Propeller rotation direction question

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Discussion Overview

The discussion revolves around the direction of rotation of ship propellers, specifically how to determine whether they rotate clockwise or anti-clockwise from diagrams. Participants explore factors influencing rotation direction, including engine configurations and the implications for propeller design.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question how to determine the rotation direction of a propeller from a diagram, noting that propellers do not all rotate in the same direction.
  • One participant mentions that marine engines can rotate either clockwise or anti-clockwise, with specific examples from their own experience with a Volvo Penta engine.
  • Another participant highlights that the sense of rotation is related to the propeller's design, which is akin to a screw pushing water backwards.
  • There is a discussion about the ambiguity of clockwise and anti-clockwise rotations depending on the observer's perspective, emphasizing the need to specify the reference point.
  • Some participants note that twin screws on large ships typically rotate in opposite directions to mitigate gyroscopic forces.
  • Concerns are raised about the efficiency of gearboxes if opposite rotations are achieved through gearing.
  • Participants share experiences regarding the configurations of marine engines, including the possibility of converting engines from one rotation direction to another.
  • There are mentions of larger engines and the complexities involved in their configurations, including torque compensation in setups where engines rotate in the same direction.

Areas of Agreement / Disagreement

Participants express differing views on the standardization of propeller rotation direction and the implications of engine configurations. There is no consensus on a definitive method for determining rotation direction from diagrams, and multiple competing perspectives remain on the topic.

Contextual Notes

Participants highlight the importance of specifying the reference point when discussing rotation direction, as well as the potential inefficiencies in gearboxes related to engine configurations. There are also references to specific engine types and their characteristics, which may not be universally applicable.

Who May Find This Useful

This discussion may be of interest to individuals involved in marine engineering, boat maintenance, or those curious about the mechanics of ship propulsion systems.

voyager221
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How can you tell from a diagram of a ships propeller the direction it is moving ie clockwise or anti-clockwise.

Am I correct they don't all move clockwise?

What factors determine it?
 
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voyager221 said:
How can you tell from a diagram of a ships propeller the direction it is moving ie clockwise or anti-clockwise.

Am I correct they don't all move clockwise?

What factors determine it?

A don't know about larger 'ships' but there are marine engines that work clockwise and engines that work anticlockwise. My boat has a Volvo Penta engine which goes the opposite way to most possible replacement engines (meaning that I will need a new propellor as well - for a couple of hundred quid extra!)
A propellor is a 'screw' and the sense of rotation for forward motion will be such that the diagonal face will be pushing water backwards. Diagrams are usually clear enough to show that.
 
clockwise and anti-clockwise rotations depend on where you are observing from, so it seems useless to ask unless you specify "observed from..."

If looking at a diagram of a propeller I can see the "twist" or the orientation of the leading edges and trailing edges of the blades, and if I know the direction of rotation, I can answer the question, "this propeller is pushing the ship forward, or, this propeller is pulling the ship in reverse."

No, there is no standard or preferred direction of rotation. Ship's propellers may move in either direction of rotation. Twin "screws" are usually opposite so as to avoid gyroscopic forces affecting the ship.
 
I think it's specified looking towards the stern.

Are you saying that the two engines rotate in opposite senses? If the opposite rotations are achieved with gearing then I imagine one gearbox would be less efficient than the other?
 
sophiecentaur said:
I think it's specified looking towards the stern.

Are you saying that the two engines rotate in opposite senses? If the opposite rotations are achieved with gearing then I imagine one gearbox would be less efficient than the other?

Looking towards the stern from on board or looking towards the stern from behind the ship? Without specifying the reference point clockwise and anti-clockwise are ambiguous.

I don't know about gearboxes or how the engines are connected to them, but I can say without any doubt that many large ships have two screws, one on each side, and that they counter-rotate. I have measured their acoustic signatures and other characteristics many times.
 
When you are "looking at the stern", I assume you are abord the ship.
If you are sure about the counter-rotation (and it would make a lot of sense for manouverability) then I reckon that the two engines would have to be identical and gearboxes different.
 
Last edited:
Many marine engines are sold in right and left configurations, and can be easily converted from one to the other. A few are kluged up to make the change in the gear box, but each engine has its own box. I've also seen two engines turning in the same direction, but with the shafts mounted at strange angles to compensate for the effect of the torque.
 
These would be BIGG engines, I suppose. No one ever suggested that I could get a Betamarine / Bukh / Watermota of around 20kW to go the same way as my Volvo Penta. I suppose that a different starter motor and cam shaft would be all that's strictly necessary.
 
I've seen this on engines as small as 120 HP. But all my experience is with diesils. Sometimes I forget that they put gas engines in boats.
 
  • #10
Mine's a diesel - but yachts only have tiny engines in them. Engines are far too noisy and smelly. They do come in very handy at times, though.
 

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