Manouvering Of Ocean Star

Welcome, Guest. Please login or register.

[condor ferries]

Im having real issues with the manouvering of the Ocean Star, When I want to turn the vessel the ROT only goes upto a maximum of 20 degrees (at full speed) with 90 degree rudder. Even at low speeds the ROT isn't fantastic. The bow thruster when I do use it gives a few degrees more turn but not much.  Is there any tips on how to manouver this vessel correctly?

[Bottman]

Hi!

First of all it makes no sense to turn the pods to 90 degree whilst running ahead with significant speed! There is no rudder (and 90 degree wouldn't make sense too!), there are thrusters! The props of the pods will mix up the water, maybe they are cavitating, but there will be no thrust at all. Turn them both to 30 degree at full speed conditions or slightly more and you will see a sufficient ROT. Above appr. 5 knots the bow thruster has no effect, neither at the OS nor at any other vessel. For course changes you should always use both thrusters and beware of the opposite direction of the thrust! Turning the controls to port will see the vessel is turning to starboard...

Cheers

[budbud]

hello!
Well I agree with you Bottman when you say turning pods at 90° is not efficient to turn but I somewhere disagree with the fact there are not rudders. Actually, azipods in big ships have a shape that allows to use them like rudders at significant speed. But turning them at 90° would have the same effect as turning normal rudders at such speeds at 90° that means no turning effect and a huge drag.
And I find also according to Condor ferries that the Ocean star doesn't turn enough at low speed. Azipods are really much efficient at low speed than rudders.

[Bottman]

Hey,

of course you have to use the pods as rudder! Because there is no other rudder  . Depending on their type, you can use some of them rudder like, but at slower speed there is not enough water stream along their shaft, so the effect is less, compared with a conventional rudder. And, thats for sure, the OCEAN STAR modell has no azimuth thruster of that kind (Mermaid, ABB-Azipod...). It's more like the original Schottel-Rudder-Propeller with just a turnable propeller and without any rudder effecting shapes or blades.

Cheers

[mvsmith]

hello!
Well I agree with you Bottman when you say turning pods at 90° is not efficient to turn

That depends on whether you are driving a real ASD ship or the Ocean Star in the simulator. In the latter case, maximum ROT is obtained at 90 degree azimuth regardless of ship speed. The effect of transverse water flow on propeller efficiency is not modeled.
Regards,
Marty

[condor ferries]

Thanks for the help, I've tried moving the azipods to 35 degrees and still I only get a ROT of 20 degrees, is there a tutorial anywhere on how to use the azipods in SS08?

[mvsmith]

Some quick tips on Ocean Star:
She is ASD, with two azimuthing propellers in the stern.
The joystick controls are interesting, and allow more flexibility, but you don’t need to use them for this mission. Just note that the white arrows point in the direction of the shaft toward the screw. (That would be toward the stern of a fixed-screw ship.)
The unit pushes the stern in the opposite direction from the way the arrow points.

First, disable Precision Steering so that you can hold the Right-Hand Shift key to make incremental changes with the arrow keys, and also have rapid return-to-center by pressing one key.

Speed control:
Hold the Shift key while pressing the Up/Down keys to the desired RPM, then release the arrow key before releasing the Shift key. This controls both screws at the same time.
To stop both screws, just tap either U/D key. Ocean Star has a very long stopping distance, so it is usually necessary to go full astern.

Steering:
Hold the Shift key while tapping the L/R keys to rotate both units together.  Each tap is 18 degrees.
A quick tap on either L/R key will return both units to center.
Remember that the stern will be pushed in the opposite direction to the arrows, so the ship will turn in the same direction as the arrows.
To turn to port, use the right arrow key.
To turn to Starboard, use the Left arrow key.
Maximum ROT is at 90 degrees. You can go beyond that if you want to lose way at the same time.
This ship has a very high moment of inertia, so meeting her takes practice.
The ROT indication is your best friend; learn the maximum ROT to achieve in order to turn the ship a certain number of degrees. Set rudder amidships when you reach that ROT.
Use small rudder angles for small course corrections.

You can do fancy stuff by operating the drives separately with the joysticks, but that’s for another day.

On most ships, the two joysticks can be locked together so one joystick controls both screws. This is the normal mode while underway. Independent control is used for critical maneuvering.

[TUPilot]

Thanks for those tips...

I find that when I try and move at low speeds, either using the pods to turn or pods/thrusters for lateral motion, I get a lot of forward speed--with 90 degrees on the pods, the ship will accelerate quickly to 3-4 knots forward speed in addition to the lateral drift.  I'm not sure if this is unusual, or what the best way to correct for it is.  I'm finding ways to cope, but would be interested in a expert explination/procedure.

[budbud]

I think that the pods have been programmed like rudders with great angles. I mean that if you put the thrust ahead even at 90°, the game considers a engine ahead with a rudder at 90°. If you put the thrust astern you will get aft speed.

[mvsmith]

Bottman says they are more like Schottels or Z-drives. In any case the Ocean Star in the simulator does not have truly accurate dynamics, so applying theory to learn how to operate the OS is less fruitful than simply messing around. In order to operate any ship in the sim, you need to learn how she behaves under various circumstances.
My notes were intended specifically to get you through Michael’s excellent mission “Ocean Star serves French Toast”, which gives you a good opportunity to practice.
If you find you pick up forward way at 90 degrees, try 108.
As I said, there may be no “real World” explanation for what a model does. The only conjecture I have is that it would be similar to stern walk in a single screw ship, but that is unlikely as the screws would probably be counter-rotating. Since at 90 deg, the screws are in tandem—one seeing the other’s wash—who knows?
Marty

[mvsmith]

If you also look at the Course over Ground with screws at 90 degrees, I think you will find that some of that velocity is at right angles to the heading; she is being pushed sideways. You can remove the forward component by making a fine adjustment to the angle with the mouse on the blue circles. Something around 100 degrees, perhaps. You will not get the way to zero because of the athwart ship component. You need to watch the COG to see when it changes direction.
The sideways movement is common with most bow thrusters because the thruster torques about a different CENTRO than the water force on the hull.
Marty

[LucAtC]

Having remapped the keys to have a standard configuration, I tried to "reverse engineer" the behavior of OS. I mean, looking at the movement of the ship with bowthruster only, and trying to imagine what kind of lateral thrust could induce that.
The movement of Ocean Star is very similar to that of PoR and of P6 in the same circumstances. It is as if the thrust of the bowthruster was applied at the center of the ship, or very near to it, ie not far from her would be pivot point, thus greatly reducing the turning moment. The transverse movement is likely correct, so that the thrust of the bowthruster has been correctly applied at the center of mass, the torque having been simply forgotten.
The phenomenom being investigated, it could probably be corrected in a very near future? I think so, but it is just a guess.
Regards,
Luc

[mvsmith]

Luc,
Every ship, when being turned by a single thruster, must move sideways. The moment of inertia of the ship resists the force of the thruster. This effectively transfers part of the thruster force to the center of mass. The center of rotation is shifted aft (for a bow thruster):
For Vermaas, it moves to a point about 3 containers from the stern.
For POR, it moves nearly one half length aft of the stern.
Think of the ship resting on a frictionless surface, or levitated in space. How would it move if you were to push on the spot where the thruster is? Newtonian mechanics dictates that it would translate as well as rotate.
Regards,
Marty

[LucAtC]

Hello Marty,
Yes, no doubt about that, of course, the Newtonian dynamics in the game is realistic in its essence, but the question is not so much the realism of the movement resulting from the external forces, but well the realism of these appplied hydrodynamic forces.
Like you write, the thrust of the bowthruster is equivalent to this thrust and a torque, both applied to the CM. This transverse thrust less the side resistance will accelerate the ship laterally.
Let us imagine a ratio of 15% of the bowthruster to the main propulsion, the acceleration sideways will move the ship's mass at 15% of the possible longitudinal acceleration, if only the transverse hydrodynamic resistance was the same, and that although the frontal area is 10% of the lateral one.
As a consequence, this transverse acceleration of the CM must be around two orders of magnitude smaller than the longitudinal one.
I made a quick estimation, Ocean Star being in the middle of a channel 300m wide, and the displacement of the center of the ship was around 75m in about 70 sec with bowthruster only. Neglecting any resistance, this lateral accelerated movement of 3cm/s² is 15 to 20 times too much for such a ship, meaning that it can eventually represent a scaled model, but not a ship with a 45000 ton mass and bowthrusters of 5000 kW.

A second objection to the movement of Ocean Star, is that for displacement hulls, the pivot point stayed within the length of the ship, as long as the only acting force was that of the propellers, whichever their direction. It is indeed difficult to prove, or only indirectly. If one desires to move the (any) ship with an orientable propeller, without turning her (ie crabbing), one directs the thrust to the pivot point, the center of rotation, where the counterclocwise resulting torque balances the clockwise one. If forces acting on your ship have their point of application outside the hull, they can only originate from added masses or the ship's inertia force, both being only reactions to movements.

It is not surprising, as the simulation of hydrodynamic forces in the game environment is such an uneasy task, but I this time I wonder what happened to the forces.

Regards,
Luc

[mvsmith]

Hi Luc,
I don’t defend the ship dynamics as being correct. I only point out that the observed effects are present and explainable even though the relative magnitudes of the effects might not be correct (and seldom are, especially those due to viscous forces).
Regards,
Marty

[Bottman]

Wow, how many "expert-latin" here! For me the OCEAN STAR works fine. I can do everything I want with her and it looks and feels very good! Thanks to VSTEP! I wonder, how long the discussion will be, if we got an azimuth driven tug some times!?! Not as large and heavy as the cruise liner it could be much more difficult to meet your dynamic expectations..

Cheers

[mvsmith]

Michael,
I’m not complaining about Ocean Star. I think she is easy to handle. And the French toast is delicious.
Regards,
Marty

[LucAtC]

Hello Michael,
The initial post was that the RoT of Ocean Star was 20°/min at its maximum. In fact, it comes steadily to a maximum of 40°/min at a thrust direction of around 60°, and remains at that value up to 90°, the diameter of the circle diminishing consequently with the speed.
Although my latin  is long forgotten (last time 1959), it looks like Ocean Star works indeed fine with her propulsion system.
Did you mean that such a discussion was meaningless? My knowledge of cruise liners and tugs is superficial, but not so that I know there are facts they cannot escape. And if I feel something is not as it ought to be, I try to be clear enough in my questions or answers, reason for the expert-latin. Although I don't try to be at all exhaustive, I try to write in such a way people understand what I mean, sometimes a difficult task. The trade-off is probably not always optimal, that is right. But how else discussing the bowthruster of Ocean Star?
Regards, 
Luc

[mvsmith]

Wow, how many "expert-latin" here!

Michael,
Luc & I can always go back to discussing Marmite.
Regards,
Marty

[Bottman]

Hey Guys,

don't worry about my post! I just wanna kidding you a little bit and I'm still looking forward to your opinion about the above mentioned tug, if it will be finished some times for the simulator...

Best regards