SJR for "Cirrus" State of the Art and Next Step?

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  • 30 Jun 2019 23:20
    Reply # 7711574 on 7584488

    Robert, further on the question of increasing the camber for the mains: I found this post from Slieve in an interesting thread from 2016. It gives one of the reasons why Slieve increased the camber and sheeting angle for the jibs but did not increase the camber for the mains, when designing the new rig for Amiina.

    "For the main panels we used 7% camber at 37% chord for panels 2 to 5, and 5% camber for the top panel. This was only a fraction more than we used for earlier rigs.

    It may be good to increase this figure, but it is never a good idea to change two variables at the same time as you will not know if either change has been beneficial. It is close hauled performance we are trying to improve and theory suggests that it is the jibs which effect this the most, and this seems to be the case in practice."

  • 24 Jun 2019 23:07
    Reply # 7644741 on 7584488

    I like your belayjng pins and partner arrangement.

    Here's a sort of visual comparison between the 10% and 8% mains models. You can see the difference. (Jibs are 10% and 12 degrees.)




    Also, here's Dave D's Wayfarer SJR sail, this looks to have a large camber in the mains, maybe equal to the jibs. It would be good to know how that particular detail worked out - Dave?  (Dave also has some good photos of making the sail in his public photo album.)


    Robert, I'm looking forward to the write-up.

    Last modified: 25 Jun 2019 00:33 | Anonymous member
  • 23 Jun 2019 15:52
    Reply # 7621011 on 7584488

    Hi

    A bit more opportunity to play today and a few photos to explain last winters modifications.

    The original mast is a 76mm OD 6082T66 Aluminium tube, 3.25mm/10swg wall thickness, with a wooden insert at the base. To convert this to a free standing mast I have increased the wall thickness by 3mm for the first 3M by getting a slightly smaller tube turned down to be a snug fit. I wouldn't do this again. Getting the 3M long tube inserted into the mast was problematic. There was a significant risk of the inserted tube getting irrevocably stuck before it was fully home. The day after doing this I read that getting a smaller tube and building up the diameter in 3 or 4 places to fit, with epoxy and tape would have done the same job. This sounds a lot simpler and is way I would try next time. The mast has then been extended by 1M by fitting an insert into the end and pop riveting the extension on. There are a lot of fittings on the top of the mast so that it works for both the existing gaff main and jib set-up as well as for the future SJR. The other photos below show the new mast base and partners being dry fitted and the extended mast in position on the boat. The mast is now 6M long with a bury of 0.64M.

    The 1:4 sail model has been enhanced today with a 10% camber @ 37% chord main panel. A very unscientific 15minutes spent with a piece of wool on the end of a stick hasn't shown any obvious flaws in how today's light winds flow across the panels.

  • 21 Jun 2019 23:34
    Reply # 7592897 on 7584488

    The thought and preparation you are putting into this project inspires me! I have created a “Cirrus” folder and look forward to more drawings and photos. Instead of paper models, I made a full size one out of plywood and used it as a “tailor’s dummy” when making the sail (did you see that here?) Slieve says that’s a waste of time, but does recommend the paper model.

    I have a feeling you will have no problem making the sail, I found that part rather fun. I still say the challenge will be to get a set of running rigging which all fits, and works, and “folds up” easily between outings. The trailer sailer does seem to present its own special challenges in this regard, and I look forward to learning about your solutions. Enough from me on that subject, but do keep a little redundancy in the mast length, if you can.

    Out of interest, would you please, when you get a moment, describe your mast (how it is made, how it was extended etc.)? And the bury, looks to be about 10%, is that right?

    Moving on to the “Next Step” part of your thread. As Slieve developed his ideas, from Poppy through to Amiina’s second sail, he has progressively increased the jib cambers and sheeting angle, and he wrote: “As to whether it is better than the earlier figures we cannot be sure, but in theory it should be better and there are no stalling problems. Even with the 'blunter' entry the boat still points well so there must be more 'upwash' which is desirable, and there seems to be more drive into a seaway. If building another rig for myself I would certainly be looking at even increasing these numbers, but for the moment these numbers are known to work well.

    Regarding the mains cambers, he wrote: “… , it is normal practice for the main to have less camber than the jibs…”

    This suggests to me that there may be room for some further development, and Slieve is the person I would go to with ideas. Maybe Slieve or Scott D will chime in here. Or, Robert S. and others who also have been developing this rig.

    Slot width, as you say, is rather easily altered on these sails, if you allow for it when you make the battens. Don’t forget though, if you alter slot width you are also altering the geometric so-called "balance" of the sail. In fact, any of the changes you might make, are likely to affect that elusive actual aerodynamic centre of pressure of the sail as a whole – whatever you do, that has to stay aft of the mast (axis of rotation of the rig), sail balance being exactly what we don't want.

    Your "37%" point of maximum camber: I guess there are people with theoretical knowledge who can comment on that. (see "aerofoil shapes and entry angles" including a delightful post from Arne on 25th April). I drew my aerofoil more or less freehand and I think that the maximum point was about there - by the time I had made the pattern and sewed the sail I am not quite sure if it was still there though! You realise, of course, your soft fabric sail when inflated will not end up looking exactly like the stiff paper model.

    We should also probably at this point remind ourselves of a common sense statement made by Arne in the thread referred to above, to the effect that this is not rocket science and people should not be put off making their own sails, from these hypothetical discussions which are so interesting to some people and maybe not so to others.

    You are at the cutting edge Robert. The beauty of these little home-made sails is that we can afford to “suck it and see.”

    And down in the garden where you have set up your model - I expect you will be planting a row of runner beans there next spring?


    Last modified: 22 Jun 2019 03:28 | Anonymous member
  • 21 Jun 2019 18:54
    Reply # 7592520 on 7584488

    Hi Graeme

    I may be should have finished off the sail design first but the mast did get extended this winter. On the basis that the mast is now as long as I can sensibly fit on my trailer the sail design will have to flex to fit :). To check out space for blocks and drift etc I have made up a 25% scale bamboo model which does look like everything should work out.

    I am currently using 30mm blocks and think these should still be fine for this sail. So there will be plenty of room at the top of the mast. If needs be I will be able to use just single lines and hoisting will be by hand.

    The photos below also show a couple of jiblets made up from paper patterns. The lower one is a second design iteration with a slightly tighter leech. Both Jiblets are made to a 10% camber with a 12degree sheeting angle.

    Thinking about the camber on the main panel. The current view seams to be that this is best designed at 7% camber at 37% of chord. Could the camber be increased to 10%? From drawing out the sail profile it looks like the air flow across the sail would still work and it would bring the Centre of Effort for the sail towards the centre of area which would be useful for keeping this boat balanced. Is there a reason I shouldn't try this?

    The other iteration on the SJR that seams worth trying is reducing the slot size to the minimum for the jiblets to fit around the mast. My mast is a 76mm tube so with a 12degree sheeting angle there is plenty of space between the mast and the jiblet leech. This is also a low risk trial as the slot can be easily extended as long as I have a bit of spare batten length.

    Any thoughts welcome.

    As I have had this week at home with time to play and think my thoughts on the sail design have moved on. As I'm back at work next week progress will be back to a more sedate rate.



  • 21 Jun 2019 00:31
    Reply # 7591206 on 7584488

    I think most people will agree with Scott in all his reasons given - and bearing in mind this little junk rig is scaled down from the diagrams we are normally used to looking at, your latest version looks to me to be a much more practical and workmanlike arrangement for a small boat. Well done.

    Have you made your mast yet? If not then you might want to allow a little extra height - either that or shorten your sail plan a little. I did the same thing as you are doing and designed a mast and sail with a view to trying to get every square inch of sail possible onto a mast which was as long as I dared to make it. The result was, just not quite enough room up there between yard and masthead. As it happened the mast was a  bit longer than planned and buried deeply in a sunk tabernacle, and I had not quite got around to cutting it shorter - so I had a way out. I have now a rig which is perhaps a little too big, we will see - but luckily at least it works. 

    You say that there is 0.6m of mast above the yard - but it doesn't seem to look quite right - and putting a ruler on the computer screen makes me think it is a little bit less than that, and at best, barely enough. The sail is almost brushing the deck, and if it turns out you do need a bit more halyard drift, then you have cut things so fine you might have nowhere to go. (I was previously thinking to myself that you always had the option of cutting off the bottom panel later, but with your improved sail you no longer have that available as a plan B.)

    Here is a thread from a few months ago that might be useful for you if you haven't already noticed it (minimum preferred mast height) and look especially at David T.'s typically lucid advice on this, which he posted on 27th March. His final comment: "But if possible, it's better to go for more than the absolute minimum drift that the blocks can tolerate." If you want to cut things fine, I will add that drift is not measured from the mast head. David's words again: "that drift being measured between the attachment points on yard and mast."

    Here is where I came unstuck: on a small boat you also need to remember to allow for the sail to be able to hang in the lazy jacks with the halyard released, with good clearance remaining for the sail to swing clear of the deck and/or tabernacle. With your boom just clearing the foredeck, your proposed sail plan is therefore showing the sail in taut lazyjacks (if they are the standing variety), and needs the potential to be hoisted a little higher with the halyard. You might have to try out a few different arrangements before you are satisfied with your halyard and lazyjacks. If you want running lazyjacks (as suggested by Dave D.) there is a couple more blocks and I haven't properly figured out a yard hauling parrel for mine yet, so there's another block which might have to fit in there somewhere too. There is not much room up there on a small mast, and the whole contraption has got to be able to rotate easily as well as just be hoisted.

    I suppose this is all so obvious to experienced junk sailors that it seems hardly worth mentioning. But not having done it before, and thinking I knew more than I really did, it was a trap I fell into. So, give yourself just a little bit more wriggle room is my suggestion. Your Pathfinder is going to be a great little conversion.

    Last modified: 21 Jun 2019 10:35 | Anonymous member
  • 20 Jun 2019 22:15
    Reply # 7591071 on 7584488

    Thank you for the feedback. I think I will go with a four panel design (updated pdf below) to reduce the weight and complexity. Three levels of reefing is plenty and I will use lazy jacks and a sail bag arrangement to keep the reefed panels out of the way and tidy.

  • 20 Jun 2019 15:48
    Reply # 7589630 on 7588575
    Graeme Kenyon wrote:

    I pointed out the long thin panels merely as a point of difference, and have no idea if aspect ratio of jib panels is significant. I had correspondence with Slieve some time ago in regard to scaling up an Amiina rig for a larger boat and I got the impression that he felt the aspect ratio was about right, and I was not encouraged to change it. I did wonder if there would be any significant loss from your having two low aspect jibs instead of a single larger one with the same area. I have a feeling that the right answer to this question is: no-one really knows until someone tries it.

    My intuition tells me that many small panels would be less efficient than a few larger ones.  More weight of battens, more sewing, and more of the airflow past the luff is in transition from shelf foot to middle section of the panel, more turbulence. 


  • 20 Jun 2019 00:42
    Reply # 7588575 on 7584488

    Robert you seem to have thought your way through this better than me and I would like to revise my initial comments about your sail area, which of course is a matter for you to judge for yourself. I suppose I was berating myself rather, because I think I made my mast and battens are a bit too heavy for my boat, and I think I could have made my sail a little less in area – and I was comparing my rig to the very light weight Bermudan rig it replaces. Still, not a bad idea to keep these things in mind. I wish I could afford carbon fibre – and I do intend to try to find lighter battens. (Mine are made from solid timber, same as River Rat. I am wondering now about bamboo.)

    From David W I can see now the special advantage of a hinge-tube mast arrangement when it is applied to a trailer boat – the ability to leave the rig in place, below the hinge, when dropping the mast. I can’t yet think of a way of matching that luxury. My mast is walked up from the heel in the same manner as you (Robert) are proposing, and it is so simple, easy and economical it is the one detail I am pleased with and don’t want to change. Anyway, if you can manage the mast and sail together like David D describes, then I guess you guys will get the best of both worlds. Marcus (with Freebie) tells me that he lifts out the complete rig and stows it all in one bundle (but still tends to have a bit of untangling to do at the launching ramp.)

    One thing I did not understand: your concern about “stagger” when reefing. I am not sure if we are talking about the same thing, but  – perhaps it is to do with having a vertical mast and very small batten rise, I do not know - but when I drop my sail, fully or partially, it behaves delightfully well, way, way better than any gaff or Bermudan rig I have had before - and the battens stack up with barely any stagger at all. I can’t see why yours would be very different.

    I pointed out the long thin panels merely as a point of difference, and have no idea if aspect ratio of jib panels is significant. I had correspondence with Slieve some time ago in regard to scaling up an Amiina rig for a larger boat and I got the impression that he felt the aspect ratio was about right, and I was not encouraged to change it. I did wonder if there would be any significant loss from your having two low aspect jibs instead of a single larger one with the same area. I have a feeling that the right answer to this question is: no-one really knows until someone tries it.

    Last modified: 20 Jun 2019 23:19 | Anonymous member
  • 19 Jun 2019 17:37
    Reply # 7587937 on 7584488

    Hi 

    Thank you for responding to my post. I had considered a hinged mast and if the final sail design means that the mast position has to move forward again I think I will use this option. In the current position the mast can be raised by positioning the heal and walking it up to the vertical. This seemed to present the simplest solution but fixes the position which is intimately tied in with the sail size and configuration.

    My intention is to keep the sail bundle and associated rigging attached during the mast raising and lowering by designing a bag that can keep the sail and battens under control. Now that I am thinking about batten sizes and materials I am realising that a Junk sail does have a significant weight which I hadn't totally appreciated. Thank you for highlighting this.

    I have worked out the sail size and balance by aiming to keep the centre of effort in the same position as the current rig. i.e. if the mast moves forward the sail area reduces and vies versa. I was also looking for a bit more sail area on the basis that it could be reefed easily. So a good challenge on the size of the sail, I wanted more but this mast position does result in plenty of sail area. I am not sure that it is fair to go back to John Welsford though. I am sure that he will have designed the current rig as his best compromise that can be used by a wide public. The obvious answer to the question of "is this rig too big" is "yes". I am designing this rig for me and taking the personal risk with the design. When the boat is sold on it can go with the existing gaff yawl rig as John intended. From my perspective I would like a little more drive than the current jib and mizzen provide on their own which is why the top panel is bigger than usual and incorporates the area of what would have been the second panel(I thought I was also saving a batten). I had been rationalising the overall size of the sail on the basis that it would be good in light winds and easily reefed. Is this how it works in practice? I find the current gaff main difficult to reef on the water and easy reefing is a prime objective.

    I had been pondering the amount of mast I needed above the yard are there any rules of thumb for this? There is currently about 0.6M available in this design.

    The boom height above deck level has been set at half the panel height and the 5degree rise should mean that it will miss heads of the crew. There is space and access to mount blocks etc at the base of the mast below deck level.

    The panel height had been worked out on the basis of minimising stagger of the reefed sail as I estimate there is only just enough space between the end of the battens and the stern of the boat where the sheets can be secured. This is why I have long thin panels. I am wondering how significant the 1.1 aspect ratio of the jiblets is? I had not thought about this, should this be the overriding consideration for the minimum panel height for a SJR?

    Thank you for the link to the sail balance, mast position, slot width thread. I had missed this exchanged and it helps me.

    Last modified: 19 Jun 2019 17:40 | Anonymous member
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