Speed & rate of turn of P&O ferry

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[liontiger]

Hello,

First of all, I would like to say that Ship Simulator has improved quite a bit since the 2006 version and the prospect ahead looks good, especially if new add-ons will be published. The option of expanding a simulator always makes it more interesting and exciting.

And now to my observations, which mostly goes for the P&O ferry that I have been sailing. The speed of the ferry related to it's surroundings doesn't seem to be correct. When either sailing along a channel, passing other ships that are stationary or moving both in the same way or in opposite direction the relative motion appears to be completely wrong to me? When docking with the ferry it takes forever to get alongside when using proper speeds that you would have used in real life. I found myself approaching the pier with 10 knots getting all kind of warnings to slow down speed, but to get the ship going it felt right? I have been sailing as 1st Officer on large cruise ships for more than 5 years now and I don't find the relative motion of the environment in the simulator match up with my real world experience? The simulator seems to be way too slow... Any comments to this?

Also, the rate of turn is not very accuratly depicted. Rate of turn does build up fairly quickly (though quite extremely in the simulator), but comes down again at a much slower pace when putting the rudder midships. In the simulator it drops almost instantly when putting the rudder midships which is not the case in real life with a passenger ship. This does of course vary a lot from ship to ship. On my present ship, if having a rate of turn of about 10° and putting the rudders midship it will continue the turn for somewhere around 3-5 minutes(!!) before being steady again if not applying counter rudder.

Hope someone has inputs to this, either in form of an explanation or agree upon my observations. If this turns out to be wrong parameters in the software it can hopefully be fixed in a later patch.

Happy sailings and smooth seas...

Sverre

[groennegaard]

Hi liontiger

Welcome to the forums! You are right in your obsevations. It's obvious that the SS dynamics model are far from accurate but hopefully VSTEP will improve this in the near future - that would certainly make us seafarers happier . As it is now I always play SS with a certain amount of mistrust... 

The main problem with P&O ferry is that VSTEP didn't get the dynamics data from the company. I don't know why but it seems ridiculous that P&O sponsors a ship without providing the nessesary data.

You are right about the ROT. There is also a problem with the rudder movement... Pretty fast don't you think? The delay is missing.

The speed/environment issue may have something to do with the framerate.

I hope that VSTEP will get the dynamics right soon - it will certainly make SS a lot more fun and useful. 

Regards
groennegaard

[LucAtC]

Hello liontiger,
Having made a quick trial along the 3000m splitsingdam of the Nieuwe Waterweg at a stabilised speed of 19 kts, I found a speed SoG of 16.2 kts. That could mean a stream of 2.8 kts ahead, or the need to calibrate an instrument or an error of 15% of the speed reading.
Another question is the "zooming factor" or field of view that distords the feelings, as well as the lack of 3D sight (no stereoscopic view) to explain the rest of the appreciation of the environment, the lack of some textures to help "read" the distances.
One year ago, the course stability of all ships was infinite, and since then many ships show a somewhat more realistic rotational inertia.
The rudder movements, like groennegaard rightly wrote, are far too quick indeed, I remember a time when the limit was 28 sec from "stop to stop", and that pilots ask now that it would be no more than 20 sec. (Of course not for dynamic positioning, tugs, small yachts or RIBs). The same is also true for the engines.

Improving all that is perhaps not as easy as it seems, probably due to the physics engine of the simulation, that apparently lacks hydrodynamic laws, so that the developers have to build them from scratch, and integrate them with (?) hybrid parameters, that not necessarily represent physical data. Or something like that?
They also intended to give the players the ability to tweak the dynamic parameters of the ships, it is perhaps not forgotten and would help solve these questions. The problem will then be in keeping the parameters coherent, to avoid flying ships.

Regards,
Luc

[Kenneth E. Stacey]

When the P&O ferry is leaving the berth and using the thruster 100% to starboard no headway is generated.
In my experience the use of a thruster, in either direction, for any length of time will generate headway to a maximum of about two and a half knots. Any increase in speed above this will cause the performance of the bow thruster to deteriorate rapidly.

[groennegaard]

Hi Kenneth

Is that your expirience with the for using thrusters on vessels similar to PoR?

In v1.0 the PoR gained forward speed quite fast when using thrusters alone - up to 2½-3 knots if I remember correctly. The speed was gained in a matter of seconds and it is my experience that this gain of speed was way too much for a ferry of that size. You should still gain forward speed in v1.1 but it isn't as much as it was in v1.0 and it will take longer time to gain.

Regards
groennegaard

[Kenneth E. Stacey]

Hi Groennegaard,
When using a thruster the forward speed does increase quite rapidly up to about two and a half knots. If removing a vessel berthed between two other vessels it is done in one of two ways. Either with the helm hard over towards the berth and the engines dead slow/slow ahead and the thruster full power away from the berth moves the vessel sideways, with some headway, or thruster full power away from the berth and dead slow astern, as required to stop headway if space between vessels is tight.
Best regards,
Ken Stacey

[groennegaard]

Hi Ken

Thanks for your input. The problem here is data/facts. P&O hasn't supplied VSTEP with the dynamics data of PoR and it is difficult to reconstruct the data upon experience and feeling.

Do you work on board vessels similar to PoR?

Regards
groennegaard

[Kenneth E. Stacey]

Hi Groennegaard,
Retired pilot and harbour master.
Best regards,
Ken Stacey

[Kenneth E. Stacey]

Hi Groennegaard,
I have just spoken to one of the Master's of P&O who says that the PoR has twin thrusters forward turning in opposite directions. He thinks there is also a sluice to equalise the pressure on both sides of the hull when using the thrusters. The result of this is that there is little, if any, forward movement when the thrusters are in use, technology moves on.
Best regards,
Ken Stacey

[groennegaard]

Hi Ken

Thanks for the update. I haven't spent that many years at sea (yet  ) but I have never experienced that phenomenon being that distinct. I've only been on board RORO and RO/PAX vessels.

The sluice (Anti Suction Tunnel) is a brilliant invention as the equalization of the pressure on both sides of the hull makes the thrusters more efficient. I have attached a drawing showing such a sluice located between the thrusters. The drawing is from the DFDS passenger vessel Crown of Scandinavia.

Kind regards
groennegaard

[LucAtC]

Hello Ken and groennegaard,
The AST seems indeed to make the bowthruster usable to 7 knots, it is indeed brilliant, like you say.  But how could a bowthruster give headway to the ship? It is an artefact of instruments/sensors/lochs, and recently from the position of the GPS antenna being at a distance from the gyration (instantaneous rotation?) center.
In the simulation, one reads the bow and aft speeds. The direction of rotation gives the sign of the speed (clockwise positive). You can check that there is no headway at all using only the bowthruster to get the bow away from a quay, keeping the stern "fixed" at the quay with a line (stern spring?). You will read the speed of the ship being the mean of the bow and aft speeds. These speeds being apparently linked to the ship and not to the world, because thet stay near to zero most of the time when sailing, it is in the simulation not only an artefact but a bizarre coordinate system.
I didnt dig deeper, I am perhaps wrong, but havent yet found a good explanation.
Regards,
Luc
ps: I found very interesting papers also, worth reading too
http://www.dynamic-positioning.com/dp1997/prop_deter.pdf
http://www.slc.ca.gov/division_pages/MFD/Prevention_First/Documents/2004/OSPR%20Global%20Shipping%20Perspectives/levine%20paper.pdf
  and in NL   http://www.vlaamseleerlingloodsen.net/Bestanden/simulator/maneuvreren_janssens.pdf

[Kenneth E. Stacey]

Dear LucAtC,
If a tug is towing a vessel at 90 degrees to the bow then as the vessel turns it generates headway due to centrifugal force. I assume that the same explanation is applicable to a single bowthruster.
Best regards,
Ken

[Kenneth E. Stacey]

Dear LucAtC,
In reply to your email.
As the bow tug tows the ship pivots around a point about 1/8 of the vessels length from the stern. At the same time the ships centre of gravity follows a curved path which creates an outward force due to centrifugal action. Initally this force is in line with the vessel which causes headway and this in turn is assisted with the fluid forces surrounding the ship.
Best regards,
Ken

[LucAtC]

Dear Ken,
Thanks for the explanation, however centrifugal forces being no real forces, but the perceived effect of inertia, the ship needs headway in order to have (show) inertia along the longitudinal axis.
Centrifugal force can only be the reaction to a centripetal force (in this case, transverse to the ship if you consider the tow) or a virtual (non-existent) force. So, it is the other way round.
I can try to explain my objection otherwise.
If you pull the ship at an angle of more than 90°, say 150°, the ship will go backwards although she rotates in the same direction as when being pulled at right angle (ie showing the same centrifugal forces).
I dont intend to say the phenomenon doesnt exist, but that centrifugal forces cannot be at all an explanation, they are a result, never the cause of any movement.
Also, the notion can be used as a kind of mnemonic?
As well, asymmetry of hull resistances could explain that the tow angle giving nor headway nor way astern is different from 90°, as backwards resistances are normally greater than in forward,  (entry angle, ...). Stated otherwise, the headway cannot be the result of Newton laws, well of hydrodynamic forces like you suggested.
Which ones is just what I was looking for.
Best regards,
Luc