Sailmaking detail questions

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  • 28 Apr 2016 04:14
    Reply # 3991960 on 1238226

    Yes, it's quite OK to finish a bobbin in mid seam, it happens quite a lot on big sails. When you start with the new bobbin, just make three reverse stitches over the end of the previous sewing before going forwards again.

  • 27 Apr 2016 23:32
    Reply # 3991713 on 1238226
    Please excuse my complete beginner's question. My sewing machine uses the small Singer 15 style bobbins, which only hold a modest length of V92 thread. Before starting each seam I've been making sure there's sufficient thread on the bobbin to complete it in one continuous pass. But is it essential to do the seams in continuous unbroken runs or is it ok to let the bobbin thread run out mid-seam and continue with new thread, oversewing the old?
  • 27 Nov 2013 11:44
    Reply # 1447268 on 1238226
    Deleted user

     David

    That's great - I was anticipating your reply might be more technical ie: "sail trim to windward has a narrower groove requiring greater concentration" - I was reading into your words too far.

    Once again thanks

    Ash

  • 27 Nov 2013 00:43
    Reply # 1447093 on 1238226
    I only mean that with a lower angle yard, the loads on the hauling parrels are much lighter, and so there is less work to do in setting the sail, reefing and shaking out reefs. Generally, a lower yard angle also means more balance area, and so lighter sheet loads. Gybing will be softer with the greater balance area, as well.
  • 26 Nov 2013 17:58
    Reply # 1446823 on 1238226
    Deleted user

    I have been reading and learning a lot by catching up on this thread.

    Thanks a lot everyone.

    One question for David Tyler please, on 11/3/13 on 1239162, 123910 you wrote...

    "There's a trade-off here. Steeper yard = more fan camber effect, but higher loadings on the hauling parrels and less relaxed sailing. Lower yard angle = less fan camber effect, but less load and more relaxed sailing".

    Would you just expand on 'less relaxed sailing, more relaxed sailing", I understand all the rest.

    Thanks

    Ash Woods

  • 20 Nov 2013 19:31
    Reply # 1442437 on 1442118
    Slieve McGalliard wrote: David also wrote – ‘However, I remember seeing a patent application for a stepped leech, so that in effect, there were a multiple of top edges, each generating a tip vortex of its own. It was claimed that the sum of the drag from all these vortices was less than the drag from a single tip vortex.’

    Ian Hanny (years ago I flew as his co-pilot) wrote a paper on this for AYRS many years ago. It made very convincing reading. Add in the current interest on humped leading edge whale fins, and the use of humped or saw-tooth leading edges on wind generators and even racing propellers on model aeroplanes and it seems there is gain to be made in chopping a wing or sail into span wise sections. I wonder if cambered panels with stiff battens has this sort of effect?

    There is still a lot to learn. One thing for sure, vortex drag and tip losses are a killer to performance. Huge sums are being spent on tip design.
    Cheers, Slieve.
    You could add to the list the spread primary feathers of a soaring bird's wing. Evolution has already done the experimentation, if only we could understand the results.
  • 20 Nov 2013 10:40
    Reply # 1442118 on 1238226
    David T wrote – ‘Related to this is the way that a skein of geese flies much more efficiently in a V formation than the same number of geese flying solo.’

    If you examine the vortex of the wing tip of any aeroplane (or even goose) you will see that it is possible when flying in vic formation to position yourself in the outside of the vortex, in the rising air so that you can throttle back and use less power/ fuel/ energy. What amazes me is the way the geese swop around and each take their turn in the lead position for a short period, pumping in the extra energy to keep the formation flying faster than an individual can continuously sustain. Racing cyclist teams also use rotating leaders for the rest to slipstream behind.

    David also wrote – ‘However, I remember seeing a patent application for a stepped leech, so that in effect, there were a multiple of top edges, each generating a tip vortex of its own. It was claimed that the sum of the drag from all these vortices was less than the drag from a single tip vortex.’

    Ian Hanny (years ago I flew as his co-pilot) wrote a paper on this for AYRS many years ago. It made very convincing reading. Add in the current interest on humped leading edge whale fins, and the use of humped or saw-tooth leading edges on wind generators and even racing propellers on model aeroplanes and it seems there is gain to be made in chopping a wing or sail into span wise sections. I wonder if cambered panels with stiff battens has this sort of effect?

    There is still a lot to learn. One thing for sure, vortex drag and tip losses are a killer to performance. Huge sums are being spent on tip design.
    Cheers, Slieve.
  • 17 Nov 2013 21:16
    Reply # 1440209 on 1238226
    I agree, Chris. The top edge of a junk sail, or a crab claw sail, may be developing half of the "ram's horn vortex" that is seen on delta wing aircraft, if it is at a suitable angle (though I could never seem to get any performance advantage from it, when I've tried low-angled yards), but the bottom edge cannot. 

    However, I remember seeing a patent application for a stepped leech, so that in effect, there were a multiple of top edges, each generating a tip vortex of its own. It was claimed that the sum of the drag from all these vortices was less than the drag from a single tip vortex. I can't find it now. This may have some relevance to Dave Z's design of sail.

    Related to this is the way that a skein of geese flies much more efficiently in a V formation than the same number of geese flying solo.  There is scope for experiment here - but it may all be beyond simple sailors such as me :-)
  • 17 Nov 2013 10:26
    Reply # 1439966 on 1238226
    Deleted user
    I've thought in the past about the similarity of the triangular top panel to a crab Claw sail.

    According to Marchaj's exposition of the 'vortex lift theory' the vortices roll up along the two leading edges of the sail. My reading is that the vortices develop, and are maintained by flow around the edges from the windward side. Fig 152 in 'Sail Performance - Techniques to Maximise Sail Power' shows this quite well. The leading edges (battens) should be sharp, and the sail should be axisymmetric.

    Seems to me that the lower edge of the top triangular panel fulfils neither of these criteria.
    It would not surprise me to learn that the Crab Claw works in ways other than the 'vortex lift theory' of Marchaj, however, although he seems to have investigated it more thoroughly than anyone else.

  • 16 Nov 2013 23:58
    Reply # 1439847 on 1239588
    Daniel Collins wrote:
    Kurt Jon UlmerThe top triangle alone is a usable crab-claw sort of sail, which can be set up to be taut or wind-spilling. 

    Ok, see I did kind of like that about the sails - it's sort of like what Dave Z. is doing on the Triloboat rig... just without quite the loss of area in the severely hollowed leech.
    Hi Daniel,

    The deep hollow in our CrabClaw top panel has a few points related to it:
    • One CC theory (Marchaz?) has it that the apex is a turbulent vortex generator. Turbulent whorls of 'hard air' (high pressure) roll along the limbs, generating lift. The cloth in between, in this theory, doesn't contribute much, though conic shape generate airfoil sections very naturally.

    • Some traditional CCs have extreme hollow... theory (getting speculative) goes that, in lighter winds, the whorls stick to the limbs, producing lift along light, flexible limbs. In higher winds, the whorls blow away from the limbs as sail cloth area reduces, leaving the remaining cloth acting as per normal. In very heavy winds, pressure draws the limbs together, spoiling the the shape of the remaining cloth. In all three cases, reefing is near automatic (some rigs sport a control line to draw the limbs together).

      Caveat is that most of the extreme hollows are found on tradewind craft... there's little data (reported or extant?) about how such rigs work on the wind.

    • On LUNA, with a two panel upper triangle, Hassler/McCleod standard, we found when deep-reefed that the CE of the upper panel(s) was high enough to round us up in heavy gusts.

      The deep hollowed leach on SLACKTIDE (successfully) brings that CE forward/inboard, and we haven't since had the round-up problem.

      It may be, however, that LUNA's ultra-shoal draft, full-length rocker and high-aspect off-centerboards (which make her an extremely nimble sailor) are merely a combination that make her particularly vulnerable to round-up.

      SLACKTIDE is similar, but with a long, full-depth mid-body; she is less nimble but tracks exceptionally well, and therefore may be less prone to round-up, anyway.

    One of these days, I would love to try a JR/CC sail that is a fanned stack of flexi-limbed CCs... each 'batten' would be two light limbs, joined forward and separating aft as canted Ys. Extreme hollowed panels separate and laced in place. Double sheeted. Doodles work out... maybe on a PDR?

    Totally impractical as a cruising rig, but it would be a wild treat for connoisseurs of the odd! For a small boat, it might even work out, though the reefed bundle would be a mess.  8)

    Dave Z
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