Aerojunk Folio Interim 2
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Miranda 2013 Photo courtesy of WildBute.com AeroJunk Folio Paul McKay 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. 2 AeroJunk Folio Paul McKay 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. 3 AeroJunk Folio Paul McKay 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 4 AeroJunk Folio Paul McKay 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. 5 AeroJunk Folio Paul McKay 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.