The "upside down" shape of the hull is to reduce the greatest detrimental drag factor of a "small" hull's forward movement, ie with "tumble home" (narrower at the deck than at the bottom of the hull), when the bow is depressed by a wind gust, and before it responds to the gust with forward motion, by reducing the "rough water drag factor" of the hull, (rough water factor is simply in lay mans terms, the distance that the water is parted at the surface) the greatest impedence to the hulls forward motion is actually reduced instead of dramatically increased, (the reverse is known by all sailors who have droped the bow of a hull that gets wider at the deckline deep in the water in a gust and the resistence has increased at a rapid rate and caused the "tripping effect" ie the stern wants to catch up to the bow , or as some would say "pitch pole") By reducing the rough water drag effect at the bow , as the hull is depressed, the hull actually becomes more efficient through the water instead of the reverse, and will drive down, through, and out with speed, instead of slowing and, at the least "bucking".
darryl J Barrett.