Modern whitewater rafts come in two basic flavors: 'self bailing rafts' and 'cat-a-rafts'. Self bailing rafts or 'self bailers' have the traditional raft shape, while catarafts, known as 'cats' for short, are made of a pair of pontoons straddled by a metal frame.
How is it that a whitewater raft can bail water out of itself? Well, an ingeniously simple design makes it happen; the floor of a self bailing raft is a wide flat inflated chamber, sort of like a big air-mattress. The edges of the floor are stitched or laced to the rest of the raft. When inflated, the floor is about 4 or 5 inches thick, so the top surface of the floor is above the surface of the water. When water splashes into the raft, it flows across the floor, down over the edge of the floor, and out through the lacing. This design works amazingly well. A self bailer filled to the brim with water will proceed to empty itself in just a few seconds. (Been there many times!)
If you've ever used a bucket to bail water out of a raft, you know how sweet it is to have a self bailer. In fact, if you float with someone who has an older non-self-bailing raft (known as a 'bucket boat' for the obvious trait - it retains water), you get to stop and wait for them to bail at the bottom of each major rapid. So, nowadays, when someone says 'raft', they're usually talking about a self bailer. If they say they've got a 'bucket boat', well, too bad for them.
Self bailers are the work horses on rafting trips because they can carry a lot of gear and passengers. Popular sizes are in the range from 13 - 18 feet long, with 14 feet probably a minimum for carrying gear and two adults on a multi-day raft trip. A length of 15-16 feet is ideal, and 18 footers are sure nice to have on larger rivers. From a performance standpoint, rowing a self bailer is more like driving a bus. So the longer the raft, the slower it is to maneuver. And the performance of any raft is diminished if it is overloaded, pressing it deeper into the water.
Cats are far more maneuverable than a self bailing raft but carry less weight. Catarafts are the sports cars of the river. And like sports cars, they carry fewer passengers, but provide a lot of fun for the driver. If you're new to rafting, you would be well advised to consider buying a cataraft as your first raft. Their maneuverability will help you recover when you mis-judge a rapid, and of course they're a whole lot of fun. If you live close to a white water river so that you can go boating on the weekend, a cataraft is the ideal raft for a day trip. And cat-a-rafts are also ideal for high water or extreme multi-day trips when each participant will row their own raft. Sizes of catarafts are often described as pairs of dimensions separated by "X", like "12X20", "16X24". The first number is the length in feet, and the second number is the diameter of the pontoons in inches.
In the last decade, whitewater rafts have undergone rapid evolution. Just as snow ski manufacturers have achieved radical increases in performance though ski shape and materials, whitewater raft manufacturers are constantly experimenting with raft tube shapes to achieve improved performance on the river. One of the first changes to come along was the ‘diminished tube’ concept for self bailers. With these boats, the diameter of the raft tubes is smaller at the ends of the raft than at the middle of its length. The intent is to reduce wind resistance and increase interior space for passengers and gear. Modern cataraft designs include ‘tear-drop’, meaning the tubes are asymmetrical in the long axis, and ‘fairy slippers’, meaning the ends of the tubes are turned up radically. The manufacturers tend to invent their own terminology so you'll need to do your research. If you’re buying a new cat-a-raft, you're strongly advised to try out a number of boats before you buy. And of course, ask questions of expert boaters on Internet forums.
Self bailing rafts can be set up with a rigid metal (or on rare occasions, wood) frame that rests across the top of the raft, to which oar locks are mounted and oars attached. The other configuration is to leave the frame and oars at home and just use paddles. So the term 'paddle raft' refers to this other configuration. It's just a term for a self bailer that is set up with inflated seats known as 'thwarts' (say 'wort' and add the "th") for passengers to sit on. Passengers are obligated to row, or more accurately, paddle the boat down the river. While this means work for the passengers, it makes for a very entertaining, social, and usually wet ride! Most commercial river guide services run paddle boats to give the paying customers the full immersion (pun intended) experience.
The normal configuration for a paddle raft is to have a 'captain' (meaning skilled and knowledgeable guide) sitting at the rear of the boat shouting orders to the crew and using her paddle to steer or make fine adjustments to speed and direction. The 'crew' is the rest of the passengers, with half of them paddling on one side and the other half paddling on the other.
Most of the paddle strokes that a paddle raft crew takes to steer the boat propel the boat forward. Therefore paddle rafts move down the river faster than other boats. Self bailers are next fastest. Fully loaded self bailers carry a lot of momentum and offer a lot of surface area to the river. So friction with the water causes them to be dragged along easily by the current. Since it's a lot more work to fight against these factors, the steering strokes that the person at the oars takes are usually forward strokes. The boatman just tries to adjust the direction or vector of the boats natural movement down the river. It may be surprising to know that catarafts are usually the slowest at travelling down the river as they offer less surface area for friction and are so light that it is easy for the oarsman to pull back, arresting the momentum. For these reasons, rookie oarsmen in catarafts are usually at the back of any group of rafts.
You might hear the term "R2" mentioned in the context of whitewater rafting. This refers to a paddle raft being paddled by just two persons. They usually sit shoulder to shoulder in the middle of the raft, each padding a side. Of course, they can sit anywhere they want. The term R2 implies a sportier version of paddle rafting since it is more challenging to maneuver the boat when there are just two persons aboard.
Other forms of river craft are worth mentioning. The obvious one is kayaks. These also come in two flavors; hard shelled kayaks made of various rigid plastics, and 'inflatable' kayaks, also known as "IK's" that are made of the same materials as rafts. Both types of kayaks are a lot of fun and have their vocal advocates. I'll just point out a little bit of trivia. That is, while both groups share the river, hard shelled kayakers and rafters generally don't intermingle much. They are two different sports that share the same playing field. But some hard shell kayakers shun their inflatable brethren in IKs. So, ironically, inflatable kayakers tend to hang out with the rafters. I guess it's a case of 'rubber sticks together'. ;-)
Another craft you may see on the river is the 'drift boat' or 'dorie'. These have a traditional boat shape with bow and stern rising high to break the waves. They're made of aluminum, fiberglass, wood, or polyethylene plastic. These are the craft of choice for fishing rapid rivers, as their flat smooth bottomed hull shape allows them to stop and hover in fast water and their rigid floors allow fishermen to easily stand and cast. Dories definately offer a sporty ride as they're not as stable as cats and self-bailers in big water. The best whitewater designs have sealed compartments for gear storage and to aid in flotation, should too much water come over the side. An obvious disadvantage is their allergy to rocks!
Finally, you may come across two other types of rafts, both big. 'J-rigs' are giant rafts that are very specialized passenger and gear haulers constructed of huge pontoons and are often motorized. They're used by many of the commercial rafting companies in the Grand Canyon. The second type of giant rafts are called 'sweep boats', named for the two huge oars used to steer them. They're basically very large self bailers, but the oars extend out both ends of the raft, and steer the boat like rudders rather than extending out the sides to row as a normal self bailer does. Sweep boats, or "sweeps", are commonly used by commercial outfitters on the Middle Fork of the Salmon; you'll rarely see them on other rivers. Their advantage is that they carry huge loads of camping gear and food. And, due to the huge displacement and surface area exposed to the current, they are the fastest non-motorized craft on the river. Their disadvantage is that they have no brakes! Without lateral oars to pull back against the current, they can't slow down and must ram the shore to stop. Trust that if you see one, the guide holding those big oars is both very courageous and very skilled!