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17 February 2014

Rethinking the Rig

Our first shot at SJR Ketch...
Looks good, but doesn't feel right

Second thoughts are ever wiser.
- Euripedes

Rethinking the Rig

In a previous post (Rig o' Morale), we introduced a variation on Slieve McGallister's SJR (Split Junk Rig), and the above ketch sailplan.

But a few points have been gnawing at me:

The mizzen (after sail) is most valuable to us as a maneuvering/weathercocking sail. Efficiency is welcome, of course, but not a prime consideration.

To leave anchor, for example, we raise mizzen, haul anchor, drift back on backed rudder and back mizzen to fall off reliably on our preferred tack, then raise main and sail off. No muss, no fuss, even with dangers close at hand. For sailing backward, again, we're backing sail and steering with rudder reversed. Occasionally, we'll leave the mizzen standing for a riding sail. We'll often drop the foresail, haul the mizzen close and drift, bow into wind of virtually any strength. This stops our headway, and eases motion for such chores as bailing the dory.

For each of these tasks, SLACKTIDE's flat-cut, low-lift, high drag mizzen has been perfect. Picturing these maneuvers with an extremely high-lift sail, with a large balance (area forward of the mast), jiblets cut full with a loose leech... well, it gives me pause. The high-lift can kick in abruptly, turning a backed sail into a driving one... a willawaw in close quarters could get tense. The large balance moves the mizzen's CE forward, reducing its positive effect aft. The jiblets are likely to flog in irons, which is hard on the sail and nerves... no problem for normal sailing, but... well... we're not normal!

If we want, therefore to go back to a good ol' unsplit, flat-cut mizzen, it reduces the balance by about 4ft, vacating considerable 'airspace' over the hull. Our sailplan is already snug, partly to avoid micro-burst surprises, and also as we don't like to handle very tall/heavy masts. So we want to fill that gap, at least to some extent.

Schooner rig (main mast at the forward end of the pilothouse) would do the trick, but the less efficient, after sail would be far bigger than it needs to be for our needs, and a relatively expensive tabernacle is required to deck-step the tall, free-standing mast. Also, the sheets sweep the cockpit (doable, but annoying), and we'd then want a driver (transom mounted sail) to improve balance, induce weather helm and take over maneuvering (more gear/handling/maintenance/expense). So we'll stick with ketch.

How to close the gap?

First way is to grow the mains'l, extending the battens from 18ft to 20ft. This is as long as one can get commercial lumber, in our parts. We generally prefer shorter battens for higher sail aspect ratio on a given height and less weather helm. But with this design's long mid-deck, we'll make the stretch.

Second, we'll reduce the mains'l balance to 25%. Slieve's first sail set in this proportion. He felt that more, up to 35% would improve performance (subsequent experiments confirmed this), and handling by reducing sheet stress. But even at 25%, the rig performed and handled well. We'll be rigging six part sheets, which will easily handle any extra sheet stress.

Another consideration is that our favored sailcloth comes in 60in width (5ft). At 25% balance, the jiblets can be made from one swathe, while the body can be made from three. Material efficiency!

 As a trade-off, this arrangement sounds reasonable to us.

The result now looks like this:

Note the mizzen slot is gone.

Since these changes, I'm breathing easier. That tight feeling I've gotten - trying to picture our first sailplan in an emergency exit, after dark in rising wind - it's gone away. Confidence is high. Without realizing it, that SJR in the aft position had been working at me.

Funny thing with design. We doesn't always know exactly what we want, or why. Things feel right, or not, and it may take a while to figure out why or why not. But it's almost always worth the effort.

And changing our minds on paper is cheap!




6 comments:

  1. Version 2.0 of agile sailmaking looks good! What are you going to use for sailcloth?

    ReplyDelete
    Replies
    1. Hi Jeremy,

      We love the TopNotch we used for SLACKTIDE. It's an inexpensive, unbacked, polyester cover-cloth. At 11oz, it's on the heavy side, but stands up as stormcloth. We'll use it on the mizzen, and as an upholstery cloth, below.

      For the SJR, we're still thinking. Stretchier cloth helps smooth a cambered sail... nylon does extremely well, but may not last as long. Acrylic is a contender. Either way, they improve on the grass mat that junk rig evolved around, so we're not too worried.

      The nylon, especially, is cheap enough we could easily get enough material for a second set. Between them, they'd likely last us our lives. Low UV, up here, is life-extender. Seems the sun is a sail's worst enemy. We saw no chafe on LUNA, but sun damage after 10 years (no covers, it must be said) had really reduced strength. Hers was a hi-modulus polyester sailcloth at 7oz..

      So still mulling... will keep you posted!

      Dave Z

      Delete
  2. So what mast heights and sail areas are you now looking at? ALso, from looking at the SJR, there is a significant gap at the mast, how does that fit into the calculations?

    ReplyDelete
  3. Hi Dennis,

    Mast heights are 30ft mizzen with about 275ft2 (square feet) and 34 mizzen with about 500ft2, for a total of about 775ft2 Both masts measured from deck to head.

    The gap is drawn one foot wide at the mast; each panel is 4ft, so, across 5 panels, it took 20ft2 off the area calculation for the main silhouette.

    If I were to bother calculating the total CE (which I rarely do... mostly eyeball it), I'd figure CEs for mizzen, jiblets and the after-main, then work out their common center. This method would take the slot into account.

    One reason I don't calculate CEs is that it's already a high-assumption approximation (ignores sail curvature, vectors and incidence, heeling and hull/water interactions), which then gets plugged into rules-of-thumb. In other words, all that heady calculation provides misleading precision, in my opinion, to what amounts to eyeballing it.

    In some hulls, like IOR racers, the variation is so narrow and there is so much data, calculation might well pay off. If anyone EVER calculated CEs for a barge, I'd be surprised, so any rules-of-thumb are borrowed, bent and broken from sharpies. Guesswork.

    So far, for various reasons, SLACKTIDE's CE leads the CLR too much (indicated by slight lee helm with all sail set and the boards fully forward). The easy adjustment is to haul the mizzen or reduce the main.

    It's possible that the SJR and fwd position of the main mast will increase lee helm, though we'll also be able to bring the boards farther forward, and the pilot house provides aft windage. I'm guessing, however, that the excellent SJR power vector (well forward) will reduce heeling moment, which is the force driving the bow of around the CLR.

    Here's hoping, anyway! 8)

    Dave

    ReplyDelete
  4. Replies
    1. Is this spam?

      Normally, I'd delete it, but the pics are just too cool!

      Hope y'all can read a little German.

      Dave Z

      Delete

About Me

My photo
Anke and I live aboard SLACKTIDE, our T26x7 ketch. We sail by wind, tide and muscle in the waters of mid- to northern Southeast Alaska. We try to maximize the joys of life, and minimize the chores. ........ We live between the communities of SE Alaska, but drop in to visit with friends. Lately, we've worked, every other winter, care-taking Baranof Wilderness Lodge in Warmsprings Bay. This has given us a window on Web. ........ We're working toward a subsistence lifestyle, somewhat impeded by addictions to coffee, chocolate and cheese. ........ We think TEOTWAWKI is looming, and while we won't be ready, we'd at least like comfortable seats.