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Archive for April, 2009

Lurking below the OPUS rig which has been getting all of the attention lately is my new hull design, the Dragon. Construction is a hybrid form of my usual system, EPS foam core, balsa wood sides, stem and transom with a high density foam bottom. The bottom is 5mm thick and this allows the chine to be easily sanded to give a nicely radiused ‘soft’ chine. The deck will be 1/32″ laser cut birch ply and internal structure to our usual pattern, which means simple and as little as possible.

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The Dragon shape is changing a little from the prototype seen above, the stem or bow will be 1/4″ deeper and the transom 1/8″ shallower. This gives a little more reserve bouyancy at the bow and increases the deck rake a little, the transom angle is being increased too. The trailing bulb is an important feature of Dragon which allows the weight to move aft without compromising the distance between the fin and rudder, and therefore rudder efficiency. As the kit version is being built I will be adding a dinghy style spray rail and a lift off wooden hatch as has proved so effective on the recent Pond Sprites.

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Production version of the Dragon foam core. 

The Dragon is a long hulled (fits diagonally in the measurement box) ‘new rules’ boat. She is designed to use a 3x AAA cell battery box for race performance but is definitely capable of carrying a 4x AA battery pack in a standard square battery holder. Being designed for and alongside the new OPUS una rig that is how the kit will be set up.

I am really excited about this project and will be moving ahead with it at full pace. As a result I am going to move forward with this boat ahead of the Pond Sprite project. I also intend to trial the Pond Sprite with the OPUS rig because I know that some people prefer a wider, 12″ long plywood boat and if the combination works then Pond Sprite will be a nice beamy alternative to Dragon. More news as it happens.

Graham

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housekeeping

I have been doing a little reorganising around here so I thought I should explain. There is now a new tab or ‘page’ at the top called ‘the OPUS rig’, this is where I will collate all of the information about our Scale Sailing OPUS una rig. Some content will be copied from the normal blog postings so as to keep the information easy to find. All new information about the rig, using it and tuning it will appear on that page so please check it out and watch out for updates.

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I have secured a supply of 3 cell battery boxes with integral switches. These boxes are in AA and AAA battery sizes. I am converting them for RC Footy use by the addition of a universal receiver plug.

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These battery boxes make for a neat installation and save the space and weight of a seperate receiver switch. However you must use ‘Energiser Lithium’ batteries to provide enough voltage from 3 cells rather than the usual 4. These boxes are available from ScaleSailing at $5.90 for either size (AA or AAA). Using any battery size and type you wish is now legal under the new 2009 Footy Rules.

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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.

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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.

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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.

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‘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.  

As an aside, construction of the prototypes proved difficult to get the required accuracy so I had a friend make a special tool which now allows me to make the boom/pivot/mast stub assembly quickly and with the required accuracy. When the ScaleSailing OPUS rig comes into production the boom assembly will be supplied ready made along with a prefabricated mast/masthead and sail ready to simply plug together and play.
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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 2009 by Graham McAllister Designs.

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