an illuminating article in the old Captain Basile!
"Twice the route, three times as long, four times the itch"
Reach by starting point the point of arrival as soon as possible: this is, of course, the key criterion in choosing the route of each type of boat that is going to take a navigation.
Nella navigazione a motore è in genere una scelta relativamente semplice: si traccia la rotta di minor percorso, normalmente costituita da una spezzata di lossodromie, e si cerca di seguirla accuratamente alla velocità consentita dalla potenza del motore e dallo stato del mare. Si è così ragionevolmente sicuri di arrivare a destinazione nel minor tempo possibile. Si può dire quindi che nella navigazione a motore la rotta di minor tempo normalmente coincide con quella di minor percorso.
Quest’affermazione è tutt’altro che vera nella navigazione a vela, indissolubilmente legata alla direzione e forza del vento, e più influenzata dalle correnti marine, considerato il fatto che la velocità media del veliero è inferiore a quella dell’imbarcazione a motore. Per cominciare, esiste un settore dei 360° dell’orizzonte, a cavallo della direzione di provenienza del vento, assolutamente vietato al veliero. L’ampiezza di questo settore è determinata dalle due rotte che il veliero riesce a seguire stringendo al massimo il vento sui due bordi, e varia in maniera molto considerevole in funzione delle caratteristiche del veliero oltre che delle condizioni del vento e del mare. Mentre per la Vespucci , per fare un esempio, il settore vietato è dell’ordine dei 150° nelle migliori condizioni, per una moderna imbarcazione da regata può reduced to about seventy degrees. Leaving aside the extreme cases, we can say that a pleasure craft "honest" the prohibited area is a hundred degrees, or more than one quarter of the arc of the horizon.
If the point of arrival is included in this field should make the best of a bad lot and take the pace of close-hauled, choosing the edge that comes close most of the route of least path (Fig. 1).
fig.1
And while you begin to adapt to the idea of \u200b\u200btacking, which often results in a somewhat uncomfortable navigation ( Twice the route, three times as long, four times the itch : this was defined as the tacking by the sailors of past generations!). While sailing upwind, the prudent skipper will begin to make some observations and to raise issues. If you follow a pace very close, with the sheets up to shit, yes it is true that the bow closer to see what he wants, but it is also true that it will hardly be satisfied with the speed I can get. Puggiando of 5 °, the sheets a little spinning speed improvements. Puggiando other 5 th the speed continues to improve, but the bow still deviates from the goal and the path as a result of stretching (Fig. 2).
fig.2
What then is the best compromise between speed and bow? The experienced skipper who knows his boat and knows how to bring the best will always think of this dilemma. His "uncertainty range" is, however, much smaller di quello del principiante, il quale tuttavia farebbe bene a studiare attentamente questa faccenda, se desidera salire di qualità più rapidamente.
La VMG
VMG in inglese sta per “Velocity Made Good”, che tradotto alla lettera sarebbe “velocità fatta buona”. VMG è un termine ormai entrato stabilmente nel mondo della vela, come tanti altri di origine anglosassone, e bisogna tenerselo, anche perché il termine corrispettivo italiano pare che non sia mai esistito. La si potrebbe chiamare “Velocità utile” o “Velocità di avvicinamento”, in quanto la VMG altro non è che la componente utile della velocità. Se si fa rotta diretta da A a B, la VMG coincide con la velocità dell’imbarcazione. Se invece la rotta si discosta di un certo angolo dalla congiungente AB, la VMG risulta minore della velocità istantanea: tanto minore quanto maggiore è l’angolo di scostamento dalla congiungente AB, fino ad annullarsi quando quest’angolo è di 90°. Graficamente la VMG si trova facilmente (Fig. 3).
fig.3
Just draw a line joining the ship between A and point B arrival, the route actually held and the current speed, measured on a scale as desired. Projecting the velocity on the line joining AB, we obtain the VMG, which is measured with the same scale as the first choice. In the case of Figure 3, the boat speed is 4 knots. The resulting VMG of 3.6 knots. This means that at that time the approach speed at B is 3.6 knots. Sailing is this speed that counts, and choosing the shortest route must be made with the primary purpose of ensuring that the VMG at all times is the maximum possible. For those who remember the basics of trigonometry, la VMG non è che il coseno dell’angolo di scostamento dalla rotta diretta, moltiplicato per la velocità istantanea. Con i calcolatori digitali è facilissimo trovarla. Nel caso di Fig. 3, l’angolo di scostamento è di 26° e la velocità di 4 nodi. La VMG risulta, col calcolatorino, di 3,59 ... nodi, contro i 3,6 trovati graficamente. Disponendo del calcolatore, quindi, non è necessario fare operazioni grafiche. Basta sapere la rotta che si dovrebbe tenere, quella che invece si sta mantenendo, e farne la differenza, che è l’angolo di scostamento. Il coseno di quest’angolo per la velocità istantanea dell’imbarcazione è la VMG. I tasti da spingere sul calcolatorino sono in successione: 26 °, cos, X, 4 = Returning to Figure 2, knowing what hours and how is the VMG, it's easy to establish that in the particular case, the most convenient route is 45 ° from the wind , which corresponds to the highest VMG (Fig. 4).
fig.4
In practice, when going to windward, all this does not work: she does instinctively that the helmsman with a sensitivity that has gained with practice. He means "hear" what is the best compromise between the bow and speed and will try to keep this the best compromise. In other words, keep the VMG around the maximum value, consistent with prevailing conditions. Going on board with the port tack to get from A to B, all of a sudden you will have to tack. When? To answer this question is worth considering that the VMG, as it proceeds to the port tack, it is increasingly diminishing. This is due to the fact that 'angle between the actual route, which we assume constant, and the joining point of arrival, ship-to-point is gradually increasing (Fig. 5).
fig.5
When this joining (A'B in the drawing) will be oriented exactly upwind, the VMG will be the same on both sides. From that moment on, begin to be more favorable than the other edge. To maintain the maximum VMG would then need to veer all the time. But every viramento lose speed, then you need to recover prudently, so we will go on for a while '. A good rule is to turn when joining A'B is rotated by 5 °, after passing the wind direction (Fig. 6).
fig.6
Ciò porta alla tecnica del bordeggio entro un settore di 10°, col vertice sul punto di arrivo B e con la bisettrice coincidente con la direzione di provenienza del vento (Fig. 7).
fig.7
Con questa tecnica ci si garantisce dagli effetti di un salto di vento, sempre possibile, se non addirittura probabile. Effetti che talvolta possono essere benefici, talvolta invece nefasti. In realtà, se si fosse sicuri che il vento si mantenesse constant in direction and intensity throughout the area affected by tacking, it would be worthwhile to carry out many of viramenti board long enough to pull a tack board to either the left or straight, and turn when you are in a position to reach B along with another board. But this rarely happens, even in races in a triangle, where the routes are relatively short and times of a few hours only. In practice, the wind sometimes scarce, sometimes gives good. For example, if, after a hypothetical border along with the port tack (Fig. 8) the wind was running about forty degrees counterclockwise, at the time of turn would be coming straight from the goal and you should start to tack.
If he had remained in tack in the field of 10 °, the turning of the wind could be achieved with one B edge.
fig.8
In summary, having to tack from A to B you should: 1. Choose the board closest to B. 2. Continue on the board until the detection of B exceeds 5 ° wind direction. 3. Tack, and if the wind turns, start over from step 1. If, however, remains constant, continue in turn as described in Section 2. 4. When you are close enough to B, that in view of the goal and no more than a quarter of a mile, continue on a board, leaving off the field (Fig. 7), turning when the angle between the bow and the detection of B is twice what the boat is able to forge with the true wind (not to be confused with the apparent wind, which is shown to the flag at the masthead or ribbons tied to the shrouds). To find out what holds the boat, just take note of the compass heading on the two sides and divide by two the difference. This is also the most practical way to ensure true wind direction. An old rule said to veer when the goal was abeam, or a little 'abaft the beam. The boats average now shaking a little more, so by adopting the old rule then you may want to puggiare to the point of arrival. This is not a bad thing, but the sailors are well aware of how much progress may be lost!
The tacking downwind
The pace in the stern is not the fastest. This is particularly true as the wind is light. The beginner realizes that the rate increases considerably when dall'andatura aft luff is full of only twenty degrees. The reason is that the apparent wind, the minimum intensity nell'andatura in the stern increases immediately luffing and, what is more important, visibly rotates more than was barley. This greatly enhances the aerodynamic efficiency of the sails. Having to go from A to B with the wind in its sails, it is thus said to be the direct route that will get to B in the shortest possible time. This will only happen if the wind is strong enough to keep his boat maximum speed. In medium wind conditions, light winds and even more, you'll always luff for a route, the wind, making walking the boat much better. Even in this case the pilot will be plagued by doubt: "If barley go faster, but then I have to jibe, and then a path higher. Should I? And what should I luff? "Here the considerations on the VMG are easier to implement than in tacking upwind, where the sensitivity of the helmsman to play the role. It should however have a speed sensor as well as the compass. Starting from the wind in their sails luff of just 5 ° 5 °, each time waiting for the speed stabilizes, and take notice. During these tests is to govern well the compass, taking care to keep the bow steady as possible for as long as is necessary to stabilize the speed. A simple graph, like that of Fig 9, or an even simpler operation with calcolatorino electronically immediately clear which of the four or five experimental routes allows greater VMG, and it is soon given the quickest route, although longer direct one.
fig.9
Wanting to get things done "so much per pound, luffs is about fifteen degrees with average wind, and up to about thirty degrees with light winds. If the wind is light you can also head up to 40-45 °. Suddenly he will present the question of when to jibe. The concept is simple: it is weird when, taking the wind on the other tack, under the same angle, you also have the bow on B. In practice, this occurs when the angle between the heading and the direction of B (which is the detection of polar B) becomes twice the angle between the stern and the true wind direction (Fig. 10).
fig.10
This can be appreciated by eye with a fair approximation, watching the small ripples in the wind drawing on the surface of the waves. While these do not always have the same forward direction of the wind, the ripples are always perpendicular to the wind itself. The detection of polar B can be appreciated by eye. The portable detector, however, is far more accurate than the naked eye. To use it properly need to get their reading of the compass when you take the survey. The difference between the two values \u200b\u200bis the polar tracking. Also to appreciate the true wind direction can help you with the portable detector, pointing in the direction perpendicular to ripples. If the wind is not exactly in the stern, you will choose the first edge closer and closer to B, and antics following the simple rules shown earlier (Fig. 11).
fig.11
Finally, if the wind shifts, it is necessary to jibe once, or continue on the same tack depending on how it runs, in order to maintain VMG always to the highest available. In practice, when the wind gives puggia little weird, and when he gives you good. Everything that is independent of the presence of current knowledge and more or less reliable of the local winds, for example, knowing that there is a high probability that the wind speed at a certain point in a certain way, or the current could influence, may well be a convenient start tacking in both bow and stern in, starting with the negative edge. These are the cards that play the competitors. However, considering that local practices, rather than the fate groped should certainly follow the rule of trying to approach the goal with the best VMG.
Giancarlo Basile (October 2007)
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