Sails: the Source of Power

Sails: The Source of Power An Introduction to allow extra cloth to be added to the luffs are cut with a convex shape, with sail between the straight line distance some amount of extra cloth outside the Model Yacht Sails, Part 1 between the head and clew. This extra straight line between head and tack. by Rod Carr cloth is called leech roach and is re- The amount of extra cloth outside the he first job of a new skipper is to ported as the maximum distance out- straight line is called luff round, or learn the specific nomenclature side of the straight line. To prevent this Main Luff Allowance (MLA). On a Tthat describes the sails. There extra cloth from folding over, support sail with a 67” luff, the MLA is typically are a handful of terms, and the quicker is added in the form of battens, short ¼”, and is reported as such. The extra they are added to your vocabulary the lengths of a stiff material attached to cloth, when set on a straight mast the easier it will be to communicate with the sail, or slipped into pockets that extra cloth moves back into the body of your sailmaker and understand the spe- terminate on the after edge of the leech. the sail increasing its camber or depth. cific techniques described for sail set, The number, length and placement of Adjusting the mast by bending it for- trim and tuning. battens are controlled by Class Rules. ward in its midsections pulls the extra A suit of sails consists of a main- In general, maximum batten length is cloth forward, thereby flattening the sail sail and a jib. The word is “suit”, like about 2.5 x leech roach. Modern sail for higher wind speed condition. pants and a jacket, not “suite” which is materials do not require batten sup- Mainsail foot – Same comments an expensive multi-room arrangement port as much as the old cotton sails apply as for the jib foot. in a hotel. did, so they are very often eliminated Mainsail leech – Same comments Figure 1 provides a profile view of from jibs to prevent the jib leech from as for jib leech. In addition, most main- a sloop-rigged mainsail and jib. The being “hard” or flat. sails will be constructed with battens, as names of the corresponding edges of Mainsail luff – Most mainsail mainsail leech roaches are usually larger each sail are the same: luff (leading HEAD edge), leech (trailing edge) and foot (bottom edge). The names of the corners likewise correspond: head (top forward corner), tack (bottom forward corner) FIGURE 1 and clew (bottom after corner). Now, each edge of the sail may or may not be cut straight. Starting from HEAD the bow: Leech Jib luff – if cut concave, the maxi- Roach mum concavity is referred to as jib hol- F low, or jib luff allowance (JLA). It is F L U E L usually measured in 1/32” increments, E Luff C and reported as a 4 or a 6 meaning H Round 4/32” or 6/32” of concavity near the midpoint of the curve. Single panel Leech sails may have no concavity, or they Luff Roach Allowance may have some convex JLA to provide the extra cloth needed in the sail to de- H C MAIN velop camber, the airfoil shape required F E F E U L for good performance. Remember that L the jibstay will sag under wind pressure and that the jib luff will be forced to JIB assume that shape. Jib foot – most jibs are cut with extra cloth outside the straight line drawn W between the tack and clew. The amount E L TA T outside is referred to as foot round, and C W AC C K LE K C FOOT Foot Round is usually reported in inches. Most jibs FOOT Foot are rigged with “loose feet” meaning Round that the sail is attached to the jib boom or club at the tack and clew only. If D the sail will be attached along the jib Luff Edge Leech Edge m club, there will likely be very little foot L round. Camber Cross-section Jib leech – Class rules typically 18 Issue 130 <www.ModelYacht.org> Model Yachting Magazine © Winter Issue, 2003 than jib leech roaches. Care must be speed capability and are recommend- At the very least, he should inform you taken that the battens are not so stiff ed for medium to heavy winds. Draft of the JLA and MLA measurements so that they flatten the after third of the stripes are often put on model yacht that you will have a starting point for sail to look like a barn door. Battens on sails to assist in visualizing the shape in setting up your initial mast shape, and modern sails assist in shaping the after the sail when full of wind. backstay tension. Single panel sails parts of the sail, the cloth is generally Sail Construction must be full of air, so that the cloth can stable enough to hold itself fairly well, Sails come in two general varieties stretch under the wind loading and can needing just a bit of help. of construction; single panel, where begin to take on the cambered shape Sail Shape the entire sail is one piece of cloth with- necessary for drive. Figure 1B shows a cross section of out seams, gores, or cuts in the body of Paneled sails are used in the greater a sail. The wind blows from right to the sail; and paneled, where each sail is proportion of racing classes, and also left. The dimension “L” is the chord made up of panels or strips of cloth, at- in scale models that are going to be op- across the sail, a straight line from luff tached edge to edge with tapered seams erated on the water. They provide su- to leech. The % camber is found by di- to induce three dimensional shape in the perior performance because the airfoil viding the maximum depth “D” by the sail when filled with wind. shape that produces drive for propel- chord “L”. Model yachts sails typically Single panel sails of a woven ma- ling the hull is built into the sail. Most have cambers which vary from 5% for terial are typically encountered in the use a membrane material such as a my- flat sails, to 15% for quite full sails. The construction of “kit” boats, because lar sandwich with load carrying fibers, position of maximum draft is found by single panel sails are much less costly for or mylar film. These modern materials dividing “M” by “L”. Mainsails gen- the kit maker to provide. They are also don’t stretch, so the cambered shape erally have maximum draft located at found in several AMYA racing classes, must be built into the sail with tapered 40% to 50% of chord, while jibs can be such as the CR 914 and Soling One seams that hold the panels together. successful with maximum draft located Meter where the Class Rules require ei- Sail Attachment at 35% to 40%. Sails with maximum ther kit sails only, or aftermarket single A sail is attached to the spars by draft forward have good acceleration, panel sails cut to a specific size. The de- each of the three corners and by the don’t point particularly well, and have tails of successful set, trim and tuning luff of the sail to either the mast or a fairly low top speed potential, gener- of single panel sails is an entire subject the jibstay. (See Figure 2) In the case ally a good form for light winds. Sails in itself. Your sailmaker should be able of the jib, almost all jib luffs are fitted with maximum draft aft have good to provide you with guidance if you are with a hem into which the jibstay slides. pointing ability, have a good top end purchasing sails of this type from him. This supports the sail along its length ,JU354 ,JU354 American Model Yachting Association © 2003 <www.ModelYacht.org> Issue 130 19 .PEFM:BDIUJOH.BHB[JOF *TTVF toria, Fairwind, International One Me- ter and U.S. One Meter, that use round mast materials like aluminum or carbon fiber tube, use the mast loop method (3A) with good success. Aerodynami- Jibstay cally it provides the best leading edge, which gets traded off against a shorter Halyard sail lifetime since the luff is attached (fixed) at discrete points. Jackwire methods (3B) work well, with various methods Mast employed for holding the wire to the mast. We show a row of cotter pins set through the mast, with “windows” cut in the sail for clearance. Dress hooks have been used for attachment to the HEAD jackwire, but seem typical of only vin- tage boats these days. Extruded masts like the Bantock “Groovey” and the Ozmun “Goldspar” are amenable to the standard bolt-rope (3C). This method produces generally poor sail shape at the exit to the mast slot, and poor hinging ability in light air. Jack- wire methods using mast slides or oth- er attachment methods (3D) produce better hinging and longer sail lifetimes. CLEW TACK Mast slugs or beads directly attached Down to reinforcement patches on the luff Haul are also used (3E). Sail Setting Assessment Jib Club Jib Club Success in sail setting requires an understanding of the match between Figure 2 the flexible sail and its mounting com- Sail Corner Rigging ponent. For mainsails, the mast shape must be matched to the shape of the mainsail luff for a good match.

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