Hello Guest March 28, 2024, 15:04:28 *
Welcome, Guest. Please login or register.
Did you miss your activation email?

Login with username, password and session length
 
Pages: [1]   Go Down

Author Topic: Steering Astern  (Read 7980 times)

Pilot Moore

  • Forum member
  • Posts: 8
Steering Astern
« on: January 08, 2008, 15:13:26 »

My one big bug bare with SS06 & 08 is the fact that the ships steer astern.  This is totally unrealistic and does not happen in real life and big ships.

For a rudder to be effective it needs a flow of water over it.  The prop is positioned infront of the rudder for this reason.  When going astern there is minimal direct flow over the rudder which means a vessel would not readily steer. 

Small boats such as yachts steer astern as they have comparitively very large rudders for the size of vessel.

I apologise if this has been mentioned before.  If it has guide me to the posts.

Thanks

Simon
Logged

Mad_Fred

  • Administrator
  • Posts: 8689
  • ✝ In Memoriam
Re: Steering Astern
« Reply #1 on: January 08, 2008, 15:23:21 »

Hello Simon,

Going backwards is being discussed in this thread too;
http://www.shipsim.com/ShipSimForum/index.php/topic,2713.0.html

I don't know if it's precisely about what you are saying here, but you could have a look at it.

Regards,
Fred
Logged

LucAtC

  • Ship Simulator Developer
  • Global Moderator
  • Posts: 2218
Re: Steering Astern
« Reply #2 on: January 08, 2008, 15:55:14 »

Hello Pilot Moore,
You are quite right, a search (from the forum root, for instance) with as keywords "rudder astern" will show that the problem is very well known, from the beginning of ShipSim2006, as is the link given by Fred.
It has been insisted upon by the beta-tester many times, as far as I know, and repeating it can do na harm at all, to the contrary.
Nevertheless, many progresses have been done, compared with 2006. The question is that of the realism of the hydrodynamic forces to be incorporated in newtonian mechanics, certainly not an easy one.
Regards,
Luc
Logged

TerryRussell

  • Guest
Re: Steering Astern
« Reply #3 on: January 08, 2008, 20:40:23 »

Hi Luc.

I think the physics are actually quite simple to model. For small ships with outboards or steerable drive legs, they will turn in reverse.

For any ship with fixed propellors, there will be virtually no steerage in reverse. Bow and stern thrusters will of course work.

In real life, any good captain/helmsman will give the bow some moments of inertia (i.e. "swing") in the required direction while going for'ard, then go astern while the bow is still swinging round. Depending on size and weight etc of the ship, you can get quite a bit of turn accomplished. I've done it when taking a 60 foot cruiser through Loch Ness and the Caledonian Canal a few yers ago (I usually work with smaller vessels with steerable propellors or legs, etc).

The simple algorithm above isn't perfect, but would be a lot closer to reality. SS08 ships already retain the moments of inertia. Simply disabling the effect of the rudder in reverse or drastically minimising it would imitate reality much closer.

Somehow, I don't think the coding for that would be earth-shatteringly difficult, compared to the multi-player games and such. This is an "over a cup of coffee" modification for a reasonably competent programmer, I suspect (OK, several cups for each ship, plus testing).
« Last Edit: January 10, 2008, 16:47:28 by TerryRussell »
Logged

Tavares Junior

  • Forum member
  • Posts: 329
Re: Steering Astern
« Reply #4 on: January 10, 2008, 15:41:53 »

Terry,
You are quite right.

Minimizing the rudders effectiveness would bring the steering of the vessel's closer to the reality.
As also looking for the ship's interactions and hydro performance too.

http://www.shipsim.com/ShipSimForum/index.php/topic,3367.0.html

Have a nice day,
Tavares
Logged
Tavares Jr
** Open Sea - Radar observer - SMCP - GPS / Chart Plotters / Certifyed yatch Master

LucAtC

  • Ship Simulator Developer
  • Global Moderator
  • Posts: 2218
Re: Steering Astern
« Reply #5 on: January 10, 2008, 18:56:18 »

Hello Tavares,
Saying that
Quote
Minimizing the rudders effectiveness would bring the steering of the vessel's closer to the reality
(engine reversed or ship backing) is indeed very interesting per se, but this remark  is probably not valid for all ships, whether in ShipSim2006 or in ShipSim2008.
One could think that indicating which ships need a change of their rudder performance would help the developers. Also, giving some goal values would be nice, and would greatly enhance the remarks.
Regards,
Luc
Logged

TerryRussell

  • Guest
Re: Steering Astern
« Reply #6 on: January 10, 2008, 21:03:41 »

Hi Luc.

I did give guidance:

Quote
For small ships with outboards or steerable drive legs they will turn in reverse.

For any ship with fixed propellors, there will be virtually no steerage in reverse. Bow and stern thrusters will of course work.

If you think it's useful, I'll try to identify the vessels which will steer in reverse, and then the rest won't, of course. Before anyone objects to the list, please remember that any twin-screw vessel can be turned by putting one engine for'ard and the other astern. Here I am talking about steerage by rudder alone.

WILL STEER IN REVERSE:
sailing yachts
historical yachts

AND:
The Red Jet doesn't actually have rudders but SS08 uses the rudder controls to pivot the drives, so will also steer in reverse, via the rudder controls. (In real-life, the Hovercraft won't turn by much if you reverse the engines and use the rudder control)

That's it, so far as I can see. I'm working from the list of ships that you kindly sent me, Luc. I've found that there are a number of other vessels in the game that aren't on the list, so I may have missed something.

Hope this helps.

If anyone is convinced this is wrong, please say so, but please give yourr source (makers web site, scan of specifications, etc) - TIA!
« Last Edit: January 10, 2008, 21:06:35 by TerryRussell »
Logged

LucAtC

  • Ship Simulator Developer
  • Global Moderator
  • Posts: 2218
Re: Steering Astern
« Reply #7 on: January 10, 2008, 23:35:16 »

Hello Terry,

I agree with you quoting that there is virtually no steerage in reverse, although surely not for all ships with standard rudder/propeller arrangement.
There are fishing vessels, seagoing tugs, pilot ships that can be manoeuvered in reverse, although only when there is not too much wind, and by skillfull captains. Like you mention, sailing yachts and historical yachts too.
Foreseeing movement and track of a such ships when reversing and then backing is certainly difficult. The pivot point (center of giration) of the ship moves to the stern, reducing an already weak turning moment, while setting too great rudder angles, stalls the rudder now taken aback.

Nevertheless, as once there is a distinct flow of water on the rudder, the side thrust exerted by the rudder is the same as when going ahead engine stopped, for any practical use. The resulting turning moment is of course different, and will probably not counterbalance the transverse thrust effect of the propeller if needed. To gain speed backwards, you cannot evidently do without some propeller...

So, there is a definite effect, that can and must be simulated or computed, but its direction and the resulting turning moment are so uncertain that most people think it is unpredictable. Yet, all ships turn readily when backing, very often even too much, and not in the desired direction. And is that not in fact the problem?

The simulation of such characteristics (rudder thrust, its lift and drag as a function of speed of ship and of propeller rotation, propeller thrust components) can make a realistic simulation, much more than neglecting existing forces, I think.

About the list, could you please tell me which ships aren't in it, so that they could be added?

Regards,
Luc
Logged

TerryRussell

  • Guest
Re: Steering Astern
« Reply #8 on: January 11, 2008, 01:39:52 »

Hi Luc.

I live in a fishing village on the South Coast, and have a several boats of my own (7 metres to 15 metres). In reverse there is just about nothing! Two of the boats have planing hulls and one has a semi-planing hull. As soon as the engine goes off, they tend to slip and spin a bit. In reverse they go more or less straight but no steerage. (wind affects them greatly of course)

But that's why most of us have twin propellors. That provides steerage. Some of the fishermen here have large rudders, but they have quite small boats (less than 5 metres) and are mainly catching crab and lobster so going over the many sand banks and shingle banks.

In my experience, most large vessels (certainly those above about 15 metres) using a standard small-rudder-aft-of-propellor arrangement will have virtually no steerage in reverse whatsoever. They will have just as little (i.e. almost none) when going forwards with propellors off.

There may be a slight "paddle wheel effect" in some cases when reversing, but it is very little. As many boats have duo-props, these of course have no paddle wheel effect.

Vessels with steerable drive legs or outboards are OK, since the propellor gets turned. In those cases, the boat doesn't actually have a rudder, usually.

Fo rthe missing ships in the list, I'm compiling the list for you, but it includes the 9 types of yacht, and so on. As I come across them when writing missions, I am adding them to the list, so hold on a week or so, please!  ;)
Logged

LucAtC

  • Ship Simulator Developer
  • Global Moderator
  • Posts: 2218
Re: Steering Astern
« Reply #9 on: January 11, 2008, 03:46:13 »

Hello Terry,
I understand better now. Sorry, I didn't consider such small boats and light planing hulls, although I had some years experience with bridging launches (Selve, Fairey Marine, Schottel and M&S), certainly not to be compared with those boats, and a naval experience gained as a deck officer on ships of various sizes but relatively large, and lasted only 5 years.
Thanks to a longer experience of North Sea fishing vessels, particularly mine, not as small as 15m I must confess, I am sure that fishing ships and tugs answering to the rules of classification societies (including steering gear arrangement) can be sufficiently steered backwards as needed. Otherwise, they would need to be forbidden to sail.
You are certainly right that a small-rudder-aft-of-propeller can be unsufficient if less than 4% of the lateral area for such small ships.
Although duo-props show no paddlewheel effect, they (ignore but) use it to steer astern, and the lack of rudder on sterndrives is sometimes compensated for by the relatively big flat gearcases.
So, in fact we do agree about these things, and the same goes for the missing ships list. As these are no player ships, I'll rather change the title to reflect it.
Regards,
Luc
Logged

TerryRussell

  • Guest
Re: Steering Astern
« Reply #10 on: January 11, 2008, 10:33:44 »

Hi Luc,

There are also the Inland dry cargo, Inland tanker missing from the list. In shallow waters, I assume that they could run aground if used as AI (when my player vessel is in close proximity, the "sleep" dynamics go into "real life" mode).

Sorry if that is unclear. I don't know what terms the programmers are using for the fact that AI ships will run aground when they are near to the player ship but will pass through eah other and through shallow aters when further away ("Never-sleep" when it is disabled, I know).

This has caused many of the Custom Missions to malfunction when I have tried them. So so guidance on draught, turning circle, sped etc would be very useful when designing missions.
Logged

budbud

  • Forum member
  • Posts: 292
Re: Steering Astern
« Reply #11 on: January 11, 2008, 11:06:46 »

Hi Terry,
I'm not sure to have well understood what you mean by:
Quote
AND:
The Red Jet doesn't actually have rudders but SS08 uses the rudder controls to pivot the drives, so will also steer in reverse, via the rudder controls. (In real-life, the Hovercraft won't turn by much if you reverse the engines and use the rudder control)

The Red Jet is a hydrojets driven boat and not a hovercraft. And the hydrojets allow you to steer going aftward and even with no speed.
And regarding the hovercrafts they have orientable air propellers that allow them as well to steer in every direction whatever the speed can be.

Sorry if I didn't understand properly what you would like to say.
Bud
Logged
C'est curieux ce besoin que les marins ont de faire des phrases!

TerryRussell

  • Guest
Re: Steering Astern
« Reply #12 on: January 11, 2008, 21:13:42 »

You've almost just said what I said!  ;D
Quote from: TerryRussell
uses the rudder controls to pivot the drives, so will also steer in reverse
The Red Jet ferries don't have rudders. They steer by moving the drive mechanism.

But in the case of the hovercraft, you have very little steerage in reverse. Ther equivalent of a rudder is the vanes at the propellor exhaust. If you reverse the engines, sucking air over the "rudder" vanes won't work, becuase a lot of the air doesn't pass over the vanes. It gets sucked in from the sides and so on.

So a hovercraft will steer by using the vents on the top (the "thrusters"). Trust me, I have driven hovercraft and used the Isle of Wight one many many times. They actually steer them by parking at a slight angle when they go up the slope. When they raise the skirt,the hovercraft falls backwards and fspins round, assisted by the vents.
Logged
Pages: [1]   Go Up
 
 


SMF 2.0.14 | SMF © 2017, Simple Machines