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Counter-rotating propellers are installed on some twin and multi-engine, fixed wing aircraft. They are called counter-rotating propellers because the propeller(s) on one wing turn(s) in the opposite direction to the one(s) on the other wing. more
On most twin or multi-engine propeller driven aircraft, the propellers all turn in the same direction, usually clockwise when viewed from the rear of the aircraft. In a counter-rotating installation, the propellers on the right wing turn counter-clockwise while those on the left wing turn clockwise. The principle advantage of counter-rotation is to balance propeller torque effects thus eliminating any problems associated with a Critical Engine. [links]
Counter-rotating should not be confused with contra-rotating which refers to two propellers on the same engine which spin in opposite directions. here
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Jan Adkins for WoodenBoat Magazine
A propeller only “walks” when its shaft is angled downward. The upstrokes become longer than the downstrokes, creating a sideways force.
The initials RH (for right-handed) or LH (for left-handed) are usually stamped on the prop, along with its diameter and pitch. That’s how you can tell which type you’re dealing with. If the initials aren’t there, it’s easy to determine a prop’s direction in the water. Simply watch which way the prop turns when the transmission is put in forward gear. If it’s clockwise when viewed from astern, then the prop is right-handed. Counterclockwise means left-handed. [links]
Slow speeds and high rpm amplify prop walk, as do greater propeller diameter or pitch. Shallow water, on the other hand, reduces upward water flow from beneath the vessel, and can reduce or even cancel out prop walk.
The backward motion illusion occurs when the speed of rotation is such that--in the interval between each frame--a new blade moves nearly into the position occupied by a blade in the previous frame. If the timing is precise, it looks like the propeller did not turn at all.
This phenomenon is not common in direct vision, but often seen in movies and television. Film and TV actually consist of series of still photographs shown in rapid succession to fool the eye into seeing motion. The individual pictures do not actually move. Instead, the position of a moving object shifts with respect to the background in each successive still frame; when viewed rapidly, one gets an impression of smooth motion.
The same phenomenon can be observed by adjusting the number of light flashes from a stroboscope. Under certain circumstances, objects illuminated by some kinds of fluorescent lights will appear to rotate backwards to the naked eye. The effect occurs due to the 60 cycle per second flickering of the light, which is normally too rapid for the eye to register. [links]
But if the blade interval doesn't exactly match the camera interval (or a multiple of it), then the propeller will seem to turn slowly forward or backward. When the next blade is a little slow in reaching the previous blade's position, the rotation appears to be backward; when the next blade arrives a fraction early, the visual impression is that the blades are turning forward.
Result: Better ship performance in the range of 8% to 10%. More flexibility in the engine arrangement and more competitive ship performance.
For controllable pitch propellers, operation at a constant number of revolutions over a wide ship speed reduces efficiency. Reduction of the number of revolutions at reduced ship speed will give fuel savings. here
CRP propellers can either be mounted on twin coaxial counter rotating shafts or the aft propeller can be located on a steerable propulsor aft of a conventional shaft line.