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Sailing and Understanding Sails
By Johnny Routledge Published: 11/11/2008
Sails today are very different from their ancestors in the period before the polyester revolution. Shape has always been the most important factor in sail efficiency, and time was when you chose your most suitable canvas for the conditions, pulled it up and sheeted it in. Some cruising sails are still made like this. They work well enough, but the cloth from which they are cut often means that their performance potential is nowhere near that of a modern sail whose geometry can be modified to suit the wind and sea. Such equipment has worked its way into cruisers following the lead set by racing yachts, whose hi-tech vanguard have now moved on to cloths of such sophistication and stability that the shape cut into their sails is barely compromised until they literally burst.
The maximum camber of a sail should be somewhat forward of the middle of its cross-section. In practice this varies to a degree with what sort of sail it is and how hard the wind is blowing. The power of an aerofoil depends upon its depth of curvature, so a baggy sail will drive you along in light airs far more effectively than a flat one. As the breeze hardens, the power of the fullcut sail will become too great for the boat. It must then be flattened or reefed, if either is possible; or changed for a different sail if not.
This requirement is underlined by the fact that as the wind increases, a sail naturally becomes fuller and the point of maximum camber is blown aft towards the leech. Both these results are the opposite of what is desirable, and something must be done to mitigate them.
In addition to the question of camber control, there is also the matter of twist. Most sails twist away from the wind in their upper sections. This tendency is built into them deliberately and can be controlled so that it works to your advantage.
Twist is a shut book to many sailors, but to ignore it will measurably compromise your boat speed. The reason for its importance is this: wind blows more strongly aloft than near the deck, because surface friction with the sea slows it down. When a boat sails along, the wind she actually experiences is a composite known as apparent wind. She may be powered by a true wind from abeam, but she is making a ghost breeze from dead ahead in an equal and opposite direction to her own progress through the flowing air. This phantom combines with the true wind to generate the actual breeze across the sails. The apparent wind which they form comes from further ahead and is stronger than the true wind, so long as it is not blowing from well abaft the beam.
Clearly, the faster the true wind for a given boat speed, the less will be the interference caused by the boat's movement. Because the true wind aloft is a little stronger than at deck level, the apparent wind up there is somewhat more `free' than the air lower down. If the upper part of the sail can be twisted to take advantage of this, its resultant force will produce a larger forward component than that being delivered by the lower section of the same area of canvas.
Furthermore, the whole of the sail will be setting cleanly, with no part either lifting or stalling.
In the case of a fractional rig, the upper section of the mainsail cuts undisturbed air, while the lower parts receive their wind already bent further aft by the headsails. Twist control is vital if the top of the sail is not to be stalled completely.
Too much twist can generate a fearsome loss of power if it is allowed to go unchecked while you are reaching on a windy day. The boom kicks up in disgust, while the upper third of the mainsail dumps its air unceremoniously to leeward over its tortured leech.
Shaping the headsails
In most boats, the primary tool for headsail camber control is the halyard winch. Some traditional craft are equally well served by a tack downhaul, but whatever method is employed, the crucial feature of the sail at any given time is its luff tension.
Hoist the sail, then steer the boat on, or nearly on a closehauled heading. Now look up at the mid-part of the sail. If it has a 'go-fast stripe' your task is made easier. If not, you'll have to judge its shape by looking at the seams. The camber should swell out to a maximum 35-40% of the way aft from the luff. If it is too far aft, tension up the halyard and watch the draught move forward. If the luff is too `hard' (ie, the camber is too far forward), slack away a few inches and keep looking.
If the sail seems susceptible to this treatment, check it again once your boat speed has built up. The apparent wind will now be greater and the sail may require some adjustment. It's important to do this with your fully open roller reefing genoa as well as a hanked-on sail.
As the wind picks up, keep hardening the luff until your efforts to maintain a good camber become fruitless. The sail should now be overpowering the boat if the sailmaker and the designer got their sums right. Change it for a smaller one, which should also be flatter cut, or roll some away.
The converse of keeping your sails reasonably flat as the wind hardens is that a sail can sometimes be set up to be too shallow-cambered. It will then lack the power to drive the boat in light airs. If the sail seems lifeless, ease the halyard, and the sheet too if necessary, so as to power up the canvas.
Attention to the luff of the sail may cause the leech to require service. The leechline, if fitted, is a light piece of small stuff sewn into the trailing edge of the sail. It should be gently 'tweaked' just far enough to stop the leech beating, and no further. Too much tension causes a hooked leech, which is hateful to behold. If the leech is already hooked, slack away the line as far as the sail will let you.
The twist of a headsail is determined mainly by the position of the sheet leads. Most boats have these on sliders. If yours doesn't, the sail must be cut to the position of the fixed leads.
Sheet-lead positions are crucial. When the helmsman brings the sail a little too close to the wind from closehauled or a close reach, the luff should lift evenly all the way up. If the bottom of the luff lifts first, the lead is too far forward, making the leech too tight so that the sail is not twisting enough. If the top goes first there is too much twist, caused by the lead being too far aft. The best position can only be found by experiment, but luff 'tell-tales' are a tremendous help. If you don't have any, install them now. All you need are three 8 in (20 cm) lengths of wool pushed through the sail with a sail needle, about 6 in (15 cm) abaft the tuff (in a 35-footer) and knotted on both sides. The windward ones will always flick up just before the sail lifts. If the leeward ones go dancing they tell you without room for argument that the sail has stalled either from oversheeting on a reach, or because the person steering the boat to windward is driving her to leeward of her best course.
Shaping the mainsails
As in a headsail, mainsail camber is largely controlled by luff tension. However, sails on boats with any pretension to performance generally also offer a clew outhaul. The effects of this will extend approximately to the lower third of the sail. Haul it out to flatten the sail as the breeze fills in.
A mainsail that is set behind a genoa will emphatically not require a hard entry. Such a form will often result in the backwinding of the main luff when the boat is closehauled. Instead, a gentle curve aft to a maximum camber virtually in the centre of the sail will work well if the boat is masthead rigged. The more powerful sail of a fractional rigger should carry its maximum camber somewhat further forward, but still with a flat, gentle entry.
Mainsail twist is highly controllable on a modern yacht. Leech tension, the essential element, is determined by the mainsheet when closehauled. With the kicking strap (or kicker, or centre boom vang) let off, juggle the sheet tension until the top batten of the sail lines up with the boom when viewed from directly underneath. There is no need to lie in the bottom of the cockpit, a glance will suffice.
Once you have the twist you are after - and if the sails are well cut, the leech of the main will now sweetly follow that of a well-trimmed genoa - the mainsail's angle of attack can be determined by using the mainsheet traveller, so long as the wind is well forward of the beam. This means in practice that when you are beating or close-reaching you shape the sail with halyard, outhaul and sheet, then trim it with the traveller. If you are far enough off the wind to want to ease the sheet, set up the kicker to maintain leech tension when the sheet can no longer supply it.
On a race boat, the powerful kicker may be brought into service even closehauled to help flatten the sail. Such fine tuning is a waste of effort on most cruising mainsails, but the basics should never be neglected. I've heard people complain along the lines of `All this sail shape nonsense is for the boy racers. Who cares about Y knot?' I do, for one. At 6 knots it is worth 6 miles over 24 hours. To be an hour later than you might have been could lose you a tide, resulting in a further three hours' delay. It may also be the last straw for a fatigued crew, causing a fatally bad decision in the face of a rising gale which you would otherwise have missed. Or you might merely get in after the shops have closed.
Whatever the result, not to give your boat her best chance to perform well is unseamanlike. You don't have to thrash a boat to extract that extra Y knot, yet carried to its logical conclusion, 6 miles lost in a 24-hour passage is the best part of two days wasted on the average ocean crossing, though in fairness I have met people who don't press on because they seem to like it out there.
Una-rigged craft often sail excellently. A single, well-shaped aerofoil set from a lightly stayed or unstayed mast can be shaped with great precision and can be remarkably closewinded. Two notable examples of the truth of this are the Finn dinghy and the North American cat boat. As yachts become larger, a single sail becomes a worse proposition for reasons of handling and of shipping a spar of sufficient proportions to carry it. From time immemorial, therefore, sail plans have been divided.
In addition to the benefits above, split rigs offer two further advantages. Because individual sails are set forward and abaft the centre of lateral resistance (CLR) about which the boat effectively pivots, they can be sheeted so as to balance her steering characteristics. At speeds too low for the rudder to be useful, the sails can even be used to persuade the vessel to point where you want her to go. Secondly, the slot between two sails produces a venturi effect, accelerating the air which is squeezed through it. This raised velocity increases the power not only of the rig as a whole but also of its individual components. Those who doubt that this is happening have only to stand in the slot of a yacht sailing to windward in 15 knots of breeze. Tell them to hang on to their hats, though. It's breezier in there than they'd ever have believed back in the cockpit.
As skipper, you have the balance of the boat's rig at your fingertips, and assuming that she is well designed, there is plenty you can do about it. The yacht should be easy and light to steer, showing a gentle tendency to turn into the wind if left to her own devices. If she has too much mainsail on and only a small jib, a sloop will want to round up. The result is weather helm. This tires the unfortunate who must steer, as well as slowing the vessel down through the drag of the rudder. The dreaded lee helm, on the other hand, is the lot of the sailor whose boat is carrying too much canvas forward and too little aft.
Lee helm is a dismal condition. It makes manoeuvring difficult, steering frustrating, and it has a debilitating effect on the yacht's capacity to sail close to the wind without making excessive leeway. A touch of weather helm holds the rudder a degree or two to leeward, which diverts the flowing water, just enough to help the keel lift the vessel in opposition to the sideways forces. Lee helm achieves the converse effect.
In addition to the disposition of her sail plan, a yacht's helm balance varies depending on how much she is heeling. As she heels to leeward, any tendency to weather helm will increase. Rolling to windward generates lee helm. This makes sense when you consider that the whole outfit is being pulled along by the rig. As the boat heels, the centre of effort of the mast and sails moves outboard. If you dragged the boat through the water by a rope on the end of a beam lashed athwartships across her deck, she would try to swing away from that side. The same thing happens with the rig.
Boats with flat, beamy midships sections such as are found in many high-performance modern cruisers and racers suffer from a more subtle source of heel induced weather helm. As they lean over, the leeward side of the immersed hull becomes rounded to a point of exaggeration. The weather side is correspondingly flattened. The imbalance produces weather helm which, in certain cases, appears suddenly and uncontrollably above a critical angle of heel. You need to watch out for this in such a vessel, particularly if you are sailing in a river on a gusty day, surrounded by expensive moored yachts.
Shortening sail as the wind strengthens is part of the sailor's everyday life. It is not something to be put off. The process should be as natural as shifting gears in the family car. Not only does carrying the right canvas for the conditions give you a drier ride and increase your chances of arriving with the boat in one piece, it keeps the yacht more upright. As we have just discussed, the less a boat is heeling, the easier she will be on the helm. This benefit is assisted by the fact that the centre of effort of a smaller headsail or a reefed genoa is further forward than that of a larger one. Similarly, a reef in the mainsail moves the leech inboard along the boom. The sails are therefore generating less weather helm than if the yacht were spreading everything she carried. With a sensible awareness of the principles of sail balance in your mind, it isn't difficult to reduce your canvas in such a way that the boat's behaviour remains docile.
It would be unwise to generalise about where to begin sail reductions. A masthead cruising sloop will usually set out by tying one reef into the mainsail. This may be followed by one or two changes of headsail before going for the second reef, and so on. A fractional-rigger often reduces the size of her headsail first. A ketch or yawl has a mizzen to consider as well, but the principles remain the same.
The years since the mid-1970s have seen the rapid rise and general acceptance of patent reefing systems. The best of those offered for headsail roller reefing have by now achieved high reliability and are able to reef the sail to a moderate degree without too much sacrifice in shape. Poorer gears produce a dismally reefed sail which looks more like a flour bag than a number 3 genoa. With the canvas rolled away to storm jib size the result is execrable. None the less, all such arrangements give the benefit of instant sail area adjustment. In a short-handed craft this sometimes more than compensates for what is lost in pure performance. No boat must go to sea, however, without making at least some arrangements for the day when the gear fails. The most satisfactory answer is a separate forestay that can be readily set up and to which a storm jib may be handed. Indeed, this produces the best of all worlds because such a jib will invariably set better than the deeply rolled genoa. It can therefore be used routinely for heavy weather sailing.
Mainsail reefing systems now exist which are way in advance of the old 'round-the-boom' roller reefing. Such a method was never ideal on the bermudan rig, though it remained in use for decades. By far the simplest and best way to reef the main is with `slab' reefing, but if you cannot bring yourself to make even that much effort, in-mast and in-boom systems can be bought off the shelf. In-mast gears put considerable weight aloft and add to the awful sum of the rig's windage. They may or may not be reliable, and a sail built for such a setup will probably have a straight leech with no battens. On a contemporary rig this looks downright sad and it's certainly less powerful than the elliptical trailing edge of the conventional mainsail. The Spitfire didn't have those beautiful wings just to look pretty.
Mainsail reefing options therefore subdivide into three choices: in-mast roller, inboom or round-boom roller, and traditional gear for reducing the sail in `slabs' at the foot. Of the three, slab produces by far the best sail shape; it's extremely reliable and, in any case, is readily repaired at sea. Mainsails of under 500 sq ft (46 sq m) are easily handled by two healthy adults and can be dealt with singlehanded without major inconvenience. To compromise this vital sail out of laziness or lack of stomach for getting wet seems odd to me, especially when the latest fully battened mainsails and lazy-jack systems make the job of stowing child's play.
Nonetheless, the roller alternatives do have a place. They help huge yachts to be run without numerous deck-hands. They also enable the elderly or the unfit to keep on enjoying their cruising, but if these options are to be chosen, it's important to be aware of their limitations in sail shape and, potentially, their unreliability. At least an in-boom reefing system is within reach in the event of failure. Furthermore, the mainsail has a conventional halyard and can always be dropped. Although in-mast systems have improved greatly, they still represent a total commitment to the dependability of the gear. A trip to the masthead in a gale holds little appeal for any of us.
Today's Bermudan-rigged yachts have much in their favour, but sailing downwind in light and moderate going is not one of them. Ideally, this endemic shortfall is cured by using a spinnaker, but this lies beyond the comfort zone of many cruising sailors. The answer is a `cruising chute'. This has been developed from the modern asymmetric racing spinnaker and is really a light, extra-full genoa that only attaches at tack, head and clew. It works in airs too gentle for a multi-purpose genoa, and can be set on a very broad reach without a pole. Sailmakers will deliver them complete with a `snuffer' - a sort of sleeve which rolls down over the sail, spilling its wind miraculously. This removes the worry from using so powerful a tool. In short, a downwind passage in light weather without a chute is like the proverbial day without sunshine.
http://www.boatpartsdatabase.com has lots of resources for the boating trade and sailing public alike.
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