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In order to cut smoothly through the water, the lengthwise curve of the bottom (called “rocker”) must exactly match the curve of the sides, giving the water no excuse to want to flow across the chines which join them, causing turbulence and drag.
This rule can be relaxed somewhat toward the transom; making the transom wider provides a bit of weather helm, which provides built-in safety: if the steering gear fails, the boat noses up into the wind and stalls instead of falling off to leeward and maybe capsizing.
The chines should have lips that extend out horizontally, called “chine runners.” The chine runners provide enough lateral resistance to make it possible to sail without extending the centerboard(s) except when going to windward. In a tight spot, they even make it possible to sail to windward over shallows, where there is not enough draft to extend the centerboard. Chine runners work so well that one successful microcruiser design (Matt Leyden's PARADOX) doesn't even need a centerboard.
This sort of hull is easy to construct out of plywood sheets screwed and glued together and sheathed with fiberglass, and it sails just about as well as anything. My first boat, HOGFISH, which was a 32-foot square boat, could do 7.5 knots out on the ocean in moderate conditions, and that's good enough. Anything done to improve on that I would consider gold-plating, especially if it detracts from the boat's usefulness as a houseboat.
But I am improving on the traditional square boat hull shape somewhat. HOGFISH was a sharpie, with a sharp stem at the bow that joined the bottom just below the waterline, whereas QUIDNON is a scow and has a bulbous, rounded bow that bounces over the waves or, when that doesn't work, punches through them like a battering ram. Both designs are effective, and have even won races, but the scow can have much wider beam, providing lots of living space, which is essential for a houseboat.
Square boats have numerous advantages beyond low cost and ease of construction. Perhaps most importantly, they can go aground safely. Some boats go to windward well, and that's laudable, but hardly any of them go aground worth a damn. Keelboats flop on their side, bounce around in the surf and get destroyed. Washing up on the beach is a calamity for virtually every sailboat, and even “touching bottom” in calm conditions is considered an emergency of the highest order.
A square boat, with its flat bottom, can be beached, rolled ashore over round bits of driftwood, and relaunched later on without much fuss at all. Such a boat doesn't require a crane with slings, a Marine Travelift or a marine railway with a cradle to haul out or launch—just a ramp dug into a beach, some logs and a winch.
It can also settle upright at low tide, making it possible to anchor it in shallow water. Running aground in a square boat is generally a non-event: I ran aground numerous times in HOGFISH, and just waited for the tide to float me off. If that didn't work, then I “kedged off” by setting an anchor in deeper water by rowing it out with the dink, then winching it in.
Square boats make better houseboats because they have much more storage space. Using simple math, you can calculate a round-bilge boat has 20% less cross-section than a square hull of the same dimensions, but that's deceptive, because most objects you want to store are rectangular in cross-section and fit well a square hull but not a round one.
Square boats can also carry a lot more weight: because of the large surface area of the flat bottom, it takes a lot more weight to submerge a square hull below its design waterline than a round hull of the same dimensions. They also track better in the water: the long, hard chines where the bottom meets the sides prevent them from yawing around when buffeted by wind and waves, and they maintain the ability to steer a straight course all the way down to a fraction of a knot.
Lastly, they are much more comfortable in big waves: they don't corkscrew down waves like round-bilged boats do, and there is no keel to trip up on waves (once the centerboard is pulled up). Also, they are slow to heel in a gust because of the large quantity of water that has to traverse the chines, and in all but the most severe conditions square hulls adopt a certain level of heel and just stay there, while round hulls rock back and forth continuously. Even when lying ahull in big waves square hulls refuse to build up much angular momentum because a lot of the energy is dissipated by water swilling over the chines, and this limits the rocking.
And, so I can shave a very large amount of the cost off simply by declaring that the hull will consist of exactly 5 planar surfaces, 3 of them simple curves (bottom and sides) and two perfectly flat (deck and transom). It won't look like a yacht, but it will still look like a boat, and that's good enough for me.
The biggest complaint about square boat seems to be their lack of sexy, sleek lines. Now, being a family man, I am not overly concerned about such matters, but in case you are, I have never found women to be particularly concerned about the lack of sexy, sleek lines in a boat—especially the sexier, sleeker women. They are mostly concerned about whether the toilet flushes, whether there is plentiful hot water for the shower, whether there is enough room for all their shoes and hanging lockers to hang their evening gowns, and whether there is enough flat surface on deck for them to stretch out and model their bikinis. And so the problem with the lack of sexy, sleek lines should be easy to remedy through the usual means, without resorting to trying to sculpt them out of plywood. Seems commonsense to me.