# Propeller rotation direction question

1. May 22, 2012

### voyager221

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?

2. May 22, 2012

### sophiecentaur

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.

3. May 22, 2012

### Bobbywhy

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.

4. May 22, 2012

### sophiecentaur

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?

5. May 22, 2012

### Bobbywhy

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.

6. May 23, 2012

### sophiecentaur

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: May 23, 2012
7. May 23, 2012

### Pkruse

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.

8. May 23, 2012

### sophiecentaur

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.

9. May 23, 2012

### Pkruse

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. May 23, 2012

### sophiecentaur

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.