In order to propel themselves, boats use rotating propellers similar to what many aeroplanes use. Driven by a motor on or in the boat, these propellers lie just below the surface of the water, spinning at a high speed. While the more traditional set-up employs a single propeller, some boats have dual counter-rotating propellers. Both of these designs have their advantages and disadvantages.
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The concept of the aquatic propeller originates from Archimedean screw, which, though not pushing a boat, moved water from a lower to a higher elevation for irrigation purposes. Though no one knows who first used a propeller to drive a boat, the practice became common with the invention of the steam engine. In 1982, the firm Volvo Penta patented the Duoprop marine propeller, which places two counter-rotating propellers on the same drive.
The function of both single prop and dual prop drives is simply to transfer the circular motion of the boat's drive to the water by means of angled tines, drawing water from one side and pushing it quickly away on the other side. What the "counter-rotating" of dual prop drives means is that one prop turns clockwise while the other turns counter-clockwise. The reason the props rotate in different directions is because they would cause the boat to become unstable if they turned in the same direction.
As both types of marine drives are compatible with most boats, you cannot tell which a boat employs unless you actually get a look at the prop, which may mean pulling the prop out of the water. The props on a dual prop drive look much like what you would find on a single prop drive, except there are two of them.
Some boats have two or more outboard motors mounted side-by-side, each with its own prop in the water. One might think that having two motors in this fashion would constitute a "dual prop" drive, but this is not the case. While this situation employs multiple outboard motors, a dual prop system uses one motor to turn two props.
The main benefit of having a single prop drive instead of a dual prop drive is that it simply costs less. The initial price of the prop is much lower for a single prop drive and should your boat hit something and become damaged by it, you only need to replace one prop instead of two. Another advantage of a single prop drive is that it is much less prone to getting weeds and other fibrous objects tangled around it.
The main benefit of having a dual prop drive on your boat is that it performs much more stably at high speeds. Having just one prop turning in one direction, a single prop drive always will cause your boat to veer slightly to one side. By having two props turning in separate directions, though, a dual prop drive uses each prop's direction of rotation to negate the other, resulting only in linear motion rather than undesirable veering. Another benefit of a dual prop drive is that, having more surface area, it can move more water, resulting in quicker acceleration.
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