Pressure-driven machines
Pressure-driven swimming machines depend on one or more pumps. Discharge rates of 13 L/s (200 US gal/min) and more are possible, from motors of three or four horsepower (2 or 3 kW); power requirements are determined from pump curves, where the pump is selected to Ś volumetric flow, as pressure loss is relatively low as the water does not need to be lifted, but only overcome swimmer drag and other pressure losses within the system. One of the earliest models on the market, introduced in 1973, was the Badujet which is available only in the form of a bare propulsion system, to be installed into either an existing or newly built pool. Also in this category of pressure-driven swimming machines are a number of swim spas, usually, fibreglass shells equipped with several pool pumps to set the water in motion. Seen as more convenient since they come pre-assembled, the quality of the swim has been criticized by indoor swimmers as being somewhat turbulent, as the strength of the current comes from the speed and pressure of the discharged water, rather than its volume. Contrary to this, triathletes and other sea swimmers have praised the system due to the turbulence created by the jets mimicking the behaviour of the sea, improving stamina and general fitness, and preparing them for unpredictable conditions they may face whilst they compete. Swim Spas, as the name suggests, are a combination of a spa (or hot tub) and an exercise pool. Single-zone models are typically a fibreglass pressure-driven exercise pool which has swim jets at one end, and one or more spa seats fitted with massage jets at the other. Swimmers and athletes have praised the integration of both a hot tub and swimming machine model since they can use it for exercising and also use it recreational purposes. In the 1980s Monarch Spas developed the dual-zone ''swim spa'' so that pumps and other equipment needed for the pool could also be used to power a separate spa. Today, the advantage of the modern ''dual-zone'' system is that the two pools can be at different temperatures using different chemicals - the Hot Tub (using bromine), is hot enough for relaxation and massage, while the swim zone is cool enough for strenuous exercise (using chlorine).Volume-driven machines
In the 1980s, a new type of machine made its appearance. In an attempt to correct problems of turbulence and resulting discomfort from swimming against a jet of water, systems were devised to set the water in motion in a smoother fashion. The first, in 1984, was the SwimEx, developed by Stan Charren together with two MIT-trained engineers. This machine, consisting of a fiberglass pool with the machinery housed in an adjacent compartment, sets the water in motion by means of a paddlewheel and generates a steady stream of water as wide as the swimming pool itself. In the late 1980s, the Endless Pool was developed by James Murdock. This machine places the water-moving equipment, a large propeller encased in a stainless steel box and powered by a remote hydraulic pump, and itsHybrid systems
A number of "still-water" mini-pools have been built, designed to be used in conjunction with various resistance-swimming tether systems. These human-powered devices combine the self-contained aspect of counter-current swimming machines with the lower priced and simplicity and freedom of movement of tether systems used in athletic training. They have major cost and energy-use advantages over mechanical swimming machines. They are often used for aerobic exercise, endurance and strength training, and for stroke practice. However, they cannot replicate open water conditions, in which the water courses at speed past the swimmer, so that for competition training their use has to be combined with open-water practice. One example of such a device is the ''Swimergy Swim System'', which also makes use of wave-reduction technology.References
{{DEFAULTSORT:Swimming Machine Swimming equipment Swimming pools