RUBRICA SETTIMANALE “PAROLA DI SKIPPER” | I “CENTRI” DELLE BARCHE

La barca

7.1 I “centri” delle barche

Centro di gravità, centro di deriva, centro di carena, centro velico, centro di resistenza all’avanzamento, metacentro… Quanti centri in barca! A che serviranno poi? Premesso che la posizione di questi centri non è mai esattamente definita, anche perché è continuamente variabile, sia pure entro limiti contenuti, i “centri” servono, e come, soprattutto in fase di progettazione. Ma aiutano parecchio anche chi vuole avere le idee chiare sul comportamento della sua barca in mare, in modo da sapere come migliorarne le prestazioni. Non per niente gli sforzi del progettista e dello skipper sono tesi a creare una barca “centrata”, possibilmente in ogni condizione.

Che vuol dire “centrata” o “equilibrata”? Sono in tanti a parlarne o a scriverne, ma in pochi ad avere le idee davvero chiare. Talvolta infatti il centro velico (CV) viene messo con disinvoltura in relazione al centro di gravità (CG) o al centro di carena (CC) e la maggior parte di chi legge o ascolta non si accorge neppure dell’errore, piuttosto grave se commesso da un “addetto ai lavori”. E’ un fatto che questi centri vanno considerati sempre in coppia. Si deve però imbroccare la coppia giusta! Accade anche che lo stesso centro possa “accoppiarsi” con un altro per studiare una certa concatenazione di causa-effetto, e con un altro diverso per approfondire lo studio.

Come raccapezzarsi? Occorre innanzi tutto avere compreso bene le definizioni, che, in fin dei conti, non sono affatto difficili. Il CG, è baricentro, tutti sanno cos’è: il punto in cui si immagina applicata la forza peso di qualunque corpo, nel nostro caso della barca. Gli architetti sanno come scovarlo in sede di progetto, con un’accurata analisi dei pesi e della loro ubicazione. Va messo in relazione al CC per studiare la stabilità della barca dove si immagina applicata la “spinta di Archimede”, quella verticale, dal basso verso l’alto, uguale al peso dell’acqua spostata dalla carena, appunto, che poi è esattamente uguale al peso della barca, detto anche “dislocamento” proprio per questo motivo. Il CC della barca ferma e in assetto di progetto è localizzato nel centro di volume della carena, od opera viva, e viene individuato dagli architetti con un procedimento geometrico. CG e CC, in acqua calma e in assenza di vento o di altre forze che possono disturbare la barca ferma, sono dislocati sulla stessa verticale e, se la barca è stata ben progettata e non è stata appesantita poi in modo errato, la linea di galleggiamento di progetto coinciderà con quella reale.

E’ questa una prima verifica, da effettuare al momento del varo, ma anche in seguito, ogni tanto, perché inesorabilmente le barche vengono appesantite dagli armatori in maniera non sempre corretta. Ed è importante, perché, se l’assetto è quello giusto, la barca navigherà “sulle sue linee d’acqua”, opponendo la minima resistenza all’avanzamento. Se la barca risulta appoppata, o appruata, le sue prestazioni ne risentiranno negativamente.

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

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