[CliffordK (OP)]

Thinking about ship dynamics, why don't they use front, or bow rudders?

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With the typical stern rudder.  In order to avoid an obstruction, one turns the rudder away from the obstruction.  For example, above, to avoid an obstruction to the port side, one must turn the rudder to the starboard.

This then directs the stern of the ship further port, towards the obstruction. 

Then, the movement of the water into the port side of the ship, plus the forward movement of the thrust of the ship turns it to the starboard. 

The problem is that in the course of the maneuver, one must turn the rear of the ship closer to what they are trying to avoid.

If one had bow rudders, the turn could be initiated at the bow, and potentially save vital distance in the maneuver, as well as clearance at the stern of the ship.

I found some notes on Google Books:

Bow rudders not exceeding the draft of the hull are ineffective in ahead motion because the oblique water flow generated by the turned rudder is redirected longitudinally by the hull.  Thus, transverse forces on a bow rudder and on the foward moving hull cancel each other.  The same generally applies to stern rudders in backward ship motion.  The yaw instability of the backward-moving ship is one example could not be compensated by rudder actions if the draft angle exceeded β=1.5°.  To improve the maneuverability of ships which frequently have to move astern, eg. car ferries, bow rudders may be advantageous.

Actually, I think that last part may be it.  One can initiate a turn with a bow rudder, but one may not be able to control the turn with the bow rudder. 

There are also notes in the same chapter on bow thrusters, Section 6.2.7, p, 257.

Or, if the bow rudders need to extend below the hull, would it be worth it to sacrifice a meter of draft? 

Or...  to avoid the loss of draft, have a hydraulic rudder in a pocket that could be shoved straight down, somewhat like a movable centerboard on a sailboat.  One could even have two of them mounted at fixed angles, one for port turns, one for starboard turns.  Add an emergency disconnect system in case the system gets jammed.

There are obvious advantages of system redundancy.

Anyway, could a bow rudder initiate an emergency turn?  Perhaps not allowing precise maneuvers, but enough to have potentially saved both the Titanic and the Costa Concordia?

[Geezer]

Some smaller boats have "bow thrusters". They are propellers that are mounted in a transverse tunnel near the bow. This allows both ends of the vessel to be vectored, but they are usually only used while parking.

[CliffordK (OP)]

Some smaller boats have "bow thrusters". They are propellers that are mounted in a transverse tunnel near the bow. This allows both ends of the vessel to be vectored, but they are usually only used while parking.

Yeah...  And passive movement methods such as a rudder are the least useful at low speeds, thus the benefit of thrusters.

Do cruise ships use Tug Boats?  I suppose a tug is a type of bow thruster.

Anyway, I could foresee a panic button on each side of the steering mechanism to cause maximum lateral movement of the bow.  Or better yet a lever so there would be no confusion on the direction one is desiring to turn the ship.  Could computers be programmed to initiate a response to obstacles?

Even  with my idea of a pocket rudder at a fixed angle, like a centerboard, there would be some adjustment of lateral turning force based on depth it is inserted.

[syhprum]

There is some suggestion in the case of the Titanic that steering orders were misinterpriated and the vessel was actualy steered into the iceberg.

[CliffordK (OP)]

There is some suggestion in the case of the Titanic that steering orders were misinterpriated and the vessel was actualy steered into the iceberg.

Possibly,
But I'm seeing a few analyses that indicate just about what I had surmised with the diagram above.  The question I'm not seeing fully answered is whether the primary Titanic damage was to the side, or to the bottom.  Anyway, it is likely that ex-captain Schettino should have studied the official reports on the Titanic more, what really happened may never be fully known.

However, in my diagram, to avoid an obstruction to the Port side, one throws the rudder to the Starboard side. 
This causes the stern of the ship to shift to the Port, or closer to the obstruction, early in the turn.
Forward momentum will bring the stern of the ship into the obstruction (what happened to the Costa Concordia), and drag the rock along the side of the stern of the ship.

So, what was apparently done with the Titanic was to initially steer normally away from the obstruction.  Once the bow of the ship approached the obstruction, the rudder was then thrown to the other direction, effectively steering the ship toward the obstruction.  That pulls the stern away from the obstruction, rather than letting it slip into the obstruction, and thus hops around the rock/iceburg.

Thus, the Titanic damaged the bow, but not the stern.

Had the Costa Concordia executed a last minute turn towards the Island of Giglio, then it might have been able to avoid the impact, or significantly lessen the extent of the impact.

There is controversy with the Titanic on whether the engines were reversed, an maneuver that can decrease the effectiveness of a typical stern rudder, but would have little effect on bow, or front bottom rudders.

Simultaneously steering bow and stern rudders in opposite directions, one might be able to force the ship to shift sideways slightly.

[CliffordK (OP)]

Some smaller boats have "bow thrusters". They are propellers that are mounted in a transverse tunnel near the bow. This allows both ends of the vessel to be vectored, but they are usually only used while parking.

It looks like the Costa Concordia likely had both bow and stern thrusters.

I think this is from the Costa Favolosa, which should be similar showing the bow and stern thrusters.
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I'm not sure how effective they are at 15 knots, or if they are routinely used at speed.

Contacting the rock with the stern section of the ship means that it was in a turn at the time of the accident, but still sliding forward with momentum. 

Had the captain realized how close he was to the rock, he should have turned hard towards the rock with the rudder and stern thrusters as the bridge passed the obstacle in the maneuver that the Titanic supposedly did. 

Since it was a promontory sticking out, he might have been fine had he not over-corrected too much.

[Don_1]

Wouldn't a bow rudder come under considerably more stress than a stern rudder?

A bow rudder set in line with the ship's hull would be somewhat counterproductive. Wouldn't it?
While a bow rudder set beneather the hull would add considerably to the ship's draft.

Not that I'm a marine engineer, of course, just guessing really.

[yor_on]

Icebreakers use it, but in the form of propellers placed at both bow and stern, other ships use 'jet streams' of water for the same purpose, and yes the stress on a ordinary rudder placed at the bow should be considerable more than placed at the stern.  "Recent advances in ship propulsion have produced new experimental icebreakers. Azimuth thrusters remove the need of traditional propellers and rudders by having the propellers in steerable pods that can rotate 360 degrees around a vertical axis. These thrusters, often referred to as Azipods after ABB's brand of podded propulsion, improve propulsion efficiency, icebreaking capability and steering. The use of azimuth thrusters also allows a ship to move astern in ice without losing maneuverability, which has led to the development of double acting ships, vessels with the stern shaped like an icebreaker's bow. This allows the bow to be designed for open water performance without compromising the ship's ability to operate in difficult ice conditions."
==

Hmm tried to find verification for your view on that it must become more stress on a ordinary bow rudder than on a stern, but didn't. But I'm sure you're right anyway, the balance should be all wrong if one only had a bow rudder. Still, found this

A fine ship.

[CliffordK (OP)]

Something that could be raised or lowered like a centerboard would not increase the draft when not deployed.  The risk would be in an emergency situation where there is a possibility of grounding, then it might get badly smashed, causing severe control problems if it couldn't be ejected.  Two retractable rudders might be able to be placed at a fixed angle, with the transverse force determined by the depth and area below the hull, and the ship's velocity.

It would also seem that one could put a knife-edge rudder in front of the ship, which should at least help with initiating a turn, perhaps using less energy and delivering more lateral force than the lateral thrusters when the ship is in motion.

As far as forces, except for the protection offered by the hull shape, the transverse force on the rudder should be the same for a front or rear mounted rudder.  The difference is the amount of force applied to the hinge vs steering mechanism.  Actually those forces may be the same, except in the case of a center hinge.     And, perhaps the tendency of the hull to force water movement into a path parallel with the hull.  And, of course engine turbulence in the rear.

[Gordian Knot]

There is some suggestion in the case of the Titanic that steering orders were misinterpriated and the vessel was actualy steered into the iceberg.

That is a modern misinterpretation. Even though the rudder orders seem backwards to us, at that time (when the deck officer said "Hard aport." the man at the helm would rotate the wheel to starboard), this was how it was done, and had always been done up to that time. It would be almost muscle memory to turn the wheel the correct way.

It is like suggesting that in a modern car you were told to turn right, and you turned left by mistake. An almost impossible error, even in an extreme situation.

[yor_on]

It happens GK. Ask any driving instructor, people can 'shortcut' when stressed.

[Geezer]

It happens GK. Ask any driving instructor, people can 'shortcut' when stressed.

Actually, I really did that while I was sitting my driving test. The examiner told me to take the next left, but I took the next right instead. Fortunately, I executed the turn exactly to the book, and after I made the turn he told me he had actually said left.

BTW, I passed the test.

[Soul Surfer]

As a canoeist.   A bow rudder stroke is an important technique for fast manoeuvring on slalom courses.

[Geezer]

Come to think of it, the rafts that miners used in this part of the World to navigate down rivers used big oar like things at both ends to steer.
 
I'll try to find a piccy.

[Geezer]

They were called scows. There is a piccy here, but it's a bit lo-res.
 
http://idahoptv.org/outdoors/shows/riverofnoreturn/scow.cfm 

[CliffordK (OP)]

There is actually a good rm format movie on that page, if your system supports it.

rtsp://webstream.idahoptv.org:10554/specials/salmonriver/scow.rm

The person appears to use more of an oar stroke than a rudder movement with the paddles.  So, perhaps they would be compared to using bow/stern jets for steering.  But, the scows are also travelling at the same speed as the water, so a standard rudder would be relatively ineffective.