Citat posta Tome Bašića na web forumu nautica-portal "Što se tiče podizanja kobilice, u stvari i bolje je da se ne podiže [na Seascape18 jedrilici]. Jedra imaju preveliki bočni pritisak, pa bez kobilice bi jedrilica klizila bočno i ne bi postigla idealni pritisak na jedra koja se dalje prenose u brzinu. Također, centar podvodnog dijela broda se pomiče nazad, što nam sa velkom površinom genakera uopće ne pomaže, naprotiv."... "Kako sam prije napisao, podizanjem kobilice jedrilica se počne kretati bočno, tj. niz vjetar i kada zbrojiš komponentu tog kretanja i smjer vjetra, onda se smanjuje prividni vjetar. To je tehnika za spore brodove koji i tako imaju svoju ograničenu brzinu prema dužini vodene linije (deplasmanska plovidba). Zato spori krstaši, ili jedrilice s jednim jedrom (laser, finn) jedre gotovo ravno niz vjetar jer im se ne isplati jedriti oštrije i praviti duži put sa istim brzinama. No, sa genakerom je cijela priča zamišljena da se jedri malo dužim putem, kut prividnog vjetra dolazi dosta s boka (kod uistinu brzih brodova kao skiff čak i sa prove, do 60 stupnjeva kada jedrite u krmu). Ako bi brzi brodovi digli kobilicu, došli bi na isto ono što dođu deplasmanski brodovi, tj. brzina vjetra, otpor broda i onda se sve svede koliko jedara staviš niz vjetar pa koliko te vjetar gura, to je to. No sa genakerom se radi o maksimalnom iskorištavanju jedara i protoka vjetra, te postizanjem veće brzine broda, postiže se veći prividni, itd. pa jedra bolje funkcioniraju i više sile pretvaraju u brzinu. To je princip po kojem je BMW Oracle trimaran jedrio 33 čvora po 12 čvorova vjetra. Da je okrenuo ravno u krmu i podigao sva jedra koja ima, pod uvjetom da ga gura barem 1 čv. prividnog vjetra, išao bi max. 11 čv., zar ne?" | Translation of a post by Tome Bašić on the nautica-portal web forum "As far as raising the centerboard is concerned, it is better not to raise it at all [on a Seascape18 sailboat]. The sails create a very strong sideways push, so without a centerboard the boat would have a significant leeway, and it wouldn't achieve the ideal pressure on the sails which would otherwise be transferred into speed. Also, center of lateral resistance of the hull would be brought back, which is not helpful at all with the large gennaker area."... "So, by raising the centerboard the boat starts to slip sideways and downwind, and when component of that movement adds itself to the wind direction, the apparent wind is decreased. This is a technique for slow yachts which don't plane. That is why cruising yachts, or single sail sailboats (laser, finn) sail almost straight downwind because it doesn't pay off for them to sail a longer course at the same speed. But, with the gennaker the whole idea is to sail on a longer course, the apparent wind is from abeam (really fast boats like skiffs have it even from forward of abeam, about 60 degrees when they sail downwind). If those fast boats raised their centerboards, they would get themselves into the same situation as the slow nonplaning boats, i.e. the speed of the wind, the resistance of the hull, and then it's all only about how much sail area you have up and how much the wind pushes you along, that is all. But with the gennaker it's about using the sails and wind to the maximum efficiency, reaching greater speeds, etc. so the sails work better and convert more force into speed. That is the principle upon which BMW Oracle trimaran sailed at 33 Kt with only 12 Kt of true wind. If it went straight downwind and hoisted all the sails it has, it would have achieved only about 11Kt." |