Titanic steering issue in Camerons very dull 1997 'film'

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[Stuart2007]

Hi

Now. Before you shoot me down, READ the post.

It is well documented that at the beginning of the 20th century that you would turn the wheel to the right to go Port and left to go Starboard (just like moving the tiller on a sailing boat).

In the film the wheel is clearl shown being turned clockwise (right) to go to Port/left? With me so far???

OK. So when you are running the engines astern you should turn the rudder opposite to the direction you wish to go- to bring the stern round. So in actual fact, he would have steered rudder hard starboard to bring the bow to Port and therefore the wheel should have been turned to left (anti clockwise).

I am only guessing and I haven't sailed a boat that big... Now where's Luc when one needs mathematical data?

Stu

[mporter]

That would only be true when the ship has actual sternway, not when the engine(s) are running in reverse but the ship is still going ahead.

The rudder has an effect the moment you start the engines ahead because the propwash increases the speed of the water the rudder "sees" and thus increases its lift, but in reverse there is no corresponding stream over the rudder until the ship is moving, so all the rudder sees is the ship's motion. Until the ship stops and starts actually moving backwards, that motion is ahead, so the rudder continues to act as before, but with less effect.

River towboats in the US (and some in Europe, Eemspoort tells us) have "flanking" rudders ahead of the prop(s) for use when the engines are running in reverse.

Cheers,
Michael

[marcstrat]

Greetings,
A vessel can only react,when there is pressure on the rudder,when it's going forward,or backwards.
Only forward,the vessel reacts more faster,because the props are before the rudder(s).
In reverse,the vessel has to have a bit of speed,to get pressure on the rudder(s),before she can react.
Regards
Marc

[Captain Davies]

That would only be true when the ship has actual sternway, not when the engine(s) are running in reverse but the ship is still going ahead.

The rudder has an effect the moment you start the engines ahead because the propwash increases the speed of the water the rudder "sees" and thus increases its lift, but in reverse there is no corresponding stream over the rudder until the ship is moving, so all the rudder sees is the ship's motion. Until the ship stops and starts actually moving backwards, that motion is ahead, so the rudder continues to act as before, but with less effect.

River towboats in the US (and some in Europe, Eemspoort tells us) have "flanking" rudders ahead of the prop(s) for use when the engines are running in reverse.

Cheers,
Michael

I had always wondered about that, seeing as in the game, when the engines are in reverse, but the ship continues to go forward the steering is reversed.  But in fact this is an inaccurate part of the simulation, a bug even.  Is that right?

[LucAtC]

For what I know of the rudder arrangement of the Titanic, she had one rudder behind the center screw. This screw was driven by a turbine that had no reverse gears.
Having put "full astern" on the three shafts (ie center stop, port & stbd reverse) would have had probably the same result as giving a stopping order, due to the duration necessary for the reversing; the rudder efficiency was decreased due to the cessation of the screw rotation, although the turning radius must have been not so much changed, whatever the engines did, the hull being at high speed.
From a different point of view, the reduction in speed could have given a better chance of surviving the iceberg. Nobody knows, because the trials were never done, with none of the sisterships. Moreover, the relative position and distance of the iceberg is still unknown, as why the rudder had probably been turned before the lookouts gave alarm.

@ Stu, I am not sure if I understood well your

It is well documented that at the beginning of the 20th century that you would turn the wheel to the right to go Port and left to go Starboard (just like moving the tiller on a sailing boat).

To summarize, the order to rotate the rudder to port was given by the order "Hard-a-starboard", and  symmetrically for the other side. It came indeed from the era of the tillers, not so long ago in these days, remember also the first cars, some years before.

And Michael, you are quite right, of course, and you express it all very clearly, but marcstrat, the expression "In reverse..." gives rise to a kind of ambiguous interpretation. Is it the screw rotation, or is it the ship having way backwards. In French, it concerns the "erre", best translated by "way or stream or wake"? Of course, people acquainted with manoeuvring understand at once, but apparently even some developers in the beginning had difficulties with the meaning.
And perhaps to add also that a second source for a reduced turning moment when moving backwards is that the center of rotation of the hull is displaced to the stern the more the speed increases, what gives this sensation of sluggish response when backing, together with an enormous increase of the pressure on unbalanced rudders.

Also Captain Davies, your question is for me somewhat also ambiguous. Not per se, but there are ships with orientable propellers manoeuvring very differently from standard/classical screw rudder arrangement. Do you mean the Titanic? Sure, there are inaccuracies, but most important bugs were corrected?

Regards,
Luc

[marcstrat]

Greetings,
Yes,with my expression "in reverse",i ment the ship has to move backwards(speed),to get pressure on the rudder(s),so if you have this,than the ship can react to the rudder(s).
Maybe my explanation was a bit fuzzy,on that part.I was only thinking to make this simple.
Thanks Luc,for the more extended version of this matter.
REgards
Marc

[Stuart2007]

Thanks for the mathematical analysis Luc.

Yes, forgot about the middle prop not reversing on Titanic.

But he rudder function is based on the water moving past it- whether from the forward movement of the ship or the reverse movement of the propellor.

So IF Titanic had a reversable middle prop, then putting the rudder to Starboard would have pulled the stern to starboard surely.

Stu

[LucAtC]

Hello Stu,
Yes,  I understand what you mean, and no  it wouldnt work.
The wash (?accelerated flow) of a screw is far less on the suction side (screw in reverse, side of the rudder), than on the thrust side (hull side).
But if the "stream", created by the screw, works opposite to the speed of the ship, the net result is that the speed of water around the rudder is somewhat annihilated, depending on the wake (water being dragged by the hull, friction, etc...) and in fact on the hull efficiency. Anyway, around zero rudder moment, and the main effect would have been the transverse thrust of the (lefthanded  then?) screw.
It is only at a very low forward speed (in fact hull hydrodynamically stalled) that there is a (remote) possibility to work like you say.
Hmmm... not clear enough, indeed difficult not to be fuzzy without going far in the details  .
Regards
 
Luc

[Stuart2007]

No, that sums it up well. So he may well have made it worse after all by stopping the engines.

It works in shipsim

Stu

[Captain Davies]

Also Captain Davies, your question is for me somewhat also ambiguous. Not per se, but there are ships with orientable propellers manoeuvring very differently from standard/classical screw rudder arrangement. Do you mean the Titanic? Sure, there are inaccuracies, but most important bugs were corrected?

Regards,
Luc

I meant all the ships in the game, where if the ship was moving forwards, but the engines in reverse, then the angle of the rudder would have the opposite effect to what you would expect.  I was wondering if it was the same in real life, because I cannot see how physics permits this.

[Stuart2007]

Taking away Lucs maths,

if you put the props in reverse they 'suck' water from the back. If you use the rudder to channel the direction they are 'sucking' the water from then it should start to move the ship towards the source of the 'sucked' water.

Obviously the geometry of the ships are designed for forward movement, but newtons law holds true 'to each action there is an equal and exact opposite reaction'.

Stu

[LucAtC]

Hello Captain Davies,
If you use Z-drives or outboard motors, or any orientable screw, setting the "rudder" will direct the thrust/pull of the propeller immediately, unlike the standard screw/rudder case! In fact its function is only partially that of a rudder.
Regards,
Luc

[Stuart2007]

hence the fascination with azipod driven ships...

Then there IS a bug in SS?

Stu

[Captain Davies]

Thanks, guys!

Obviously the geometry of the ships are designed for forward movement, but newtons law holds true 'to each action there is an equal and exact opposite reaction'.

Ooops, thought that was Einstein!

[mporter]

I meant all the ships in the game, where if the ship was moving forwards, but the engines in reverse, then the angle of the rudder would have the opposite effect to what you would expect.  I was wondering if it was the same in real life, because I cannot see how physics permits this.

Yes, this is indeed a bug in the game.

if you put the props in reverse they 'suck' water from the back. If you use the rudder to channel the direction they are 'sucking' the water from then it should start to move the ship towards the source of the 'sucked' water.

Obviously the geometry of the ships are designed for forward movement, but newtons law holds true 'to each action there is an equal and exact opposite reaction'.

Stu

The problem with this, Stu, is that the rudder does not effectively channel the suction direction.  Rudders work by "flying" like an airplane wing, not primarily by deflecting the water, and the faster the stream, the more effective the rudder.  The glide path of a 727 with no power is approximately that of a rock, and the same is true of a rudder with no water movement.

When a boat is moving forward, the prop produces a concentrated stream of water over the rudder which considerably enhances the steering effect, but in reverse the prop pulls water from all directions so there is practically no effect on the rudder.  If you stand behind a prop plane you feel the propwash, but you do not feel a corresponding suction if you stand in front of it.

As Luc and Eemspoort have said, the rudder only starts to have an effect in reverse when the ship starts to actually move backward, and it only has enough effect to counteract the sideways thrust of the prop when the ship has gained consideralbe speed backwards.

Long and boring -- sorry!

Cheers,
Michael

[Stuart2007]

Not at all boring.

I'm learning to fly at the moment and find the reference to aircraft wings to be interesting.

I don't know about water rudder dynamics at all- hence the question. Whilst I suppose the tail rudder of an aircraft works basically the same (I only need to know effects going forward )

But as for reference to wings, that is a simple pressure differential and the cross section of a wing has no resemblence to that of a ships rudder.

I guess that if a ship is fitted with directional pods then the game would be accurate?

Stu

[mporter]

But as for reference to wings, that is a simple pressure differential and the cross section of a wing has no resemblence to that of a ships rudder.

I guess that if a ship is fitted with directional pods then the game would be accurate?

Stu

Well, some rudders have a foil section -- but of course it is a symmetrical foil.
The pressure diffferential is created by the steep angle between the rudder and the stream going by.  this creates a low-pressure area on the "back" side of the rudder as the water has to "hurry"to keep up with the undeflected water.  This in turn swings the stern, which swings the bow in the direction you wanted to go.

Yes, it is true that the ships act a little more as though they had pods at the stern, but this effect is not consistent (eg, they don't steer going ahead like podded vessels).

Cheers,
Michael

[LucAtC]

Also,  single-plate rudders are far less efficient than hydrofoil types, and their maximum angle versus waterflow is also far smaller. Around 10° and more than 25° respectively (google naca profiles ship rudder, or something like that).
Additionally, less known is that a well designed rudder can give a greater turning moment than an orientable propeller when the ship makes way.
And although there was only apparent wind during this  doomed night, let us remember that wind has a great effect on the handling of ships when they go backwards, or when they drift at sea. A very welcomed  effect if you intend to make way backwards to the wind. Indeed, your superstructures play the role of sails, and the rudder often cannot counter it. Otherwise, less welcomed. 

Regards,
Luc

[Stuart2007]

I was at Portsmouth waiting for the Pride of Bilbao (as one does) the day after P O Portsmouth hit HMS St Albans.

That was due to wind catching the starboard rear quarter and bringing the bow round to Starboard. Even with thrusters and praying they still smacked it.

The damage to St Albans was none too small. ( a new warship as well).

Stu