Aerofoil shapes and entry angles

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  • 07 May 2019 22:14
    Reply # 7328183 on 7302245

    Thanks Scott, it is no big deal really, just an experiment, made from scraps- a "learning how to sew" exercise. As Arne and others have recommended, it is a good idea to try something small first. Just doing a hoist today, so I can figure out what kind of running gear is going to be needed. When it goes out on the water for the first time I will let you know.`

    The tabernacle I feel very confident about. Malcolm started a thread recently on this subject, which nobody replied to, so I will chime in there with a photograph of the aluminium tabernacle.

  • 07 May 2019 15:54
    Reply # 7327195 on 7326888
    Graeme wrote:

    [...]This evening I finished making my first sail. I will be raising it for the first time tomorrow morning and feeling a bit nervous. [...]

    This is very exciting! Please let us know how it turns out. If I recall correctly you were also fitting an aluminum tabernacle to your boat without any welding. I would appreciate it if you could take some photos of the tabernacle setup and provide some more details on how it was fitted to mast step, partners and the mast.

    Last modified: 07 May 2019 15:55 | Anonymous member
  • 07 May 2019 14:14
    Reply # 7327043 on 7302245
    Anonymous member (Administrator)

    Graeme,

    when I mentioned "all the numbers and digits", I was thinking of the link that Andrew showed us, below. That surely made me grey out for a second.

    As for using CAD, I only use that for the sailplan. The required round to achieve a desired camber, is found with my simple "chain calculator" method. The round is drawn with only one station, that is, the max camber point, and the rest of the shape of  the round is found with a bendy wooden spline  -  as far from rocket science as you can get (See Chapter 4 of TCPJR).

    I wrote what I did about the SJR in the last paragraph because this was a hotly debated theme a good while ago, when some found the SJR to be unstable with the sheet released. I am convinced that this issue can be avoided or fixed.

    I remember when I made the first fully cambered sail for my Malena, in 1994 (see NL30). I worried that I might be wasting my precious holiday on producing a major screw-up. However, I kept repeating "the worse it looks, the better it sails", and in the end  it turned out to work very well, indeed. 

    Arne

    Last modified: 07 May 2019 14:52 | Anonymous member (Administrator)
  • 07 May 2019 12:06
    Reply # 7326888 on 7302245

    Arne, you made a good point by reminding us that it is possible to make a good junk sail without the need for rocket science. Your concern that the current discussion might discourage “newcomers” is well intentioned I am sure. (It might have been better though, if you had left out the last paragraph which I think might be mostly speculation.)

    Surely you are not concerned about the use of computer aided design? It’s a bit over my head too, and I don’t use it, but I think it is of growing importance and interesting to see that some people are exploring how to apply it to a junk sail design.

    You surely can’t be worried about the numbers being quoted? (“So many numbers with so many digits” ?)  These are the same numbers you quite rightly use all the time (camber and balance) plus one more number which applies uniquely to the split rig, that is sheeting angle for the jibs panel, which can't really be avoided. None of these numbers has more than two digits.

    I agree that maybe the conversation about aerofoil shape (and entry angle, which is one of the things which defines it) is “so much fuss”. I feel a little embarrassed for raising it. Worrying about that sort of thing is just what newcomers do. You may be right by suggesting it is not so very important, I am not sure yet. The problem is, when building a sail with the shelf method, unlike with the barrel method (and regardless whether split junk or any other sail) – when lofting the shelf you have to start by drawing an aerofoil shape. You can’t avoid it. So, naturally, a beginner like me would like to know what is the ideal shape to draw, even though we all agree that the final result might look a little different when it actually fills with wind. (But don’t be too sure about that either – the sail which Paul made recently for Pango looked to me to be pretty close to the foil shape it was designed to be, and I can’t see any harm in aiming for that.)

    One last thing Arne – you say that “it is difficult to get the sails wrong.” I am very much afraid that I may have succeeded in doing just that. This evening I finished making my first sail. I will be raising it for the first time tomorrow morning and feeling a bit nervous. I sure hope you’re right and I’m wrong! 

    Last modified: 07 May 2019 14:00 | Anonymous member
  • 07 May 2019 09:19
    Reply # 7326680 on 7302245
    Anonymous member (Administrator)

    Gentlemen,

    I can’t help feeling that you are about to turn the simple act of designing and constructing a good junkrig into rocket science. So many numbers with so many digits.

    I hate to discourage you; I am sure your approach is an interesting intellectual challenge, but why so much fuss? After all, no rig can sail faster than the boat allows it to, and (correct me if I’m wrong), your boats are not in the America’s Cup League, with hydrofoils and all, are they?

    The reason why I’m butting in, is because I fear that newcomers here may believe that making a junksail is only for the very bright experts. It is not. I have made several of them, and I have come to the conclusion that it is difficult to get the sails wrong. The only hard rule I follow, is to not make flat sails. As to how the ideal curve of the camber should be, that doesn’t seem to be critical at all.

    I understand that the split JR is a special case. The main thing (safety- and handling-wise) is to make the ‘mainlets’ and jiblets work together so that they luff at the same time, or the jiblets a moment before the mainlets. This ensures that the whole sail will feather in the wind and not start oscillating. My guess is that this is easier to find out on the boat than on the computer. If I were to make a SJR, I would be prepared to make adjustments to, or even re-make, the jiblets until I get it right.

     

    Arne


    Last modified: 07 May 2019 09:21 | Anonymous member (Administrator)
  • 07 May 2019 02:00
    Reply # 7326199 on 7317968
    Graeme Kenyon wrote:

    Reply to Len:

    So the closer the foresail is to the mainsail the greater the sheeting angle….  I can’t agree with that. Some years ago I had a scow which had two headsails. The jib tops'l (the outer one) made a big difference. The jib tops’l performed best with the greatest sheeting angle, the (inner) stays’l was sheeted a little tighter and the main sheeted in the tightest. The opposite to what you are suggesting.

    I took another look (this past years Sydney Hobart race in case you were wondering) and I think my mistake is that I was not taking into account the sheeting anchor point  which can not go outboard of the beam. With the Genoa being as big as it was on these boats, one has to look at the foil shape somewhat higher in the rig.
  • 04 May 2019 13:12
    Reply # 7321486 on 7302245

    Andrew:

    Using Google Docs is a good idea.

    At this stage I'm just using the spreadsheet to work out the offsets needed to get the correct camber. I'm not trying to plot the outline of the sail panel. Thus the workflow goes something like this:

    1. Plot out panel full size with straight edges on large sheet of paper, leaving sufficient space round the edge.
    2. Mark out X coordinates
    3. At the correct X coordinate, add the Yextra to top and bottom of the panel

    This doesn't take into account non-rectangular panel shapes. What to do about these is currently going round and round in my head.

    Small-angle parallelograms (i.e. the 10º batten angle commonly used) should work exactly the same as a rectangle; that is the Hp distances are the vertical distances between the battens. I would also keep the warp vertical as this will resist stretch keeping the camber to the correct shape.

    A non-vertical leach isn't a problem as the panel is pretty much flat here. However I haven't worked out what to do about a non-vertical luff.

  • 03 May 2019 17:49
    Reply # 7318679 on 7302245

    Martin:

    Trying to visualize what your spreadsheet was doing. I copied the data & formulas into a google sheet. I added a few columns to show what formula I was using for X & Y values. I need to spend a bit more time to figure out how to orientate it to have the dotted green line be the X axis.

    (Still trying to figure out how to post an image via link. It seems the message board resolves anything ending in .jpg and converts it to an <img> link, but not the google sheet's image url which doesn't have any extension, but the browser resolves as an image just fine.

  • 03 May 2019 10:21
    Reply # 7317968 on 7302245

    Reply to Len:

    The first (way to adjust sheeting angle) is to use a "hing system" much like Mingming 2 where the hing pieces are adjustable length (line instead of fabric?).

    Lines have been used. Dave Zieger rigged his scow split junk mainlets with ties instead of a shelf. (I think it was intended to be a temporary arrangement and not with the purpose of adjustability.) Evidently it worked.

    (Cropped  from a photo by Peter Frost, on http://triloboats.blogspot.com/2018/)

    So the closer the foresail is to the mainsail the greater the sheeting angle….  I can’t agree with that. Some years ago I had a scow which had two headsails. The jib tops'l (the outer one) made a big difference. The jib tops’l performed best with the greatest sheeting angle, the (inner) stays’l was sheeted a little tighter and the main sheeted in the tightest. The opposite to what you are suggesting.

    Last modified: 03 May 2019 10:27 | Anonymous member
  • 03 May 2019 09:59
    Reply # 7317960 on 7302245

    For me I guess the main reasons for barrel-cut are:

    • Simple to make
    • Easier to get my head round what the shape will be - I'm pretty confident that it will be close to an arc of a circle
    • I've already made a sail like this and it works well
    Shelf foot may well be better for the jiblets so I am going to make a prototype panel to see how it all looks.

    Regarding sheeting angle of jiblets:  I guess we're trying to maximise speed into wind regardless of the tacking angle. That is, a small sheeting angle may result in being able to sail closer to wind, but this is not useful if the hull and rig are not capable of sailing efficiently at that angle. So a bigger sheeting angle may result in lower pointing ability but higher speed towards the destination upwind.

    In theory the bemuda rig jib sheeting angle should be the optimum for the hull. I could then add on a bit to compensate for the extra drag of the junk rig and to ensure that the air can get round the mast. We'll see what that looks like.

    Regarding camber - I mostly sail in light winds with small children. Lots of camber is a good thing in light winds so I'll aim for 12% or more in the lower panels tapering down to 3% or 6% in the upper panels.

    I do need to review all the literature on SJR again; I've got a sailplan that I think works but need to go over it several times before cutting fabric. Thanks for the links above.

    My other dinghy has the mast forward so isn't a split rig; however it does have barrel cut panels with plenty of camber. The folks at my sailing club have told me they were impressed with its performance; this is despite a very traditional hull that sits deep in the water compared with most modern boats. However, the kids paddling may have had something to do with the performance!




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