Miranda!2013! ! ! ! ! ! PhotocourtesyofWildBute.com!!!!! !!!!! Miranda!!!! ...

Miranda 2013 Photo courtesy of WildBute.com

AeroJunk Folio Paul McKay

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Introduction: The AeroJunk is based on the Chinese Junk Rig. It is fully battened and reefs vertically. It has the same fail-‐safe properties. Where it differs is instead of straight battens it has wishbone battens. These allow the sail(s) to be cambered instead of flat producing more drive for the same area with a higher upwind ability. In all other respects it retains the advantages of the Chinese Junk but incorporates most of the advantages of the Western Bermudan rig. In all 3 versions the sail(s) are cut flat with no broad-‐seams or round so are easier for an amateur to make. There are 3 versions that all use similar battens. The Wishbone Junk The Aero-‐Junk The Aero-‐Wing All 3 versions require a spreader-‐bar or fixed lateral tube at the mast top to widen the lazy-‐jacks so the sail can be hauled up without the battens jamming.

A new mast for an AeroJunk should be made with the main halyard eyelet plate fitted centrally. An extra eye is recommended on the front centre of the mast for raising flags, radar reflectors etc. The new mast should have a strong tube 400mm long welded transversely. This is drilled at 350mm centres and fitted with eyelets ready for the lazy jacks. An existing Junk mast may well have the halyard eyelet offset to port. Existing eyelets for lazy jacks will be too close together so a spreader bar should be made from hardwood or aluminium then padded to protect the mast. The dimensions are the same. The spreader should be hung from the existing lazy jack eyelets or the front eyelet by two separate lines to prevent tipping over.


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The Wishbone Junk takes an original junk sail, removes the straight battens and replaces them with wishbone battens. The only change to the sail is to fit cheap plastic grommets at each batten position to allow it to slide across the compression/tie rod on each tack. The only change to the standard wishbone batten is to replace the cross bar with a S/S compression/tie rod at the 30% chord point. Compression rod = 8/10mm dia. (3/8th”) (for flexed battens) Tie rod = 6mm. (1/4”) (for pre-‐bent battens) The rig is still mounted on one side of the mast, usually the Port side. Because of the batten width deck access forward is limited on that side. Advantage: More drive and higher upwind ability. Disadvantage: Limited access forward on the batten-‐side of the mast. NOTE: a new boat conversion using this rig would benefit from having a mast offset to one side or the other to accommodate the sail. This will give scope to boat designers. The AeroJunk has the battens enclosing the mast. Here the sail area is split into a self-‐tacking jib and a main. This rig is neater and faster as it benefits from the ‘slot-‐effect’ between jib and main. On each tack the whole rig swings sideways away from the mast. Advantages: very efficient so can have a smaller sail area than a standard junk rig. Mast no longer affects the leeward airflow. Access forward on either side of the mast. Similar upwind ability to the Bermudan Rig. Faster to rig and reef than a Bermudan. Sails with just a sheet and a halyard. Ideal for single-‐handed use. AeroWing has identical battens to the square-‐topped AeroJunk and a boat with these battens can be rigged as either. Here the ‘internal’ jib is replaced or covered with an external wrap-‐round jib. Similar performance to the AeroJunk. (Not yet proved at Nov 2014. When proved as expected this paragraph can be deleted.) Advantage: Mast drag removed completely. Disadvantage: Loss of slot effect. Note: When using a wrap-‐round jib the rig becomes a ‘single’ sail, asymmetric on either tack, much like the Wishbone Rig, but the camber drops from 1:8 to 1:12. This with its enclosed mast will give a much higher ability upwind at the cost of some speed. To restore the balance the crossbar would have to be widened. To use the example boom it would have to go from 740mm wide to 850mm (33.5”) for a camber of 1:10 and 1070mm wide (42”) for a camber of 1:8.


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Here are two examples of possible sail types, peaked and square-‐topped. The square-‐topped sail with parallel battens has full adjustability for weather/lee-‐helm. Also an example of a Wishbone Junk and an American designed AeroJunk Schooner.

Fever: Eclipse 22 Mystic Rose: 26’ River Scow


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Deciding the mast position. A Wishbone Junk will work with any existing Junk or Cat Rig so no change to the mast position is required. An AeroJunk’s mast position depends upon the position of the combined Centre of Effort (C/E) of the Bermudan rig and the ‘lead’ of the donor boat. Generally speaking the AJ C/E cannot be much closer than 600mm (24”) to the mast centreline. (See main/jib balance) In simple terms if the donor boat has a masthead rig then the new mast will have to go further forward. If the donor boat was designed as a fractional rig then it may be possible to keep the new mast in the same position, even putting it alongside the original mast heel plate; eg, an offset mast to avoid a keel trunk, but this will demand a high AR. (See main/jib balance later) So the position of the mast will determine whether the new rig has a ‘High-‐Aspect’ or Low-‐Aspect’ Ratio. (AR) Basically, the closer to the mast the C/E, the higher the aspect has to be in order to balance. A C/E 600mm (24”) from the mast = High Aspect. A C/E 1000mm (39”) from the mast = Medium Aspect. More than 1000mm = Low Aspect. Miranda’s C/E is 20” (500mm) from the mast CL Miranda’s AR = 2.17:1 Peaked and now 2.06:1 Square Topped. Coromandel Peaked = 1.63:1, Coromandel Square Topped = 1.41:1 Note. If intending to convert an existing junk rigged boat or an American Cat Rig it might be that the mast is so far forward that the ‘jib’ would end up as a narrow strip of sail. Or if you added a jib with a foot of at least 1000mm (39”) it could make the ‘lead’ more than 12%. In such a case, rather than re-‐position the mast you might consider a Wishbone Junk or an Aero-‐Wing. Both these designs can accommodate extreme mast positions. ‘Lead’ It has taken me years to understand this but put simply, A high-‐aspect ratio rig needs a small ‘lead’. A low aspect-‐ratio rig needs a large ‘lead’. Here we are talking about a range of 3.5% -‐ 12%. This is because High Aspect sails have a shorter boom length than a Low Aspect boom and therefore a shorter lever. The centre of effort of the sail is determined from the flat 2-‐dimentional side view of the sailplan on the hull. (See examples above) The Centre of Lateral Resistance of the underwater hull is also determined from this plan. Perfect balance would have one point above the other with the sail in line with the boat and the boat sailing perfectly upright. Unfortunately real life means the sail is at an angle to the centre line and the boat heels.


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As the sail moves away from the centre line the C/E moves outboard and forward, and so creates a greater turning moment. As the boat heels more bow is immersed and less stern so the CLR also moves forward counteracting this tendency. Eventually the wind causes so much heel that the CLR moves ahead of the C/E and the boat tries to turn towards the wind. (Weather helm) Sailboat designers work out a static ‘lead’ that works for winds up to about Force 4. After that the heel and weather helm become so great you have to reef. For high-‐aspect rigs the C/E does not move so far out from the centre line so a smaller initial ‘lead’ is required. (Miranda’s ‘lead’ has reduced from 6.5 to 3.5% yet still balances) Designing the sail. So before touching your donor boat you should design your new sail. This will tell you where the mast needs to go. If this mast position proves to be impractical or undesirable then it’s back to the drawing board to re-‐draw the sail as higher or lower aspect. Find a side view drawing of your donor boat and work out a scale for the picture. Work out the C/E of the Original Rig. Measure just the mainsail and a triangle bound by the mast, forestay and just above the deck. Ignore any overlap of the mast by the jib/genoa. Buy some squared paper, choose a scale then draw a shape that would please you if you saw it on the horizon. (There is no point in designing something ugly or industrial looking. It costs just as much to make an ugly design as a pretty one) Work out the C/E of your new sail and place the new drawing over the original. Put a pin through the new C/E and into the old. You will discover your new C/E is higher than the original one. Move your new shape up and check it will give the helmsman a clear view forward and check the halyard point stops about 600mm (24”) below the mast top. Adjust your drawing until it all fits. Check the C/E again. Now draw an arbitrary line for the mast, at least 24” (600mm) in front of the C/E. Measure the area in front of the mast and compare it to the area behind the mast. The ‘Jib’ cannot usually be larger than 33% of the total area. (See jib/main balance). Except on small boats and dinghies the Jib foot should be at least 39” (1000mm) The jib on an AeroJunk is always smaller and slimmer than the jib on a Bermudan rig. Jib/Main Balance. If the Jib on an AeroJunk is too large compared with the main then at a certain wind speed the whole rig will swing broadside to the wind. I discovered this the hard way so think about the shape of a balanced rudder. My good friend in Belgium, Jef Verhaar, gave me a mathematical tool to check the Jib/Main balance. Work out the area of the jib in say square meters. Find the centre of this area then measure from it to the mast centreline in metres. Multiply the two together to produce a ‘Factor’. Do exactly the same for the Mainsail and produce a ‘Factor’. Divide the smaller factor by the larger factor to produce a Ratio. This will be a number somewhere between 0.06 and 0.195. If the number is 0.2 or above then there is a risk of the sail turning broadside in a Force 4 wind.


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This calculation also works for Imperial or American units of square feet and feet. Make sure you use the same units for all measurements. If your ratio is just over 0.2 you can reduce it by moving the mainsail further back from the mast, say by 25mm (1”) at a time, or by reducing the area of the jib. If your ratio is as low as 0.10 you can increase it by enlarging the jib or moving the same jib further forward from the mast, or by reducing the area of the mainsail. It takes time to work out the various balances but it is a worthwhile exercise. Finally, a low ratio increases the load on the sheet and a high ratio reduces the load. Making the sails I had my sails made for me by Saturn Sails in Largs. saturnsails.co.uk They chose a 5oz Dacron material that I can recommend. After 18months of regular use they still look good. However it is slippery and stiff and would be difficult to manage on a domestic sewing machine. During my experiments I made 4 different jibs using 170 and 200 gsm Polytarp (plastic tarpaulin) that is the equivalent of 5oz Dacron. This is cheap, strong and easier to handle. It takes double sided tape and the edges can be stuck with Tarp Repair tape. Designing the Battens Designing the battens for an AeroJunk rig simply means designing the lowest batten or boom. All upper battens are a similar shape whose length is determined by the jib and main sail chords for that batten. The upper batten widths are measured from a simple diagram. All the nose sections are made from identical lengths of white 32mm (1 1/4”) x 585mm (23”) Marine Sanitation Hose. (Boat toilet hose) First of all decide how much camber to use. A slim easily-‐driven hull will work well with 1:10. (10%) A wide-‐bodied cruiser or long keeled boat might need 1:8 (12.5%) so a camber of 1:9 (11%) is perfect for the undecided! Also a smaller camber will allow the boat to point higher than a bigger camber** (The jib angle is determined by the camber of the main as setting the jib too tight would backwind the main. Suggested jib angles: 1:8=18 deg. 1:9=17 deg. 1:10=16 deg.) ** But there is no such thing as a free lunch. Here higher pointing ability means reduced drive partly balanced by the ability to hang on to your sail for longer before you reef. All battens from the back as far as the crossbeam are similar. Then there is an allowance for the mast diameter plus 2" (50mm). After this point the front part of the batten can be a variety of shapes from rectangular, curved or triangular. I have tried square-‐ended and rounded and they both work. It is a matter of taste. The battens are made from 1”x1/2” rectangular or 7/8th” OD round aluminium tube. The crossbeams are made from the same tube; rectangular ends cut, opened out, drilled and screwed; round is flattened at the ends, drilled and bolted. The noses are all made from 32mm (1 1/4”) OD white toilet tube. Mast protection tube (behind and beside the mast) is white ABS 32mm (1 1/4”) plumbing drain pipe. Battens for a Wishbone Junk will not need an allowance for the mast or for sail adjustment inside the batten so these battens are the same length as the sail-‐foot + the sheet bearing and end cap.


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So starting from the back end of the tubes, measure 2, 7/8th” (72mm). This is the front of the sheet bearing. Add 4” (100mm) for sail adjustment and mark A. This is the leech of the main. Measure the mainsail chord from the plan then subtract the ‘Camber Factor’ % (CF -‐ see below) and mark B. This is the luff of the main. Add 1, 1/4” (32mm). This is the back of the mast. Add mast diameter + 2” (50mm). This is the position of the nylon Jib leech slide line. Measure the jib chord from the sail plan, transfer to the diagram then measure the straight-‐line length. Subtract the same CF % then mark. This is the jib luff. Add 2” (50mm) for sail adjustment. This is the back of the nose tube. Add tube diameter and this gives the overall length of the boom. Measure back 23% chord from B and draw a vertical dotted line 12.5% chord (for 1:8). The battens must be twice this width at this point. Use a springy batten (ruler) or French Curves and draw a line from the back touching the 23% point. When it is opposite the mast the line should be horizontal. This gives the boom (crossbar) width. Note. The batten tubes extend some 100-‐150mm (4-‐6”) inside the nose tubes so the nose can be moved further forward if more jib adjustment is required.


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To find the camber factor draw a line on a sheet of paper. Take a Paper Back book, open out the back cover and measure the width. Mark the line with this width. (My book chosen at random measured 127mm or 5”) Draw a parallel line 10, 11 or 12.5% of this distance away from the first line. Place the book opened as above on the line between the marks. Push the cover sideways. It will bend in a curve to meet the second line. Mark the new end position of the cover on the line. Measure between the two marks. This distance will represent the ‘shrinkage’ of the sail chord when the sail is allowed to camber. The Camber Factor or CF. The CF of the example Coromandel Boom is 58mm (2, 1/4”) or 2% Sail design (and boat design) is far from being scientific. If your batten ends up 2” too narrow or 100mm too long don’t worry. It will work.


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The end cap and the single sheet bearing are cut from 32mm (1 1/4”) white solvent-‐weld ABS drainpipe. (Plumber’s merchant). This drainpipe will also be used round the cross bar and on the batten sides to protect the mast. A cable tie is threaded through the holes on each side and tensioned. The end of the tie can be tucked away inside the gap at the back.


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These are shown for 1”x1/2” rectangular tube. For 7/8th” round tube drill two holes with a hole cutter in a short piece of drain pipe then make two vertical square cuts halfway across each hole.


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Batten assembly and adjustment

My new best friend is the nylon cable tie. (Zip Tie) Mine are 7.6mm (5/16”) wide and are available 370 and 500mm long. (15 and 20”) They are also available as locking (single use) or re-‐usable. The locking ties have a 50lb (22kg) pull and the re-‐usable about 10kg. Several ties can be joined together to make a longer tie. At the luff of the jib and the leech of the main sails a third re-‐usable tie is used to locate the sail longitudinally between the cringle and the nose tube or the sheet bearing. At the leech of the main or the luff of the jib this can also be used on its own to slacken or tighten the sail thereby adjusting the camber. Battens are assembled round the mast by unscrewing one side of the nose then re-‐attaching the nose tube. The crossbar is always behind the mast. Start by fitting the boom then attaching the lazy-‐jacks. The boom should be at a height that allows a sitting helmsman to see forward without obstruction. Then assemble the battens in order stacking on top of each other then finally the yard. Attach the lower single halyard block to an eyebolt on the yard crossbar. The end of the halyard can be attached to both the batten and the leech of the mainsail. Regardless of the sheeting system you have chosen, at this stage run a single line down through each batten end until the whole rig has been assembled. This will allow you to control the rig during assembly.


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It is easier to fit the mainsail first starting at the yard then hauling the sail up through the batten stack a batten at a time, fitting as you go. Use adjustable cable ties to fit the luff tight to the cross bar. Use adjustable ties to fit the supports to the leech in line with the start of the 100mm / 4” adjustment mark and tension. Finally fit an extra tie from the leech grommet to the sheet bearing and tension. The jib is a repeat exercise starting at the yard. The support ties are fitted across the nose through 8mm (3/16”) holes drilled through the top and bottom of the nose tube opposite the start of the 50mm / 2” adjustment point. An additional tie is fitted between the luff grommet and the front centre of the nose and lightly tensioned. At the leech a thin nylon line or cord with loops at each end is made such that the line is too short to touch the battens. The line is then passed through the leech grommet and tensioned from each side with adjustable cable ties. Ensure that the battens between the slide cords and the crossbars have pieces of ABS plumbing tube to protect the mast. The final height of the boom should be just above the parked position of the furled sail and the yard no closer than 18” (450mm) to the mast top. Adjustment. Camber. The camber of each sail can be reduced. Pulling the mainsail leech back will flatten the sail and move the point of maximum camber slightly aft. Tighten the aft tie then re-‐tension the support ties. The jib luff can be moved forward by slackening the support ties and tightening the fore tie flattening the sail. Re-‐tension the support ties afterwards. This may improve pointing ability. This can be further improved by fitting stop or spacer tubes between the jib and each batten side thus limiting the jib angle but beware of back-‐winding the mainsail. Weather/lee-‐helm. This applies to square-‐topped sail designs with parallel battens. ** To reduce weather-‐helm move the whole jib forwards including the slide line of the leech grommets. To reduce lee-‐helm move the whole mainsail backwards. This will mean new support ties across the batten behind the crossbar. Connect several ties together to make them long enough. Ensure the mainsail luff remains in the centre of the batten to prevent the batten tipping over. ** Peaked sail designs can also be moved forwards/backwards but because of the angled battens, extra measures are necessary. For the main and jib, pieces of plywood or HDPE sheet (food chopping boards) have to be bolted to the yard batten sides and drilled with holes parallel to the battens below plus 3 extra leech and luff grommets above the second last batten. For the jib a row of 3 additional grommets are fitted vertically to the luff below the grommet on the second last batten. The extra jib cloth after adjustment above the yard can be folded over and taped down.


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Sheeting Traditional Junk Rig sheeting is multi-‐part with sheetlets joining pairs of battens, usually run from the aft cockpit rail. The advantage of this is when reefed, sheet tension is enough to hold the reefed battens together. The disadvantage is copious lengths of rope sweeping across the cockpit on each tack, particularly hazardous during gybes. Many boats fit a hoop over the cockpit to protect the crew from this. Traditional sheeting is probably still the best choice for offshore and ocean junks. However coastal AeroJunks will benefit from single-‐line sheeting. Here a single sheet is run down through each batten end between the sheet bearing and the end cap. It then runs to a fiddle block on the cockpit sheet track, back up to a single block slung under the boom, down to the fiddle block and through a jammer. The other end terminates in a stopper knot on the second highest batten or the yard of a peaked sail design, or through the end of the yard on a square-‐topped design where it joins the halyard between the masthead and the yard.


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Tension on the sheet is resisted by the halyard, thus pushing the battens against the mast. This tends to reduce twist at the sail top. Coastal Junks don’t usually have to sail with more than two reefs so downhauls can be fitted to the crossbars of the boom, batten 1 and batten 2 and run back to the cockpit. I also recommend a fixed boom vang to stop the boom lifting when fully squared out downwind, fitted from the base of the mast to each side of the boom. This system has proven to be neat and effective.

Showing the sheet, halyard, end cap and an earlier version of the sheet bearing


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Coda I started this project over 25 years ago so it is the product of much reading, thinking and experimentation. I was attracted to the Chinese Junk rig for its inherent practicality and safety and always thought its performance could be improved with modern materials and the results of aerodynamic research. Since its introduction to the West by Blondie Hasler and Jock McLeod, many others have spent their time and energy pursuing the same aim, notably ‘Bunny’ Smith, Vincent Reddish and Slieve McGonagal who has adopted the ‘Split-‐Junk’ idea but the person who has done most for the Junk Rig is Arne Kvernson of Norway. He has embraced the Cambered sail or ‘Pillowed sail’ and has done much to work out mast scantlings and other details for his rig. I urge all to read his writings. Arne rigs his boats with ‘clouds’ of sail – Marena reminded me of a 19th Century ‘Wind Jammer’. I have gone down the minimalist route. Less is sometimes more. My boat, an Etap 23 arrived with a main and genoa measuring 26.88 m2. (289ft2) My AeroJunk rig for Miranda measures just 208ft2 (19.3m2) yet I can exceed the hull speed of 5.8 knots on a broad reach in a perfect Force 3 wind. (Wind speed 10/11 knots, boat 5.9 knots). I have also clocked 6.2 on a broad reach, 7.2 downwind and 5 knots close hauled in a Force 4. These figures suggest that a High Aspect Ratio AeroJunk is very efficient. Wishbone battens are still not popular. Traditionalists hate the sight of ‘all that scaffolding’. But if the shiny battens don’t put you off then I cannot recommend this rig too highly. Now for the cherry. If the icing on the cake is the increased efficiency of the AeroJunk then here comes the cherry for the top – even greater efficiency. Make a two-‐part ‘trampoline’ from sailcloth. Fill in the area of the boom under the mainsail and jib fastening the trampolines from the batten sides. Make a second two-‐part trampoline and do the same for the yard. These become wing ‘end-‐plates’ or fences. If you extend the foot of the main and jib in graceful arcs by up to 4” (100mm) they will seal against the trampoline on each tack and prevent high pressure windward air from leaking through to the low pressure leeward side. This will increase drive from the whole sail, not just some of it Paul McKay Isle of Bute November 2014


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Mystic Rose, a 26’ River Scow -‐

-‐an American-‐designed AeroJunk Schooner Picture below – Miranda 2014: now with square-‐topped battens and filthy sails. (The strange looking top to the jib and batten spacing are left over from my first unsuccessful design) Two new top battens on the way.


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Miranda!2013! ! ! ! ! ! PhotocourtesyofWildBute.com!!!!! !!!!! Miranda!!!! ...

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