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

<< First  < Prev   1   2   Next >  Last >> 
  • 27 Jun 2020 16:32
    Reply # 9064065 on 7584488

    Hi Graeme

    Yes it is a sail catcher but yes it isn't very deep due to space restrictions. I am wondering about the forward lazy jack. I'm going to see how it goes and it might not be there next year.

    Thanks for your comments. I am really please with this new sail it looks like it is going to be worth all the sewing.

  • 27 Jun 2020 15:17
    Reply # 9064000 on 7584488

    Well, that looks like a delightful inshore cruising-camper with a very nicely proportioned rig. I will be interested to know what John Wellsford thinks, and Slieve. It looks really good to me.

    Is that a shallow sail catcher you have rigged on the boom? I am not sure if it is, or if it is quite deep enough to fully muzzle the sail. If it is, then I think you could do away with that forward lazy jack which is interfering with the camber of the bottom jib panel.

    (I found with my SJR that a sail catcher is worth having as lazyjacks don't muzzle the jibs properly, whereas a sail catcher does - and if you have a sail catcher then you don't need the lazy jacks - just lifts fore and aft, and the forward lifts can coincide with the slot.)

    I thought your mast was going to be too short, but you seem to have got away with it.

    Well done. Your boat looks a treat. 

    Last modified: 27 Jun 2020 15:18 | Anonymous member
  • 27 Jun 2020 14:43
    Reply # 9063955 on 7584488

    Hi Folks

    A few pictures of the sail working. I now do have a junk rigged boat :)

    Winds were very light and I was surprised how well the boat moved. Now that I have seen and experienced the rig I am looking forward to learning more about how it works in practice in different conditions.

    3 files
  • 22 Apr 2020 22:53
    Reply # 8922825 on 7584488

    It has been a while so I thought I would share an update on the SJR development for Cirrus.

    To get myself into the method for making the sail I started of with building a few panels as a quarter scale model. This was a very useful exercise and helped my understand the material, required seam widths and how to get the sewing machine to do something like I wanted it to do. It also allowed me to do some "wind tunnel" tests in the alley down the side of my house with some interesting results. I had designed both the jiblets and the main panels with 10% camber. It was obvious from the model that air exiting the jiblets was flattening the leading edge of the main panels. So I went back to redo the design of the main panels and also discovered the thread on this site about catenary curves. I decided that a catenary curve was more representative of the shape that each panel would take. Using this approach a 10% camber turned out to be very similar to an 8% camber using the angeld shelf method. This turned out to be a long way round to get to the same conclusion others have reached that 10% camber on jiblets and 8% on the main panel is a good compromise.

    This isn't a particularly big sail (15m2). I have used 3.8oz ripstop nylon which is very flexible and relatively easy to sew in a smallish space. My dinning room was kitted out as a sail loft with a few sheets of plywood. I created patterns for each panel on QCad and had these printed out full sized on A0 sheets (sometimes more than one and taped them togetehr) to be able to transfer the shapes to the material. I found that a fiber tipped water based pen was best for drawing around the patterns and markign out the various seam lines etc. Then there was a lot of sewing. It has taken me about 15 days to construct the sail. More than twice my initial estimate. The pressure was on to get it all finished and ready for the start of the season but Covid-19 did take the pressure off and making the sail provided a nice project to do indoors.

    The sail is now on the boat, rigged and ready to take sailing once the lock-down is lifted. Note that in the photo the sail isn't fully raised or the sail bag dropped to its working position. It was a bit too windy.

    3 files
    Last modified: 23 Apr 2020 09:39 | Anonymous member
  • 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.

    6 files
  • 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.



    2 files
  • 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
<< First  < Prev   1   2   Next >  Last >> 
       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

                                                              Site contents © the Junk Rig Association and/or individual authors

Powered by Wild Apricot Membership Software