The OPUS rig is our new venture here at ScaleSailing.
This page will contain and hold parts of what has appeared in the blog posts about the rig and further details about it’s use and tuning. (OPUS is a trade mark of Graham McAllister Designs.)
INTRODUCTION
Over the winter I have been developing a different style of una rig and a new footy to test it on. Early talk about flexible rigs and their benefits for use on footys centred around the flexible mast but apparent complexities in their design meant there was little development in that direction. Later the ‘McRig’ came on the scene and it’s simplicity was infectious and it has become rightly popular. Rather than the mast bending it used a torsion bar for the mast pivot allowing the whole rig to flex from it’s base. I wanted to go back to those earlier discussions and see if I could make a bendy mast rig work.
In developing OPUS I was inspired by the una rig on the Laser dinghy. The curved mast on the Laser is shaped by the sail pocket. I decided to create the mast curve with a backstay instead which allows the mast to be shaped independant of the sail thus allowing some sail camber control. A pure Laser rig would have the mast set too far forward for a short hull like the footy so I have adopted an offset pivot on the main boom so the rig now became a una swing rig. I have been careful to allow no flex at the base and boom mounting so all of the flex and gust reaction is handled by the flexible carbon mast and sail itself.
The OPUS rig was tested briefly in the light Fall airs here before the winter freeze set in. Over the winter the idea was developed further and hand held tests in varying winds were photographed and studied to see how the rig was behaving. Clearly the rig worked nicely as a swing una rig which would make it a nice rig to use were it simply a stiff unit. What I was looking for was how would it behave in gusts as this is becoming a key element in racing footy design.
steady breeze stronger gust reaction
Well the results looked good, in a gust when the rig is close hauled or in a reaching position the mast bends away to leeward as might be expected but it also curves back more under the action of the sail and induces more twist in the sail. We know that bending away and dumping some wind in a gust is good but increasing twist is possibly even better. The movement adds upper drive due to the twist while dumping air and also keeps the lower area of the sail as set and driving the boat forward.
‘Laser’ style swing rig in action on the Dragon prototype.
The ScaleSailing Dragon/OPUS combo saw water again on the Friday of the NCR in Orlando, FL. I took the opportunity to sail the boat and with a good but variable wind blowing was able to see how the newest version of the rig behaved on water compared to land in a variety of wind strengths. It looked good on the water, I particularly like the elegant shape of the curved mast which was different from everything else on the water that day. The sail approximates to an elliptical shape which is typically considered to be an efficient planform. Tacking was quick and easy with the sail filling quickly and the boat accelerating away well. I think this initial sail size I have chosen handled the stronger wind well and should make a good ‘working’ rig to cover a range of conditions. Being used to sailing two sailed sloop rigs I did love how easy it was to turn on to the run downwind and not have to hope the jib popped out opposite to the main. Yet there was no odd behaviour as is sometimes reported for the classic two sail swing rig set up. I am prepared at this stage to say that this OPUS rig works and works well. Clearly there will be things to learn in tuning it, controls available are mast shape via the backstay, sail foot shape and initial twist shape via sliding rings on the boom.
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FURTHER TESTING
The other day we had a great time sailing the OPUS/Dragon prototype on a friend’s garden pond. The pond has a fair bit of growth here and there so it was great fun sailing around the obstacles. This was a good opportunity to test different mast locations on the hull to see how the balance of the OPUS rig compared to the expected position of a sloop rig. My original calculations were based on keeping the centre of effort (C/E) of the OPUS rig in the same fore and aft position as the sloop rig would have been. The calculated position has proved to be just about right in practice as the two more rearward positions I was testing this day increased the weather helm more than I like.
A nice easy run showing the full sail to the wind, making good speed
The OPUS rig does use quite a conservative number for the virtual ‘jib’ area as a part of the whole. This does move the required mast pivot position a little further forward than might be possible but more importantly it makes the rig well mannered and easy to sail with little chance of backwinding. I have always believed that ‘easy sailing is fast sailing’ so it should be a design goal.
I am in the process of making a larger area OPUS rig based on a longer boom assembly for light airs and will be developing a smaller ‘A’ rig too. The rig in these photos is about 140 sq.in. area and will be a good all rounder for varying conditions.
Working harder now, the OPUS rig looks good in action.
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LIGHT AIR ‘OPUS’ RIG
This is a larger 160 sq.in. rig ready for testing, the sail is cut from 0.75oz Challenge sailcloth.
And now on the water in a light wind…
A gentle run with the whole sail area presented to the wind.
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All content of this blog including text, photographs and the model designs are the sole property of Graham McAllister Designs.
Copyright 2010 by Graham McAllister Designs. OPUS is a trade mark of Graham McAllister Designs.
Hello Graham.
In a side by side run with the Opus Dragon… Is there a big difference in speed over the Pond Sprite or Kittiwake?
Thanks.
Kittiwake 2 & Pintail windler.
Skip Robb.
I have designed the Dragon and OPUS rig with the intent that there should be Skip. With luck I will be able to make those side by side tests within the next few days or at least when the weather allows. I have club mates who have a K2 and Pond Sprite to test the performance against.
When I sailed Dragon on the practice day for the NCR in Orlando she certainly looked quick against other boats. Time will tell.
Graham
Graham,
Will you be publishing plans for this rig?
Thanks,
Tim Bosma
Footy #247 “Extremely Urgent”
Hi Tim,
the rig will initially be a part of the Dragon kit and will also be available as an almost ready to use rig for use on other designs. I have been ironing out a few construction wrinkles and I will be making it available very soon. Thank you for asking.
Graham
Hello Graham
The Opus rig looks like a winner
I`m looking forward to build or buy the plans
Regards Ken Peltier
I am interested in ordering both 1 140 sg inch and a 160 sq inch OPUS rigs. How do I go about placing and order?
I am about to start building the Kittiwake 2 kit I purchased some months ago. I certainly like the looks of the Opus rig. Do you think the K2 could be modified to work well with the Opus? I was thinking of putting in mast-support tubes in perhaps three different positions, and experimenting to find the best. Graham — I would appreciate your thoughts.
Hi Kensail,
OPUS rig on a K2, good question. I have not actually tried an OPUS rig on the K2. If the mast is positioned to keep the centre of pressure of the rig about the same as the sloop rig than yes I believe it will sail fine. Nose diving on the run in a bit too much wind may be worse because the mast position will be further forward. It is something I need to experiment with when the water melts to be sure. So I don’t want to say a definite yes at this point. I will take a look at the sail plans and report back with a suggested mast location for the K2. Thanks for asking,
Graham
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