you could easily have one, but this kisses off many things that have considerable value. Most of these other things are obvious, like weight aloft and windage, but what's not so is the reward/benefit - cost/implementation trade off that must take place. Lets say you have a forged alloy or carbon composite rig made up. The stays, mast, boom, everything, dead nuts rigid, so you can have the bar taunt headstay. Now, what's this offer the performance envelop? How much higher into the wind can you get? Any real speed gains? Can you offset the windage and weight aloft issues with these gains? Etc., etc., etc. This is the *** kicker and often the deciding factor, a performance benefit that can be justified with it's cost tolerable effectuation.
     
  12. ancient kayaker
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    ancient kayaker aka Terry Haines

    The optimum would be a rigid wing sail designed specifically for known apparent wind angle, speed and sheer. Only aircraft wing designers approach that sort of control. For sailing, it can only be approximated in a one-trick boat such as SailRocket.

    Efficiency falls every time a variable shifts from its preferred value. Logically, loss of leading edge consistency must reduce efficiency further, for example, as the leading edge changes sail twist required to compensate for wind sheer becomes less optimal.

    Slack in the LE would also allow vibrations to set up and energy to be lost, increasing drag. Loss of leading edge control is most likely to occur when sailing close to the wind, just when it will reduce VMG the most . . .

    On a cat you are also battling the geometry of the boat which is hardly condusive to a super-taught rig. Perhaps it is my lack of knowledge of multis, but I would have thought a trimaran offered at least the possibility of improvement, and wouldn't be much different from a mono in this matter. But this is only a cruising boat, and already compromised by the limitations of trailering, so how much one can gain is questionable, before something breaks.
     
  13. Remmlinger
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    Remmlinger engineer

    This is a theoretical question, that can be answered using wing-theory. If the sail is cut in both cases so that angle of attack and camber are ideal, then the only difference is the planform of the wing. A slight curvature of the leading edge has practically no influence. Tho old Spitfire airplane had a curved leading edge and an elliptical planform. Today we know, it is not worth the effort. Todays airplanes have straight leading edges, independent of speed, from gliders to supersonic planes. The reason is simplicity of manufacturing.

    It is a totally different question if the ideal AoA and camber can be achieved with a sagging forestay.

    Uli
     
  14. Joakim
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    Joakim Senior Member

    Rod rigging has nothing to do with how much sag there is. Typically rod is replaced with much thicker 1x19, which actually results to a stiffer rig. E.g. for my boat Selden has specified the following wire -> rod 10 mm -> 7.1 mm, 7 or 8 mm -> 6.4 mm and 6 mm -> 5.0 mm.
     

  15. tom28571
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    tom28571 Senior Member

    You may have intended for that to be the question but it was not asked in that form.

    A problem with the saggy forestay is that is is never consistent in any degree of sag and bounces around depending on the wind force. So yes, a sail can be designed to be optimum in either straight or sag conditions but reality gets in the way of either approach and as others have said, a straight forestay is a pipe dream and can never be realized.

    I made an experiment once that could achieve a straight forestay. I had a wire sewn in a wide luff sleeve in a catenary approximating a sagged condition. The wire could be tensioned from the helmsman position. I could actually cause the luff to be ramrod straight by applying proper tension. This was definitely not legal in the racing class that I sailed and was only used as an experiment so I never got any racing results in pointing ability with it.

    I think it really could be an advantage if a sail was designed for a straight forestay. If anyone wanted to patent the idea, you can't because I still have the sail from 40 years ago.
     
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