Reinventing the wheel/snakey bending batten

  • 28 Oct 2021 18:10
    Reply # 12080273 on 12032751
    Anonymous member (Administrator)
    Anonymous wrote:

    One question I have that people might have an answer to is what is the "ideal" airfoil shape for a sail/batten? Is the upper surface of a symmetric NACA foil just about it? That's two questions in one! 

    Bonjour

    The "ideal" airfoil shape depends of many things : the geometry of the sail, the interaction betwen sail(s) masts... status of the sea (in a shopy sea you will need more power than on a flat sea) the haul performance, the force of the wind (with a slow wind you will need power and in a gust you will need less heeling moment), the point of sailing (sailing windwards request to minimise he incidence, sailing reaching is a power issue, sailing downwind is a turbulent sailing and the profile is not anymore important). Book have been written on the subject !

    Anyway the sail profile has a great impact on performance: racing boats change their sails at least once a year and for top racing situation (olympics, world championship, americ cup....) the sail are corrected from the day to day deformation every day. (the crew take some views from the sails at the end of the day for the sail makers)

    The racing crews are modifying permanently the airfoil shape to adapt it to the present situation. The crew act on the tension of the sail cloth, on the twist of the sail on the orientation of the sail to optimise the performance. On some ocean racing boats the crew is permanently twinning the sheeting of the jib and mainsail to deal with the difference of apparent wind due to the swell.

    The globaly performant sail profile  will have a camber that will reduce from the leading edge to the trailing edge with the main camber in the first third of the sail.

    It is not exally the same shape as the external surface of a Naca profile because the lack of the round leading edge impacts on the outer profile.

    Eric


  • 28 Oct 2021 17:07
    Reply # 12080102 on 12032751
    Anonymous wrote:

    One question I have that people might have an answer to is what is the "ideal" airfoil shape for a sail/batten? Is the upper surface of a symmetric NACA foil just about it? That's two questions in one! 

    There isn't one. The ideal airfoil shape is dependent on the speed of the fluid the airfoil is moving through. So in an airplane, the airfoil is chosen for the speed the plane will spend most of it's time at and even then flaps and nose movement are used to change the airfoil shape for lower speed use at take off and landing.

    With a sail boat one does not have that speed the air foil will spend most of it's time at. So being able to change the airfoil shape depending on wind speed would make a lot of sense.  So far as I can tell, nobody does (aside from sheeting tighter or not) this except multi-million dollar racing boats with pretty much solid wings rather than sails. Sails are very flexible and tend to bend more in strong wind than in light winds which is just the opposite to what is desired. To add to this, trying to flatten a sail in strong winds also  means  using stronger gear all the way from lines to battens to sail cloth.

    So what most people seem to have chosen to do, is to change the camber of the sail so that the bottom panel(s) have a greater camber than the top panels because in stronger wind the system will be reefed. The top panels often have no camber at all. This is certainly not a perfect solution because in light winds the top half of the sail is less than ideal. On the other hand, many people want to use a junk rig in the first place for simplicity and the hull has a speed limit which means once that speed limit is approached, efficiency becomes less of an issue.

    Anyway, enough background. In your case, changing the foil shape might be done manually (as in depending on weather forecast before setting out), making each batten with a different camber up the mast or some how allowing the flexible part of the batten to be tightened on the fly. While there are some exotic materials that can be stiffened just by passing electrical current through them, that is probably beyond where you wish to go. I think the best you could do is to leave the battens straight cut at the joins and tighten the middle membrane in some fashion.

    Getting back to the best airfoil shape. Assuming you want a single shape for simplicity, aim for light wind performance with the theory that strong winds will provide enough power even with poor efficiency. Remember that it is not something to loose too much sleep over because cloth is involved and no matter how close you get the batten to your chosen shape, the cloth will choose it's own shape. you can compensate for that to some extent by choosing a wrong airfoil shape (read everything you can find on both camber cut cloth and SJR for some good info on airfoil shape) but even then, wind speed will still change things... probably negatively.

    As a last thought. I find it amazing how a simple cloth with both ends held tight makes a reasonable foil all on it's own...

  • 28 Oct 2021 09:55
    Reply # 12064294 on 12025694

    I shouldn't have put bendy-batten in the thread title, I used the wrong word ingnorantly. Maybe an admin can change the title to Snakey Battens as that's the goal of the experimentation. [admin has changed title]

    Can I suggest:

    As per convention, a bendy batten is a batten that keeps bending with increasing wind strength as Arne pointed out, and is what we're all trying to find solutions to.

    A multi-hinged batten has up to five or six hinges.

    A Snakey Batten has many many "hinges" of miniscule movement.

    I rarely use a drawing board Arne, so no problems there! Straight from my imagination to the work-bench and out the door as fast as possible for new ideas especially. 

    I don't think there's anything wrong with aiming for a good foil shape on the "bad" tack, all the cambered-panel methods strive for this and don't just throw something half-hearted at it because it won't be perfect. One of the advantages of a multi-hinged approach is that the shape could be different on each tack to accommodate the aerodynamics (or lack of) of the bad tack. 

    Thanks again for the wise replies.


    Last modified: 28 Oct 2021 13:54 | Anonymous member (Administrator)
  • 28 Oct 2021 00:46
    Reply # 12040747 on 12025694
    Anonymous member (Administrator)

    "The reason is that unless the mast is hidden inside a double-ply sail, that mast will wreck any aerodynamic niceties on one tack, and will stand proud in the wind on the other one."

    True in the case of a contiguous sail Arne, but I would not be quite so sure in regard to a SJR split sail. 

    Of course, I realise this is sightly irrelevant to the sail in question, and the discussion about bendy battens which don't even apply to SJR - I just wanted to question the generalisation that Arne made about the non-importance - or should I say, less importance - of foil shape,  and the reason for it, as that reasoning may not apply in the case of SJR.

    Apart from that - an interesting conversation, and woudn't it be great if a bendy batten could make a flat-cut sail into a nicely cambered one.

    Last modified: 28 Oct 2021 03:15 | Anonymous member (Administrator)
  • 28 Oct 2021 00:01
    Reply # 12038848 on 12025694
    Anonymous member (Administrator)

    I certainly am not an expert on this, but I still know for sure:
    The worst airfoil shape is the one that never leaves the drawing board.

    I have never wasted too much energy on finding the perfect foil shape. The reason is that unless the mast is hidden inside a double-ply sail, that mast will wreck any aerodynamic niceties on one tack, and will stand proud in the wind on the other one.

    I suggest you aim for the foil shape on your upper left photo with 6-10% camber (I use 8%), and then focus making the hinges work. You can always play with foil shape later.

    Arne


    Last modified: 28 Oct 2021 00:03 | Anonymous member (Administrator)
  • 27 Oct 2021 21:33
    Reply # 12032751 on 12025694

    One question I have that people might have an answer to is what is the "ideal" airfoil shape for a sail/batten? Is the upper surface of a symmetric NACA foil just about it? That's two questions in one! 

  • 27 Oct 2021 21:12
    Reply # 12031859 on 12025694

    Thanks for the input Arne, what I like about the snakey batten idea is having very many hinges with very small movement which is quite in line with the whole junk philosophy. I believe it could be really strong. I envisage the knuckles having angled ends so the load is spread across the whole cross section rather than just the outside of the angle as in the toy snake version with a perpendicular saw cut. 

    S-bending would of course be an issue so the forward end may have to stay straight but I have another idea to deal with that! I'll keep experimenting unless someone talks me out of it! 



  • 27 Oct 2021 20:18
    Reply # 12029503 on 12025694
    Anonymous member (Administrator)

    Jan,
    I think I have seen something similar presented somewhere in a JRA newsletter, but I don’t think the design left the drawing board.

    I think I would rather call that thing a multi-hinged batten. Just like normal hinged battens, it will bend to the stops in light as well as in strong winds.
    The shape of a truly bendy batten varies with the wind strength.

    Personally, I like the rope-plus-round-disks version better (but with fewer and longer disks), but it could be that the square version is stronger.
    I found that my hinges (Newsletter 24) worked very well. You would probably save time by only having 4-6 hinges. I found that my ‘4-knuckle’ battens resulted in a soft curve in the sail.

    Remember that you cannot have a hinge too far forward, or the batten will take an S-bend at the mast on a JR.

    Good luck with your experiments!

    Cheers,
    Arne


  • 27 Oct 2021 18:47
    Message # 12025694

    A rainy afternoon in the workshop and I finally made a first attempt at an idea I've had. Basically at this stage I want to know if it's been done before. I think it must have been but I can't find anything using forum searches.

    The idea is to make a very bendy batten using the idea of those those toy snakes made from a cloth spine and a wooden body with many saw cuts. See pictures.

    I made a very rough 1000mm x 24mm x 24mm mock-up from a piece of webbing strap for the spine sandwiched between two spruce battens of about 24mm x 11mm.  I just eyeballed an aerofoil shape with 10% camber at 40% along the chord, then eyeballed where to make the cuts to make the snakey batten fit. I was surprised to find the cuts from my jig-sawblade were way too wide and had to shim out the cuts to bring the shape more or less back in line. 

    It's surprisingly strong. I can foresee lots of problems, most of which I already think there are solutions for but before getting into that I'd just like to see who did it before and what happened. 

    The goal would be to make battens with a dyneema webbing spine and incompressible knuckles glued on with epoxy and all sorts of belts and braces to make it reliable with the eventual aim of creating a planar junk sail, not a baggy one. 

    Working title is snakey battens. 


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    Last modified: 28 Oct 2021 13:50 | Anonymous member (Administrator)
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