A swimming machine is a resistance swimming apparatus, often self-contained, enabling the swimmer to swim in place. This may be accomplished either by accelerating the water past the swimmer or by supporting the swimmer, either in water or on dry land. The first type, known as a countercurrent swimming machine, usually consists of a water tank at least twice as long and about one and a half times as wide as an average person with the limbs extended. The swimmer swims unrestrained against an adjustable stream of water set in motion by means of mechanical devices, such as jets, propellers or paddle wheels.
Countercurrent swimming machines made their appearance in the 1970s, initially in the form of pump-driven jetted streams, which caused a certain amount of turbulence and an un-natural swimming environment. They were followed up in the 1980s by propeller- and paddle-wheel driven machines. These provided a smoother stream of water.
Two types of exercise machines make up the second group. Hybrid systems – self-contained micropools similar to the counter-current type but using a flexible tether to keep the swimmer in place are one type. These systems, being human powered, need neither machinery or electricity but have to be carefully designed to suppress wave formation. The second type allows a person to remain on dry land while simulating certain swimming strokes. Machines of the latter type however can not compensate for the weight of the body and the limbs and thus deprive the user of the benefits of exercise in an aquatic environment. However, the higher effort required by such machines, in the absence of the metabolic effects of immersing the body in water, makes these devices more effective than true swimming if one’s purpose is to achieve weight reduction. Similar in purpose, but not qualifying as swimming machines since they require access to a swimming pool, are various tether systems.
These systems 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 maximise 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 are a number of swim spas, usually fiberglass 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 behavior of the sea, improving stamina and general fitness.
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.
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).
Resource By : https://en.wikipedia.org/wiki/Swimming_machine