RUBRICA SETTIMANALE “PAROLA DI SKIPPER” | IL CENTRO VELICO

RUBRICA SETTIMANALE “PAROLA DI SKIPPER” | IL CENTRO VELICO

La barca

9.1 Il centro velico

Quando la barca s’inclina per effetto del vento, il centro di carena (CC) si sposta lateralmente verso il lato più immerso, mentre il centro di gravità (CG) rimane dov’era (a meno di barche dotate di ballast, di chiglia basculante o di equipaggio numeroso che in virata si sposta da una parte all’altra). In tal modo le verticali passanti per i due centri si allontanano, ed è per questo che si crea una coppia di forze tendente a riportare la barca dritta.

Il punto in cui la verticale passante per il CC a barca sbandata incontra il piano di simmetria longitudinale è detto metacentro (MC). Perché la barca sia stabile, ossia abbia la tendenza a ritornare dritta quando viene inclinata, occorre che il MC sia al di sopra del CG. L’“altezza metacentrica”, cioè la distanza verticale tra CG e MC, dà un’idea della stabilità della barca di navigare senza fare uso di angoli di barra per contrastare un eccesso di tendenza orziera o, peggio, puggera.

Secondo un’antica teoria, la barca a vela era equilibrata quando continuava ad andare in rotta per un po’ dopo avere lasciato la bara, e ciò era vero per le barche di allora, il cui timone costituiva la naturale prosecuzione della chiglia. Con i timoni staccati e appesi delle moderne imbarcazioni, le cose sono molto cambiate e, lasciando la barra, vi sarà sempre una più o meno accentuata tendenza orziera.

Il CV è il punto dove può immaginarsi applicata la forza generata dal vento sulle vele. Perché la barca sia centrata, il CV va messo in relazione agli altri due centri. Se non vi fosse l’inclinazione, sarebbe sufficiente considerare il solo CD, che è il punto in cui si può immaginare applicata la forza anti scarroccio generata dall’opera viva, ossia dalla carena, con particolare riguardo alle sue appendici.

Occorrerebbe allora che il CV e il CD si trovassero sulla stessa verticale per avere la barca equilibrata. L’inclinazione complica le cose, perché il CV, a barca sbandata, viene a trovarsi sottovento al CR, e ciò genera una coppia di forze rendente a fare orzare l’imbarcazione, in maniera tanto più vistosa quanto maggiore è l’inclinazione.

Più precisamente, le forze della coppia da considerare in questo fenomeno sono la componente utile all’avanzamento della forza generata dal vento e applicata al CV da una parte, e la resistenza all’avanzamento dello scafo, applicata al CR, dall’altra. Il braccio della coppia è la distanza orizzontale e perpendicolare alla chiglia tra i due centri.

Perché la barca resti equilibrata, occorre allora che il CV si venga a trovare a proravia del CD, in modo che la coppia puggera che così si genera vada a contrastare, uguagliandola, la coppia orziera dovuta all’inclinazione. È questo un compito dell’architetto, che lo svolge per le condizioni medie di vento, ma anche dell’equipaggio, che influisce sull’assetto, sulla superficie delle vele e sulle loro regolazioni per adattare la barca alle condizioni del momento.

Tratto dal libro Parola di Skipper, Editrice Incontri Nautici di Giancarlo Basilestabilità

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