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Author Topic: Seriously, how dare you...  (Read 17931 times)


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    • Marine Design
Re: Seriously, how dare you...
« Reply #100 on: December 06, 2012, 01:36:51 »


Your English is fine -- no need to worry -- and your explanation of the cunfusion in terms is exemplary.

People need to understand that there is a lag between engine orders (also helm orders) and execution, and then a further lag before the ship reacts, even in relatively small ships. Momwentum/Inertia, two sides of the same coin.


It is only semantics, of course, and I am not English speaking, but I'll try to explain what I think to know about the terminology used for propellers, with the caveat of the language.

Counter-rotating propellers are just that, propellers turning in opposite directions on parallel shafts, vertical like CH47 Chinook or KA-31, horizontal like twin engine aircraft, or for ships with pairs of propellers turning in opposite directions, like most but not all twin screw ships. Indeed, some modern ASD tugs, or towboats, have the same propulsion units at both sides, an interesting feature to steer on large bends on rivers.

Such counter-rotating propellers can be outward turning, like Sherpa, or inward turning like RPA12 or many other but not all CPP propulsions.
Outward turning propellers mean that the starboard propeller is right-handed, the port prop is left-handed.
Inward turning sees a left-handed one at starboard and a right-handed one at port side.
A right handed propeller turns ahead clockwise when seen from astern, the screw is right-hand. Left-handed etc.

Screw propellers like Volvo Penta Aquamatic Duoprop, or Wartsila Lips for large marine propellers have pairs of coaxial propellers, in view of increasing the efficiency of the propulsion. The turboprops of Tu-114 or the rotors of some helicopters are flying examples of the same. They have since a long time been named contra-rotating propellers.
The main goal of such an arrangement is more the increase in efficiency of the propellers, not so much canceling opposing torques. The propeller shafts are coaxial.

There are also propellers in tandem on the same shaft, with the same pitch direction, for instance at both ends of the main shaft in a pod. These propellers are of course not contra-rotating nor counter-rotating propellers.

Perhaps could an English speaking check and correct this.

Michael Porter Marine Design
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