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SAILING ON THE EDGE
John K. Walker
The sailing of advanced craft at the edge of their performance requires some understanding of the physics involved of the sails and of the hydraulics to make them excel. Catamarans are generally sailed on one hull and with the sails 'flying' and seldom with them as 'chutes' as on a dead run. The reason for this is the more efficient platform of one hull in the water rather than two and the 'flying' sails result in higher speed, even 'sailing' downwind. For example, a flying sail develops more than twice the force of a spinnaker of the same size. We will try to explain this and other factors that need to be considered for high performance sailing. Also see the excellent article by Bryon Anderson on ‘The Physics of Sailing’.
Flying sails produce an extra aerodynamic force by the wind blowing smoothly across the sail. The way to understand this is that in a laminar (smooth) flow over a surface, the pressure of a fluid decreases if its speed is increased (Bernoulli's theorem). The expression for this relation is
P + ½r V² = constant (1)
where P is the pressure, r is the density and V the velocity of the wind. An airfoil, such as a sail or a wing, generates additional lift or force from the decrease in the pressure when the air going smoothly over the top (leeward side of a sail) moves faster than that going over the bottom (windward side). The faster the flow, the greater the drop in pressure and hence the greater the lift. Careful studies indicate as much as 75% of the lift (force) can result from the flow over the leeward side of a sail. However, the magnitude of this additional force critically depends on the shape of the airfoil, as well as the speed of the flow over its surface and the size (area) of the airfoil. To first order, the lift L generated by a wing or sail is given by
L = CL(q )r V²A/2 (2)
where A is the area of the sail and the coefficient of lift CL(q) depends on the shape of the wing and the angle of attack q (direction of the apparent wind). Note the lift doubles when the area of the sail doubles, but the lift quadruples when the wind speed increases by a factor of two.
The optimum shape for maximum lift (largest CL) for a single surface airfoil, such as a sail, is when the draft, which is the ratio of the 'depth' to cord (width) of the wing, is ~11% at 35% of the cord. Remember the whole surface of the sail is under a significant pressure differential and so it should not have any holes in it. Note for a given area, a long thin wing, such as that of a high performance glider, is more efficient than a short wide one.
The wind speed is seldom steady and to keep a catamaran just flying a hull requires active control of the variables that affect lift. The first step in obtaining the right amount of power to just fly a hull is to have active control of the sail area. A good method for such control of the area (potential power)
is with a furled sail. On una rigs the sail could be furled onto the boom or reefed down with ties as the wind speed increases. However, on a sloop first furl the foresail in as the winds increase in force. This will also permit the boat to point higher as less wind is being deflected onto the main sail. Then for very heavy conditions reduce the area of the main sail to further reduce the lift by furling or reefing the sail or using small storm sails. Finally, in storm conditions, sail with solely the wing mast and a storm jib or the foresail nearly all furled.
The next step in controlling lift is for the helmsman to control the angle of attack (direction of the apparent wind). The lift increases linearly with the angle of attack until the wing (sail) starts to stall at about 20° to the apparent wind for a una rig. At this point of sail the lift begins to decrease but the drag dramatically increases. Hence the 'groove' for optimum lift and minimum drag is at most a few degrees, say from 17 to 20 degrees. For a full rig the jib provides additional lift but it also deflects the wind onto the main sail. While this increases the lift for the main, it also alters the angle of attack by perhaps 15 degrees. Hence the apparent wind for the main sail is now ~35 degrees. However, for a partial rig the jib deflects the wind onto only the lower half of the main sail and the large angle of attack stalls the top of the main sail above the hands and hence reduces its lift and increases its drag. Thus the top third of the main sail for such a system must be 'twisted off' about 10° for it to obtain maximum lift. The Tornado '89 Worlds champion team of Australians Booth and Forbes initiated such increased twist off of their upper main, which has become a feature of many sail designs. Note the jib is offset about 12° from the centerline when close hauled so it is near the optimum angle of attack for maximum lift. The jib must of course also lead the main sail by about 10° on other points of sail and should be trimmed and adjusted in concert with the main sail. The jib can help to maintain laminar flow over the main sail when on a broad reach. For heavy conditions the slot between the main and the jib should be increased to avoid restricting the airflow and hence reducing its velocity and thus lift (eqn. 2).
The draft of the sail is important. It can readily be checked for a fully battened sail with the mast and hoisted main sail in a horizontal position, on say two sawhorses, and a long tape and ruler to measure the cord and depth for the determination of the draft. Set the draft by adjusting the tension on the battens. Mark these positions on the battens, as it can be useful to adjust the draft for different wind conditions while underway. Note tapered battens help to move the draft forward to the optimum position (35%) of the chord. For light conditions the draft might be increased to approximately 15 %. Furthermore the velocity of the wind generally increase with height above the water. Hence the draft of the lower part of the sail should be greater than that of the upper part because of this gradient in the wind profile. However, for heavy conditions reducing the draft can reduce the lift of the sail. This can be accomplished by letting out the battens, particularly those for the top half of the sail to reduce heel. Stiffer battens can also be used to flatten the sail. The sail can also be flattened with the downhaul, outhaul and main sheet. Some Tornadoes now have a 16:1 ratio for the downhaul, which is used to actively control the draft, and hence lift, of the upper main sail in order to keep the windward hull skimming the waves. This significantly reduces the hydraulic drag while keeping the sails nearly vertical for maximum lift.
The leading edge of a wing (mast) is also critical in determining the lift and drag ratio of an airfoil. A tapered mast with controlled rotation can be used to optimize the lift or drag for different conditions. For high speed and for pointing or close hauled sailing to the wind, aligning the mast more with the apparent wind reduces the drag. For low wind conditions rotating the mast into the wind (to leeward) increases the lift. For example, the mast should be at ~100° from the centerline when the apparent wind is abeam on a broad reach for low or moderate wind conditions.
Finally, the enhanced pressure and vacuum on the surfaces of a wing or sail deflect the flow around the ends of the wing (head and foot of a sail). This reduces the lift in these regions and it also causes a vortex, which increases the drag. These effects can be reduced by winglets or by having the foot of the sail sweep the deck. The latter also increases the aspect ratio of the mainsail, which improves pointing capability. However, as neither of these systems is very practical the only option is to use the boom to try to reduce this 'end effect' flow under the sail. By raising the boom and keeping the foot of the sail as close as possible to the boom the flow around the foot of the sail can be reduced, thereby increasing the lift. However, a higher tack and a different downhaul system may be required.
WIND and LIFT INDICATORS
To check that windflow over the sail is laminar it is necessary to have telltales in the critical regions and to continuously monitor them in order to ensure smooth flow and hence the most lift and least drag. A streaming telltale indicates smooth flow while a fluttering one indicates turbulent flow and hence less lift and more drag. I find woollies make good telltales as they flow well in most winds, although magnetic tape is better for very light conditions. Several sets of telltales are necessary for monitoring the flow over the whole sail. The first set of telltales should be at about 9% of the cord from the luff. Telltales at about 40% of the cord and at the leech help to indicate the amount of turbulence (drag) in these regions. The telltales at the luff of the jib should be used to monitor the angle of attack to the apparent wind. Steer to high and the windward telltale will first lift and then begin to flutter. Sail to low and the leeward ones will begin to flutter and then drop. Generally, the best performance is when both telltales are streaming in the wind but with the windward ones flutter occasionally. On a reach trim the sails toward whichever telltale is fluttering. The telltales on the main sail are used mainly for trimming the sail. For instance, if those on the leech flutter back towards the leeward side indicating turbulence and over sheeting, then the main sail should be eased out to improve the flow.
The best way to optimize these different factors is by match sailing and trial and error adjustments and carefully comparing effects and results. Tweaks can also be made while racing with the same boats in your fleet. Good sailing and keep those telltales flying!
PROPOSED UPDATE FOR TORNADO II
The standard Tornado has been significantly updated with a spinnaker, flat top main sail and double trapeze. Note the jib should also be updated so that it can be furled. Such a jib could be furled when on the layline and a header would otherwise force one to make extra tacks to hold the course. The sail area for a boat with a furlable jib could also be readily reduced under heavy conditions to provide safer sailing. However, the new Tornado is still a displacement boat and its speed is limited by hydraulic drag. Having the centerboards changed to lifting foils can reduce this drag. These foils will be cantered inward when flying a hull which will then provide some vertical lift to the boat and hence reduce the hydraulic drag on the hull that is in the water. The higher the windward hull the more the vertical force becomes and the more lift for the boat. However, the leeway is reduced somewhat and the optimum will need to be determined experimentally. Such hydraulic lifting dagger boards have proven successful on other catamarans in France and Latvia and the boats are significantly faster. Such a boat would of course be somewhat faster and more challenging to sail and be competitive for the next decade or so.
PROPOSED TORNADO III
Goran Marstrom has developed a new 20 foot catamaran, the M-20, a una rig of 105 kg with a gennaker that is significantly faster than the Tornado but it has some limitations. A more versatile boat would be similar to the M-20 but with a furlable main sail and gennaker/genoa. The area of the main sail should be readily changed for different conditions by being furled onto the boom. The tension on the battens can be removed with quick release strings at the ends of the batons to alleviate problems with the batons when furling. However, a furled gennaker/genoa should be explored for the foresail as it has many advantages. A furled gennaker can be used as a small foresail to provide high pointing ability and extra lift when close hauled and for tacking, especially in heavy conditions. A small foresail at the bow would not reduce the high pointing capability of a una rig, as it would not deflect wind onto the main sail. However, the forestay should be near the tip of the bow and extend to the top of the mast to keep the foresail away from the main sail. The jib leads should be near the centerline at such times for high pointing ability. The foresail could be extended when on a reach or when on the layline and a wind shift has provided some lift. It can be extended further to a gennaker when on a broad reach. We also suggest the length of the boat be extended to 23 feet and the beam to 11 feet as the new Marstrom construction techniques have significantly reduced the weight of catamarans. Such a boat would be faster than other comparable catamarans and go through chop and swells better. It would also stand out among the other 20-foot cats.
The boat, however, is still a displacement sailboat and more speed can be obtained by hydraulic lift of the hulls to reduce drag. This can be accomplished by either planning or with hydraulic foils. We might explore the possibility of having flexible 'fins' along the sides of the bow. These would extend from the bow to about the main beam and they could be deployed when above about 10 knots. At this speed they would provide some lift (planning) that would reduce the drag and hence increase the speed. A second method is to have an inward cantered dagger board/foil, which should be in front of the main beam for stability. Such boards have proven successful in France and on the new Catri 23 trimaran being developed in Latvia. The rudders also have a small horizontal foil to provide stability and additional lift. Sailboats with such foils significantly outperform other comparable boats. Numerical models of these concepts might be made to test the different possibilities before a trial boat is made. Hopefully a demonstration fleet would be available for the 2008 Olympics.
The International Tornado Association should be responsible for the development, measurements and specifications for all three Tornado's and possibly for the A cat and for overseeing international regattas. We suggest that all three boats be raced together at national and international regattas for the next decade or so.
Catamaran races should have a least one reaching leg for the thrill, fun and show that often occurs on this leg.
TUNING THE TORNADO
by Roland Gaebler
The Formula One of Catamarans
One of the most thrilling classes at the Olympic Games, the Tornado has climbed to a level of technology and quality that other classes of this size are yet to come close. World and European champion, Roland Gaebler, tells his tricks on how to sail the formula one of these two-hulled racing machines.
Advanced manufacturing and construction technologies have produced a design that fulfils the harshest requirements in both, fun and race sailing.
Since its birth in 1967, the Tornado Class has developed to a true one-design class, which means that 95% of the worldwide crews are using identical equipment, fully developed and easy to handle.
To keep the new boat purchase
price stable, use of expensive materials is consciously restricted and in some
cases forbidden. Over 2,000 crews from
the Tornado Association and boats are being sailed in over 30 countries
worldwide by mixed crews ranging in weight from l20 kg to
The large number of sailing events does not only comprise of World, European and National Championships but also ISAF World Cups, Eurolymps and many national races. These races occur practically every weekend.
HULLS AND RIG:
The three major boat yards that produce Tornado Catamarans are: Sail Centre of Sweden (SWE), Reg White Ltd (GBR) and Scheurer of Switzerland (SUI).
The Sail Centre of Sweden (Marstrom) has the largest share of production (about 90%) of the Tornados mainly due to the fine craftsmanship and use of the best materials and advanced pre-preg auto-clave baking technologies.
When deciding to buy a Tornado, a careful selection has to be made between two types of sheeting. On one hand, the aft sheeted type consisting of two triple blocks attached to the rear beam. This system offers advantages in medium to strong winds. On the other hand, the central sheeting system that leads the main sheet forward along the boom and down to a block in the middle of the trampoline is easier to handle in light air, but needs more time to get used to it.
A decision is easy when buying sails. Basically only one cut exists for both main and jib. A set of sails costs about 2,900DM including battens. Hyde of England or Ullman Italy and Oilman USA offer competitive complete sets which only differ in very minor details. These sail makers have an even market share of about 30% each. Special sails are also made by both Randy Smyth USA (Silver Medal winner at 1984 and 1992 Olympic Games) and also by Chris Cairns AUS (1984 Bronze Medal winner at 1984 Olympic Games) depending on crew weight and preferred wind conditions.
About 95% of the crews use Marstrom tapered masts. These require fine adjustment of both spreader rake and pre-bend before standing upright. Once set, they require very little adjustment over the wind range.
Spreader Rake: The difference in spreader rake is related to both the brand of mainsail used and crew weight. For Hyde, Ullman and Smyth sails they all require between 25mm to 45mm of spreader rake depending on crew weight. The lighter the crew, the more spreader rake required.
Pre-Bend: Similar settings and concept of spreader rake applies for pre-bend of the mast. The basic tension (between 25mm and 45mm) can be used but excessive bend while sailing can distort the main. The sign of too much pre-bend is usually if the mainsail starts to go very flat (and possibly invert) in the lower 1/3 of the sail. If this occurs, reduce the pre-bend by approximately 25mm.
Another method of testing the tension is to sail in very strong wind with full tension on the mainsail Cunningham and the windward wire should not be seen to be loose. If the windward wire does start to go slack, tighten the bronze turn buckle on the front of the mast about 4 or 5 full turns.
Forestay and shrouds are tensioned so much that the rig gets very tight when fully adjusted prior to leaving the beach. This helps reduce forestay sag and thus helps the boat point. It is also to help the mast stay as vertical as possible and avoid unnecessary spilling of the air off the top of the rig.
The easiest way to tension the rig is by using the shroud terminals with the crew hanging from the trapeze wire at the rear quarter of the boat. Two people are better than one hanging from this wire when tightening the rig.
After sailing, it is recommended to untension the rig again to protect the boat from being fully loaded (with tension).
When the rig is tensioned, the mast rake is critical for the boats overall balance which gives the helm neutral feel. That is, not steering too much to windward and not at all to leeward if the tiller is not held.
To measure the trapeze sheerline, take the trapeze wire forward (not attached to the stretch cord in the beam) and touch it against the inner deck radius at the point where the bridle joins the hull. Then, with the mast fore-aft, walk backwards along the side of the boat until the trapeze wire meets the outer gunwale. This should be at the recommended measure point 10cm forward of the rear beam.
The rudder assembly from Swedish maker Marstrom, fitted on most Tornados, needs no adjustment. Rake and parallel are already set by the factory. The kick-up system on the Marstrom rudders is also set at the factory and works well if you run aground. As the system settles over time, the kick-up adjustment may needs to be adjusted slightly by turning the + screw clockwise two or three turns under the arm to adjust the lock-down mechanism.
On non-Marstrom boats, with the
rudders set up on the boat in the fully down position, you must ensure the
leading edge is parallel to the trailing edge or up to
Rudder rake is measured off the transom. By placing a straight edge down the transom, the leading edge rudder should be raked forward about 30mm. This measure may vary from boat to boat and should be adjusted until the helm becomes neutral or with slight weather helm (boat tends to point in to the wind) if the tiller is released.
If too much weather helm is experienced, then the rudders need to be adjusted further under the boat (that is, greater than 30mm). If lee helm is experienced (boat wants to bear away when tiller released) then the rudders need to be raked back (less than 30mm).
Those who want to sailing for fun should not worry too much about the battens. Simply tie them in the pocket so no wrinkles are showing in the sail and enjoy.
But for those who race will find that the correct battens can be of great advantage with boat speed. No matter who made the battens, they should have a certain stiffness, so the mainsail profile produces the correct forward forces.
By using a 10kg spring scale, the batten stiffness related to the tension on the leech can be measured the easiest way. The lower 5 or 6 battens can be used in any weather. Their stiffness (when pushed or pulled against the scale) should be between 1 and 1.5 kilos and a camber of between 35-40%. The upper 3 or 4 battens should be adjusted according to the wind forces. The stronger the wind, the stiffer the batten.
Between light wind and strong wind the very top batten may vary between 3kg and 6kg. The #2 batten may vary between 2kg and 5kg and #3 between 2kg and 4kg and all with a draft of between 45-50%. For optimum ratings on your battens - ask your sailmaker!
ADJUSTMENT OF THE SAILS ON THE WATER:
Mainsheet, Cunningham, foot tension and traveler. Upwind.
In light wind the main needs very sensitive adjustment. Most of the crews sail with the sheet too tight which stalls airflow. The telltales in the upper third should fly astern for 50%, and monitoring the aft leech is of big help, to find the correct sheeting. The aft leech should not flap leeward.
Using a mark on the main sheet helps to retrieve the right position quickly, especially under pressure, in hectic moments, at the start or when founding marks.
The mains Cunningham stays firm to keep the profile of the sail. The traveler remains centered and the foot is slightly opened about 2 - 5cm distance when measured from the mid of the boom.
As soon as the crew can use the trapeze (due to wind strength), they should cleat the jib sheet and take over control of the main. In gusts and waves, good crews permanently work with the sheet to keep the boat flat and fast.
Medium to strong wind - upwind: In case the lee hull commences submerging the boat will stop immediately, so the crew has to ease the sheet at once to prevent capsizing head over heels. When the boat accelerates again, the sheet has to be hauled in quickly again - the only way to gain optimum speed upwind on a Tornado.
The Cunningham can be kept loose as long as the helmsperson doesn't hike outside the gunwale. If the wind increases it has to be hauled tighter simultaneously to keep the hull flat and the boat powering forward otherwise the Tornado will yaw windward, loose its top speed and tends to stand high upwind. In excessive winds, this may require two hands to pull the Cunningham very tight. If the hull seems to fall 'flat' in the water, ease the Cunningham up to increase boat speed again.
The traveler remains centered always upwind, but the foot tension should be hauled out to maximum in strong wind. With the wind abeam, the trim is similar to compensate the gusts and keep the boat stable.
After the traveler car is placed in its correct position, about 10cm inside the inner leeward gunwale, the foot tension is eased 15cm to 20cm distance from the mid of the boom and the Cunningham is fully released. The main should be opened as far as the telltales are flying 50%. The 5th batten from the bottom also acts as an indicator, which should touch the leeward shroud, while the 4th should be just free from it.
The downwind constellation can be kept at all forces and only needs correction according to the wave height. In choppy seas and disturbed waves it is better to haul the sheet slightly closer.
Jib Sheet, Cunningham and Barber Hauler:
Jib control is not tricky, because the diamond spreaders on the rotating mast provide a good measure. In light breezes, the jib needs to be about 5 or 6cm distance from the spreaders. More wind requires hauling 2 – 3 cm closer. In varying wind force, the crew has to concentrate very much on hauling and easing relative to the spreaders. In light winds, the Cunningham is loose enough to see small wrinkles up and down the luff. As the wind increases, it is hauled tighter so that a smooth profile is always being kept. The deepest point in the sail always has to be in the first half (about 35-40%). Full length stripes at the 1/4,1/2 and 3/4 distances help with this adjustment.
A better drive is obtained by using the barber hauler which fully pulls the jib outside. The jib telltales on the aft leech are flying 50%, similar to the main, requiring permanent attention. When the leech telltales are flying always, the sail is basically too opened. When the telltales never fly, the sail is too closed. The forward telltales also act as a guide for the helmsperson. Once the jib is more or less set, the steering can revolve around the forward telltales. If the closer (inside) telltales are not flying then the helm should be pulled away - and vice versa. If the steering course is correct, then the sheeting will need to be adjusted according to the forward tell tales. If the inside telltales are not flying then sheet in - if the outside telltales are not flying sheet out. Simple rule: Inside not flying = sheet in. Outside not flying = sheet out.
Jib sheeting hauling points. The hauling points are adjustable along and abeam with a simple device. In light winds they are about 130 to 135 cm backwards and 40 cm to centerline, measured from the aft of front beam to jib wire start.
In steps of 5 cm, both these points are hauled outwards and backwards, in strong breezes 145 cm to 150 cm backwards and 15 cm to 20 cm outwards.
Upwind and abeam.
The perfect airflow of the main is achieved only when the mast rotation allows a good angle of attack, in light winds the mast controller should direct to the leeward shroud. As soon as the crew enters the trapeze, the mast rotator has to be pulled slightly towards the centre. In moderate breezes to about 45 degrees of centerline. In strong winds you must be careful not to pull the mast too close to the centre of the boat so that the diamond spreaders on the spar do not pass behind the jibs aft leech.
After easing the foot leech, the mast is rotated outwards and cleated using the rotation limiter. The rotators lever direct forward of the front beam (90-100 degrees).
In light air the crew lies on the windward hull before the front beam and concentrates on watching the jib sail. The helm sits upright in front of the shrouds, just behind the crew. When the wind increases and the windward hull tends to rise, the whole crews weight has to be moved backward. When the crew starts hiking on the trapeze, the helm should sit behind the shroud and start using the foot straps.
When the crew often has to come in and out due to minor gusts, the trapeze wire has to be adjusted in a higher position. As the winds increase and become more constant, the crew has to hag deeper, of course. When hanging stretched on the trapeze, the aft foot has to stand just behind the shroud beside the helmsperson.
The helm firmly holds the shroud with their fore-hand, while the crew controls the sheet from 3 Beaufort on and cleats the jib sheet. By doing so, the helm can fully concentrate on steering without disturbances. Apart from that, the crew gets a firm, rigid hold when standing just beside the helm.
Trim along and abeam similar to upwind, but to prevent submerging in gusts the crew weight has to be moved aft ward. In gales the crew might even hike out behind the helm.
The crew should clip on to the aft retaining line, fastened to the transom, to avoid dashing forward when submerging the lee hull. The lee hull must not dive too deep because the Tornado could somersault. The jib's sheet must be hand-controlled, for quick and easy release when submerging appears. The same applies to the crew and the mainsheet.
Wind abeam and broad reach.
In light air the crew weight has to be moved far ahead and to windward. With increasing winds it is again moved further backward. One should not stick to their position, but try to move along with the waves.
FLYING THE LATEST DOWNWIND TECHNIQUE - THE WILD THING
In the past few years a new technique for trim along and abeam at forces above 3 Bft has succeeded more and more. Gybing downwind and flying the windward hull out of the water to reduce water resistance has become the latest craze in boat speed. The get windward hull out of the water, the crew sits just in front of the leeward beam and leans outboard in the foot straps, while holding the jib's sheet.
The helm stays on the windward hull and steers about 5 to 10 degrees higher upwind than on the ordinary downwind course. The traveler car is fixed near the rear leeward hiking strap. The jib is not completely hauled out (with barber hauler) - about 20 - 30cm from maximum out.
In this way the jib and main get a different angle to the apparent wind than on the direct downwind course. In winds around 3Bft to 5 Bft the leeward centerboard remains down, but the technique also works without centerboards. This new technique downwind is not recommended for beginners. It requires much training due to the many risks involved. For further technique hints on the Wild Thing see additional report by John Forbes in the addition Website tuning guide.
Highest upwind speed is obtained by having the windward hull just leaving (or kissing) the water, not affected by touching a few wave tops. Thanks to a perfectly working rudder-assembly, steering the Tornado is a true enjoyment. The Tornado practically "sails on tracks" when correctly trimmed and adjusted.
THE TORNADO CLASS: PERFORMANCE AND FUN
Due to its sheer size and high speed, the Tornado Catamaran is not a cat for beginners but more a boat for sailors who already have gained experience in other classes and want a greater challenge from the speed experienced. Those who decide to sail a Tornado will be welcomed into a community in which performers and free timers share a lot of fun together. Although it is about Olympic Medals in this Class, the atmosphere amongst the sailors is conveniently cool.
Sailing a Tornado is simply more fun.