To start with, I will only be covering some basics about hulls here. Hull design is an incredibly complicated subject, with different concerns for different hull configurations and a good deal of trial and error in the process of making improvements. One thing I will say, however, is that a boat's speed is highly dependent on hull design. The surface area of the hull that is directly exposed to the water is the single largest source of drag for a boat.
Monohulls
When most people think of a "boat" they typically think of something like what is shown in the picture on the right. This type of boat is the most common, has only one hull and is called, accordingly, a "monohull". With a single hull that is typically much longer than it is wide, monohulls tend to roll considerably, as can be seen in the picture. They also expose a large surface area to the water, and so tend to be slower than multihulls. In races there are often separate classes for monohulls and multihulls for this reason. In order to counteract the rolling tendencies of monohulls, most are built with a "keel", which is basically a large, lead-filled fin under the center of the boat. Both the weight and the hydrodynamic action of the keel help keep the boat righted.Hydroplaning
At a certain speed where the drag forces on the boat equal the propulsion forces, a boat can no longer accelerate further. This is called "hull speed". While there are several ways of circumventing this limit, the most common method for monohulls is to force the boat to climb over its own wake, thus reducing the surface area exposed to the water and thus drag. This is called hydroplaning, and requires a special hull design. Hulls designed for hydroplaning tend to perform worse in non-hydroplaning usage, and additionally a considerable amount of force is required for the boat to climb over its own wake, which makes hydroplaning less efficient than displacement mode (the normal way a boat travels). Sometimes air bubbles are pumped under the boat to further reduce friction. It should be noted that most high-speed boats like jet boats and even racing sailboats use hydroplaning hulls.
Hydroplaning hulls cannot use keels like regular monohulls do, since the keel and the shape of a keel hull interfere with hydroplaning. Instead they use a "daggerboard", which is an unweighted, retractable fin. Multihulls also commonly use daggerboards, although for somewhat different reasons.
Self-righting Design
When a ship, for whatever reasons, finds itself upside down it is called "capsizing". Capsize is considered a disaster for a boat, since a capsized boat cannot be controlled or propelled intentionally, and because the crew can become trapped inside the hull. For a monohull the most common mode of capsizing is rolling over, consistent with their weakest counterforce.
There are some monohulls which have been designed to automatically flip themselves right-side-up again following a capsize in order to prevent the crew from needing assistance, so called 'self-righting hulls'. I mention them here because, while such a design sounds good on paper, in most situations where a capsize happens conditions are bad enough that it is likely to happen again. Normally a boat that capsizes will stay that way and remain fairly stable, however with a self-righting hull a boat can instead end up doing barrel rolls through a storm, which produces a sort of cat-in-the-dryer effect on the crew (pardon the mental image). Any (non-self-righting) well designed boat is livable while capsized, so at least the crew can have stable living conditions while awaiting rescue.
Multihulls
Thousands of years ago, the ancient Polynesians crossed the pacific to Hawaii and Easter Island on boats that were little more than multiple canoes tied together. The most primitive of these was the proa, pictured left. A proa has two hulls which are asymmetric, with one much larger than the other. The extra hull provides additional (buoyant) support against sail forces, which allows a larger, more aggressive sail to be used, but they can only turn away from the wind and handling a proa is quite complicated.
As a result proas are rather uncommon and exotic today. I should note, however, that there are some modern, overpowered, high-tech sailboat designs which use the proa configuration in order to achieve maximum leverage on the sail, but at the expense of only being able to sail in one direction. In general, though, proas are not very practical and most multihulls use other designs instead.
Unlike a monohull, the wide base created by multiple hulls makes well designed multihulls very resistant to rolling over. Instead, multihulls have a greater tendency to bury their bow into a wave, causing the boat to flip end over end, called "pitchpole". Also unlike monohulls, no well-designed multihull ever uses hydroplaning. The hulls can be so efficient that the hull speed of a racing multihull can exceed the hydroplaning speed of a monohull. However, like a hydroplaning hull, when hit from the side by large waves, a multihull skipper (captain) can retract the daggerboard and simply let the wave carry them sideways without flipping. The high resistance to capsize makes multihulls particularly attractive for rough conditions and open ocean use.
Catamarans and Trimarans
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| a catamaran |
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| a trimaran |
Most modern multihulls use symmetric hulls as opposed to the proa configuration. Boats that have two equal hulls are called catamarans, whereas boats that have three (symmetric but not necessarily equal) hulls are called trimarans. Note that higher numbers of hulls are possible, but basically irrelevant for engineering purposes. Catamarans and trimarans are fairly common and even familiar in modern times, seeing applications in both civilian and military settings.
While both create a wide base that helps to balance out various forces on the boat, trimarans in general have a higher total stability than catamarans, and are known for having higher performance overall. However, depending on other design factors that difference may not be very important in practice.
Between the two, catamarans actually offer a surprising amount of interior space when used for a large (30ft+) boat, whereas a trimaran is the opposite since its outrigger hulls typically don't provide enough space to be livable. For a smaller boat, a catamaran offers few advantages since the interior of the hulls are too small to be usable, whereas the main hull of a trimaran can potentially seat several people comfortably. Additionally, most multihulls feature nets stretched between the hulls to provide safety and (perhaps extra) walking space on the boat.
Due to their various advantages I will only be considering catamarans and trimarans for the design that I will propose later. Additionally, any disadvantages they carry are easily mitigated with few and small modifications, which is advantageous both for design and construction.
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