Split rig - luff, leach, sling point, downhaul...

  • 07 Nov 2016 17:04
    Reply # 4368611 on 4195606
    Deleted user

    Next question on the two top panels:

    Compared to the other four (split) panels, what kind of area should I be aiming for?  I'm going with the consensus recommended full panel for the top two. But should panel 5 be about the same area as the main four?  What about the final top panel 6? Am I looking for just a wee scrap of cloth for a force 6-7? Should panel 5 be cut with a bit of camber and panel 6 flat? 

    The green lines on the image below match the model on my wall, which looked good there, but look a bit too aggressive when put over the boat.  Is the red better?

     

  • 03 Nov 2016 17:31
    Reply # 4362916 on 4362227
    Deleted user
    Slieve McGalliard wrote:

    Hi Scott,

    It would appear to be best if the rotating air is pushed right up to the tip of the yard, and in the absence of wind tunnel information my best guess has been as used on Poppy and Amiina...  I feel the 30° yard gives good control of the tip flow and vortex...

    The model will show that you do need the tapered luffs to balance the tapered leeches. Any cambered sail drawn with straight luffs but tapered or fanned leeches will end up requiring parrels to maintain the straight leech or have high stress at the leeches of the upper panels, as the angled leeches push the battens forward like a bow pushing an arrow. The model helps you get a balance between the forward pressure on the battens at the leech and the aft pressure at the luffs.

     

     

    Great answers, Slieve.  That clarifies a bunch of my questions about why you draw the top panels the way you do.  The summary seems to be:

    1) Best guess is a 30° yard angle reduces tip vortexes.  Which means fanning the top section(s).

    2) Fanning the top panels requires tapering back the luffs to balance the yard tip tensions and minimize skewing vertical forces into horizontal ones.

    3) Then do what we can to make it look good.

    I'll make some tweaks to the model (so easy to do with my fancy setup) and take some more pictures. 

    Once I get the Stick & String model set, I planned on adding sewn fabric to get a better sense of what that requires.  Paper will be a good starting point.

  • 03 Nov 2016 16:58
    Reply # 4362882 on 4359124
    Anonymous member (Administrator)
    Steve Liebig wrote:

    Please direct me to the 'child's table' for rig doodling.  Imagining a cambered-panel-split-junk conversion of a local, fin-keel day-sailer I did the following:

    1.  Placed the mast in the same location as the original.

    2.  Drew the jibs and main panels to approximate the size of the originals.

    3.  Drew a halyard sling-point in about the middle of the yard.

    4.  Came up with an aspect of about 1.0 in order to achieve about 10% bury:  21 foot mast (above the deck) with just over 2 feet of bury from the mast step to the deck.

    Two questions

    1.  Is it idiotic to proceed like this without maths?

    2.  Does a low-aspect rig really result in poorer upwind performance than a higher aspect rig?  I've never heard an acceptable rationale for that assertion.

    Warm regards,

    Steve

    Link to photos of the original and my doodle.

    Original marconi sail plan:

    My rig doodle:

    Besides maths, I'm also incapable of inserting a photo into this post!


    Steve's links don't work with me either, but I guess he refers to these two diagrams in his member's photo album:

     

    Correct me if I am wrong, Steve.

    Arne

  • 03 Nov 2016 16:35
    Reply # 4362826 on 4195606

    Thanks for your kind words, David.

    You are right in saying that the yard will bend a little, but I have found the braced yard that Arne first used and which I have copied is remarkably stiff. When the top panel is full length with no split then there seems little point is adding a little guessed amount of round onto an already estimated round, so I ignored it. The problem has been considered when using split panels right to the top, by ensuring that the length of the top main panel luff is very slightly longer than the drawn length. This means that when the sail is hoisted a tug on the downhauls, and in particular on the lowest operating batten will pull down on all the luffs of the mains and slightly bow the battens downwards and keep not only the main luffs tight but also the main leeches and the jib luffs tight. It is difficult with an inexact material like sailcloth the ensure all luffs and leeches are the exact same/ correct length, so the building in of a deliberate error makes it self adjusting.

    With the full length top panel the battens were able to bow downwards taking a tiny increment of camber out of the top panel which is of no significance.

    Designing and building rigs can be great fun and I think more people should try it. It's not difficult and is very rewarding. (And if you choose the material from the sale, very cheap)

    Cheers, Slieve.

    PS. Chris, I will write on the other thread when I get a moment.

    Last modified: 03 Nov 2016 16:37 | Anonymous member
  • 03 Nov 2016 15:36
    Reply # 4362764 on 4195606

    Slieve, congratulations on an excellent posting in reply to Scott, explaining some of the whys and wherefores very well indeed. 

    Just one small comment: you say, rightly, that a yard must be stiff, but of course it can never be 100% stiff, it will always bend a little. That's why I say, quite frequently, that the head of the sail must have some convex rounding on it, corresponding to the probable bend of the yard under load, even when aiming for a flat top panel. The amount can only be a guess, but it's always better to have a little too much rounding than to have too little.

  • 03 Nov 2016 10:55
    Reply # 4362256 on 4362227
    Anonymous
    Slieve McGalliard wrote:Look at the trouble aerodynamicists (spelling?) have put into tip design, and the structures on the tips of modern airliners.

    Not wishing to drag this thread off topic, I have posted separately in this forum some questions about comparing aircraft wings to sails. Slieve, your response would be very much appreciated.

    Chris


  • 03 Nov 2016 10:47
    Reply # 4362249 on 4195606

    Hi Steve,

    The links to your photos didn't work for me so I'm only guessing at your ideas.

    I'm sorry but I don't know where the 'Child's table' for rig design is, but it's all child's play anyway and it doesn't require real maths but just some simple 'sums'.

    1. I would suggest that you do not start by placing the mast in the boat, but rather draw the rig on a separate piece of paper to the same scale as the Bermudan rig and then slide it back and forward on the Bermudan diagram to get the centre of areas in the right relationship for the rig you've drawn and accept the mast position you then get. This is almost always further forward that the Bermudan position, except in extreme fractional rigs.

    2. In talking about drawing jibs to the same size as the Bermudan sails then you must be thinking split rigs, and again I would start off with a required area and work from there. Bermudan headsails come is such a variety of sizes, and with overlap that you must end up in trouble with your approach.

    3. The sling point is often started around the middle of the yard, but probably will drift a bit aft in practice to get best set.

    4. With a split rig it can usually end up with the mast about the same height as the original Bermudan mast above deck. I've read that a bury of 8% or more will do the job. If the aspect ratio of the full sail is very low then is will be extremely low with a couple of reefs taken. Photocopy your doodles and cut them off with two reefs down and see how they look.

    In theory higher aspect ratio is better as there should be less induced drag. Think of a glider wing as being efficient, with the long narrow wings whereas the powered plain can waste fuel with a lower efficiency low A/R wing and still fly satisfactorily. Look at the long slim wings on modern medium weight airliners and you'll see where the fuel efficiency is coming from and which generally mean lower fares.

    Do these answers help?

    Cheers, Slieve.


  • 03 Nov 2016 10:14
    Reply # 4362227 on 4195606

    Hi Scott,

    Your comments in your mail on 23rd about napping strikes a chord. In the 2009 Island Race in Poppy I told my daughter and her husband off for working their way backwards through the fleet while I had been below and she replied that I should not have had such a long siesta. Ungrateful child (aged about 35 at the time). You just can't get the staff! In the 2007 race I was sailing master on a Hunter Pilot 27 called Romarin, and we came 6th out of some 700/800 boats entered, so it can be done, but I didn't allow myself a nap in that race.

    Your model rig looks very good, and it should help you envisage how the full sized will work. Please don't take my comments as criticism, but as observations based on other viewings. The batten rise of 10° looks high to me now that I've seen Edward's 5° rise battens which is easy on the eye and looks as if it encourages airflow in the right direction. The yard angle looks very high and I don't think it will control the tip vortex. The split junk is an effort to get a high Lift/Drag ratio, so where it is possible to improve the lift with camber it is equally important to minimise the total drag. Total drag is made up from a number of components, and the major one is 'Induced Drag' (or tip drag or vortex drag which are not quite the same but are similar). This is one area we must try to get right.

    The elliptical Spitfire wing was supposed to be the most efficient shape possible, and aesthetically is most beautiful, but since the 1940's no-one has used it as better results can be achieved more simply. Look at the trouble aerodynamicists (spelling?) have put into tip design, and the structures on the tips of modern airliners. Arne's tip plates on rudders is a simpler but also useful effort to help the L/D ratio, so we should also look for gains at the head of the rig, or to be more precise, less losses with inefficient sail profiles. As tip plates seem to be impractical though some have tried them, the best simple solution is to sweep the tip up at an angle that will discourage excessive airflow rolling over the sail and letting the vortex form further down the leech. It would appear to be best if the rotating air is pushed right up to the tip of the yard, and in the absence of wind tunnel information my best guess has be as used on Poppy and Amiina, at around 40-30°.

    I now prefer the lower angle and feel the lower angle gives a better shape to the top tapered panels when it comes to defining the material patterns to get the desired camber. I feel the 30° yard gives good control of the tip flow and vortex.

    I didn't worry about the angle between the yard and the luff of the top panel. The model will show that you do need the tapered luffs to balance the tapered leeches. Any cambered sail drawn with straight luffs but tapered or fanned leeches will end up requiring parrels to maintain the straight leech or have high stress at the leeches of the upper panels, as the angled leeches push the battens forward like a bow pushing an arrow. The model helps you get a balance between the forward pressure on the battens at the leech and the aft pressure at the luffs.

    Yes, the yard has to be stiff. The weight of the whole rig is being carried by the halyard close to the middle of the yard, but the actual weight is carried at the tips of the yard, putting it under heavy bending moments. I tend to start with the sling point at 50% yard, but it usually ends up further aft, with the adjustable yard hauling parrel peaking it up and keeping it snug. A simple yard parrel keeps the yard against the mast, but lets it fall aft when lowered. You don't want anything tight when hoisting. Try hoisting the model rig from the lazy jacks.

    At one stage I referred to Amiina's rig as being 'brutal' as it was almost a parallelogram and aesthetics weren't considered, but when sailing with the cambers filled it looks powerful and doesn't look bad at all. The small amount of taper in the top panel softens the shape nicely in practice.

    Arne doesn't worry too much about the orientation of the cloths as it is not that critical with lowly stressed material, but I have always tried to follow normal sailmaking practice and place the thread line parallel to the leech. My reasoning is two fold. Firstly I want a clean run off for the air with no risk of a hooked leech, which will cause significant drag. And secondly, I form the leech with normal leech tablings and leech lines as is standard sailmaking practice. This way the cloth itself starts to take the structural loads towards the leech and must not develop bias stretch just inside the tabling. With a sail of any size I make the individual panels up from vertical cloths which is economical in material and with careful planning can provide the seams to use for broadseaming, killing two birds with one stone. It is really very easy when you can picture it. Try to make a model test panel with vertical strips of paper. Even angled shelf foot can be made up with vertical cloths, and be economical. There's not that much sewing involved, and when you use basting tape it's dead easy.

    Enough for now. Does this make sense?

    Cheers

    Slieve.


  • 02 Nov 2016 13:27
    Reply # 4360702 on 4195606
    Deleted user

    Thank you David and Arne - the "unless other factors are important" is a good rule of thumb.

    Working with the model, I can definitely see the need for some kind of yard parrel - if only to maintain basic control of it's location against the mast.  I don't think it needs to be running rigging - I'm guessing with careful placement, I can use just the halyard and combo downhaul/batten parrels to get the right set.

    With 35% balance, though, it might be that I'll end up putting the halyard placement more than 55 to 60% aft on the yard length in order to get the leverage needed to peak things up.  Also, since both the forward aft portion of the yard is shortened, I bet that yard needs to be extra strong.

    Are there other rigs that have the upper panels' luff tapering back like Slieves?  Reading through his write-up, it seems that there are two separate suggestions at work - the split rig for the jib-lift, and this top-of-the-sail profile modification.   I'll wait for him to chime in later this week as promised. 

  • 01 Nov 2016 18:49
    Reply # 4359382 on 4195606

    Hi Scott, I've just seen your posting and will try to get an answer in the next couple of days.

    Cheers, Slieve.

       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
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