Influence of linked rudders on DP capability

Consider the following case:
- Ship around 140m LOA;
- Twin screws and two linked spade rudders;
- Two tunnel thrusters forward;
- One tunnel thruster aft.

I was told that if the rudders were not linked (i.e. each of them could move independently of the other) the DP capability would be improved.

I fail to understande that uncoupling the rudders would produce more than a very slight improvement at the most.

Is there around anyone who can shed me some light on the subject? Is it possible that the improvement might be linked to the way the controlling DP software operates and not to the propulsion/thruster system itself?

Any help would be welcome!

Rodrigo

 

Assuming you mean Dynamic Positioning, I can see a few advantages of being able to decouple twin rudders. Specifically it allows you to vector the thrust of each engine separately.

For instance when backing the starboard engine, and ahead on the port to turn the bow to starboard. It could be nice to deflect the rudders separately to help turn the boat. However unless you have an electrically controlled rudders I don't think the advantage would be worth the complexity of coupling and decoupling rudders. With electrics, and with some of the advanced dynamic positioning software it coul very well do it automatically.

 

Thanks for your input!

Yes, I'm talking about dynamic positioning.

My point is that the propeller going astern would generate very little lift on the rudder due to poor flow. A ship moving astern only starts to have lift on the rudders after having some speed...

 

never seen a dp vessel with rudders linked as there is a massive advantage of having them separate including a dp2 vessel that has a loss of the aft tunnel can still stay on postion as one rudder can go full lock (especaily if you use becker flap rudders)to create a sideways force whilst the other is straight for the engine that is in reverse to conteract the forward force of the other engine.
I would guess you wouldnt get dp2 class with linked rudders

 

It's a DP class vessel and the rudders are linked (there are no requirement on rudders not being linked).

I just don't see the massive advantage as the rudder behind the propeller in reverse wouldn't produce much lift hence its angle being more or less indiferent.

I can see that there might be a slight improvement but not a massive one.

 

yes but which DP class?
The rudder behind the prop in reverse will shut off a huge amount of water flow if on full lock
The operator will have less control by not having separate control of the rudders.
You should watch what every dp system does with the rudders when you are holding station and you will see.