Ship Simulator
English forum => Development corner => Ship Dynamics => Topic started by: LucAtC on June 16, 2007, 17:30:36
-
Titanic - Ocean Star - Agile Solution - Nordkap
4 Tables summarizing the stopping distances that can help avoid too many engine orders, and could help in keeping the Chief happy.
The distances are expressed in nautical miles, consistent with the DTW of the GPS tab.
Also in lengths of the ship as a visual aid and in meter for those who prefer it.
The column "Throttle" gives an idea of the somewhat stabilized corresponding speed.
No wind has been taken into account, and in full reverse all the ships keep a straight course and heading.
No streams, of course.
Such speed tables will probably lose any meaning in ShipSim2008, as soon as there will be an editor for the performances of the ships.
Open to any improvement!
Regards,
Luc
Edit: Obsolete data removed
-
I just read an article that the Spirit class ferries of Dover (as in Herald of Free Enterprise class) on sea trials went from 23kts to stationary in 400yds.
Surely not. Even with 23,000 SHP that's not possible....Luc?? It had twin screws at the rear and a single screw in the bow with a rudder forward of it.
Stu
-
Good question, thank you!
There were not many details about this ferry I could find.
I suppose a mass of 11000 t (of 1000kg). The difficulty is that ferries are also designed while taking reserve power and accelerations into account, so that one cannot infer water resistance from engine/screw data.
That is why such ships seem to be "overpowered", and the limits to torque, power or thrust available are not so clear as with other designs? Anyway, a very wrong guess, but not unreasonable, gives indeed your number!
I can add, in 5 minutes. But the Chief Engineer must have had sour thoughts about this trial.
Moreover, it is consistent with other data of ferry trials I found (with some difficulty...)
I almost forgot to answer: 400 yards is possible.
Regards,
Luc
-
Thats incredible. I was on the Pride of Bruges/POSL Picardy/ former Pride of Free Enterprise which held the fastest crossing Dover to Calais after the Herald (which was marginally faster).
I went on the last official passenger crossing and the capt went full bore. The speed was incredible and we overtook the SeaChance no probs.
Amazed at the 400yds though! But yes, SHP was 23000. Sure about that.
Stu
-
Stopping distances of Pride of Rotterdam and container ship Vermaas.
Very good news, the accelerations of both ships are realistic, and so are also the decelerations of PoR.
The stopping distances of Vermaas are alas 1.7 ship lengths while reversing, and 2.5 lengths "free going".
Luc
Added Latitude stopping distances, much too short for a VLCC of this type? The IMO rules of 15 lengths are (were?) difficult to obtain, also due to the time needed to stop the screw.
By comparing the dimensions of VLCCs, Latitude must be a 200,000 ton tanker, and rated at 16 knots, "only" 30,500 hp.
To get the characteristics of the simulation, it would need almost 3 times this power. Not impossible, of course.
So, good news too: Acceleration and deceleration are consistent, but no transverse thrust of the screw when slowing full astern.
Luc
Edit: Obsolete data removed
-
Luc,
I haven't got 08 yet (shop has it bu won't sell until tomorrow :( ) But I saw in Bottman's video of PoR leaving Hamburg that it seemed to turn very quickly when using bow thrusters.
Is that realistic or is it a bit that needs refinement?
I've never seen a ship turn so quickly on thrusters, but I guess it is fitted with multiple bow/stern thrusers so may be right- I don't know (tell you tomorrow ;) )
Stu
-
I noticed this too and wonder the same. I also noticed that it turns pretty fast in general for such a large ship.
-
Actually it is realistic. The PoR is designed to be very manoeuvrable as it has to manage it's daily berthing without any help from tugs (that will too expensive and difficult).
It is equiped with pitch propellers and this specific type of propeller is capable of going from full ahead to full astern in a matter of seconds. This is why it stops quite fast.
However, the rudder effect is not realistic. Normally a vessel of PoR's type would be much more agile.
This is my professionel opinion as a ship's officer on board RO/RO ferries.
Regards
groennegaard
-
Hello,
Indeed, the rudder effect is greatly underestimated for all the ships. The effect of the rudder(s) is at its maximum at an angle varying from 7° (flat plate, low side thrust) to 25° (for more elaborate profiles, high effect) between the rudder and the flow of water. 8)
In the game, the rudders have almost no effect at 15°, gaining ;D only a few degrees/minute RoT.
Will there be an editor for the ships parameters? ???
Regards,
Luc
-
Will there be an editor for the ships parameters? ???
I really hope so! :) But I think VSTEP should take the dynamics more seriously. When they make these deals with P&O Ferries, Red Funnel and Fairmount, it should not be that difficult to get the dynamics data of the ships, and I am sure that the masters of these ship are more than willing to test the models... :D
BTW: Nice tables you have made... :o They are very usefull. :D
Regards
groennegaard
-
This surprises me actually. I thought the PoR rudder would have been to high turn rate. But I don't know as I am not qualified- I will take your expert knowledge.
Groennegaard- What ship(s) do you serve on? (Understand if you can't say for security reasons etc). I've always had a fascination with ferries.
Stu
-
Just checked the RoT of the PoR ( :D ).
At 15 degrees rudder, RoT 12°/min, ie 15 min for half a turn? A not so impressive rudder.
Strangely, the RoT seems not to depend on the speed? :o
With thrusters, I have no idea. Only people on board could tell how they affect the rate of turn at various speeds. One can think that could be linearly added to the effect of the rudders?
Regards,
Luc
PS: Hm, edited my previous post and added the table for the Latitude.
-
Having recorded the accelerations and the decelerations of Titanic, I can confirm two news, one good and another one:
When 100% (ie full ahead) is given, the acceleration is very consistent with the data of Titanic, of course such values are probably near the maximum possible values. The development of this characteristic is very good, congratulations to the developers!
It makes no difference whether, being at top speed, one puts full astern or simply put to stop. That is of course more than bizarre, the stopping distance being 4.2 or 4.4 ships' lengths from 22 knots to stop...
Regards,
Luc
Edit: Obsolete data removed
-
I found a Google link http://www.geocities.com/hollywood/theater/7937/smith.html
It tells that the stopping distance from 20 knots to stop, full reverse, was 850 yds. The number I found was 841 meter, so the stopping under power is excellent.
Regards,
Luc
-
I found a Google link http://www.geocities.com/hollywood/theater/7937/smith.html
It tells that the stopping distance from 20 knots to stop, full reverse, was 850 yds. The number I found was 841 meter, so the stopping under power is excellent.
Regards,
Luc
That is quite phenomenal, and suggests that the hull resistance was enormous. This is consistent with your earlier result -- that the vessel stops in about the same distance with power simply shut off. And with positive acceleration being very poor, she is either under-powered or the relationship between engine power and propellers was not well worked out.
Cheers,
Michael
-
Having recorded the accelerations and the decelerations of Titanic, I can confirm two news, one good and another one:
When 100% (ie full ahead) is given, the acceleration is very consistent with the data of Titanic, of course such values are probably near the maximum possible values. The development of this characteristic is very good, congratulations to the developers!
It makes no difference whether, being at top speed, one puts full astern or simply put to stop. That is of course more than bizarre, the stopping distance being 4.2 or 4.4 ships' lengths from 22 knots to stop...
Regards,
Luc
Greetings Luc,
Yes,i find it also very interesting,and strange to.I was realy thinking that it would helped but it seems not,realy!
Maybe,it has something to do with the props,because a propeller is build to go forward,in reverse the prop looses a lot of his quality.
However,i'm not so educated in this matter,it's just a idea of mine.
Regards
Marc
-
Hello Marc,
Moreover, having reversed, the turbine is disconnected and the center propeller gives no longer any thrust.
There remains the power of the reciprocating engines, "However, this was not an instantaneous process. In general, and with the engineers ready at the controls, it would typically take from 10 to 20 seconds from the time the order was received in the engine room for the engines to come to a stop and first start to reverse their direction of rotation. It then would take another 30 to 50 seconds before the engines would be backing hard.12 The lifting of the changeover valves to redirect exhaust steam from the reciprocating engines to the condensers directly, thus taking the turbine engine off line, would take only about 10 seconds once the changeover-valve reversing lever was swung over." (cit. Encyclopedia Titanica,titanic_prime_mover.html )
At 83 RPM, the engines would deliver 41000 hp, but the efficiency in reverse being estimated (in these days?) to be half of that in forward, the thrust in reverse is greatly reduced. The fraction it represents to slow the ship becomes greater the more the speed is decreasing. Of course this backing power is increased by the water resistance.
Anyhow, the stopping distance ought to be much longer when you simply cut the power than when being full astern. In fact, the proximity of both curves lets me think of a simple mistake. Not mine, I had to recheck everything, I couldnt believe it.
I shall try to find out what is wrong?
Regards,
Luc
-
Having searched for a good guess, I came to the (evident) conclusion that one needs correct data about Titanic hull water resistance vs speed to begin with. The only values to be found were the estimation of 165 ton (long tons?) thrust at 21 kts. Whether it was correct or not, difficult to tell, as are the values of acceleration to be found here and there.
What I could find is the shaft horsepower (shp) vs speed of SS America during her speed trials in June 1940 on the measured mile at Rockland, Maine, from a McGraw-Hill book "Introduction to steel shipbuilding" (by Elijah Baker III, 1943, rev. 1953).
It just remains to scale the data to Titanic, both ships sharing some characteristics.
Regards,
Luc
Edit: Curves extended to zero power - speed. Why not? and speed 21 kts, not 21.5
Edit: Obsolete data removed
-
Hey Luc, your second name isn't Einstein is it?
Seriously, I find your mathematical reseach really useful, as I'm sure many people do. Even if there is no dynamics editor, I bet some of your research will be included in future ships here!
Stu
-
Seriously, nothing I intend to do is more difficult to do than designing hulls with sketchup or blender! Also, everything is well documented. But being no member of the distinguished SNAME (waw!), I have to find shortcuts (simplistic formulas) and best guesses of missing coefficients to evaluate the resistance.
For instance, frictional resistance of ships hulls (the term is self defining!) has been subject of many experiments, leading to many methods of determination through the ages, improving together with computing power. What is well known is that it increases as the (approx.) square of the ships speed, and consequently the power needed to overcome it increases as the 3rd power of the speed.
Residual resistance (the rest, self defining too...) is more kind of an "ink bottle", but for geometrically similar hulls at corresponding speeds (square root of linear ratio), this resistance is proportional to the displacement ratio.
There is nothing particularly complex to that, thanks to Excel, but the devil is in the details: Validity of the data and of the formulas, not confusing units, pertinent applying of methods, ... hats off, naval architects coming!
Regards,
Luc
-
And you are explaining all that in a second language... hats off indeed :)
Stu
-
I need to go sleep.
I think he just said that he makes ships hulls different by there real life statistics so the drag under water has the same force as in real life and the ship stops in the same distance.
Am I right?
:|
-
I need to go sleep.
What do you mean? Any link with the topic? :D Falling asleep? ::)
Well, if I understand 8) what you meant, you are indeed right. Of course, the real data of Titanic are lost, but I think the acceleration data from the simulation can be made fully consistent with what we know of Titanic power, speed and hull, and moreover these data could be confirmed through the scaling of SS America data.
But decelerations in the simulation are doubtful, and there are also doubts about the claim of the 850 yards from 20 knots to stop measured during the trials. In fact, even if Titanic was light, the numbers dont simply fit with what the engines were capable of achieving.
To sum up, Titanic 2008 uses floating anchors or drag anchors to slow down... :o
Regards,
Luc
-
Scaling having been done, I didnt succeed without setting the propulsive coefficient at 0.9 in view of obtaining 165 long tons at 21 kts. This being the ratio between towrope and shaft horsepower, would mean more than adequate hull and propeller efficiencies. It could also (perhaps) mean or that SS America was remarkably better designed than Titanic (15%?), or (more probably) an unallowed scaling.
Nevertheless, the power at 21 kts, 24280 ehp, means a correct ratio of some 0,53 vs indicated (brake?) hp, and that this value, as well as the corresponding curve, are a good estimate of the total resistance of Titanic's hull.
Also, comparing the data collected during the "trials" with the the curve, while acceleration is due to the thrust available vs (ie minus) resistance divided by mass, I can confirm that the acceleration curve is very good up to 20 kts, becoming somewhat optimistic above this speed.
As a conclusion, I think that the comparison with SS America showed that the resistance curve is enough realistic, and that decelerations, whether free or with propellers (stbd and port, not center!), are simply not possible.
Regards,
Luc
Edit: Obsolete data removed
-
I think reading that again when I'm fully awake would be a better idea. :)
Stu
-
I have to admit Luc, you are VERY knowledgable with regards to all-things-shipping ;D
Forget BASIC details, with yourself, we get everything.....VERY IMPRESSIVE :D
Regards.
DJM.
-
Hello,
Now that Titanic is renewed (1.1), I made a check of the accelerations/decelerations. It looks like the stopping distances have been somewhat increased (4.6 and 6.4 lengths from 22kts to stopped).
If the deceleration full astern still follows a nice curve, without power it is linear, ie 0.4 cables stopping distance for each knot. It has been modified, yes.
Regards,
Luc
-
Agile Solution dynamics have changed since the 2006 add-on. The table attached gives the new approximate values of stopping distances in the sim. (Version 1.2, Build 1015)
The acceleration and deceleration values seem to indicate less power than in 2006, as I couldnt reach more than 14 kts, compared to 16.5 in 2006, and a speed of the real ship 17.5 (Deo Volente). Also, the curves are nice.
Transverse thrust effect when going astern, as well as an improved rudder (efficiency, kick effect) are still needed to improve the realism of the manoeuvrability.
Regards
Luc
Edit: Obsolete data removed
-
Hi Luc :)
You are a keen observer! :D
I can tell you that all that you mention will be improved. The max speed has been raised to 17 knots, the kick ahead effect has been improved as well as the transverse thrust.
Regards
groennegaard
-
Hello groennegaard ;)
Going a little bit further, I tested a turn (only to port...) full ahead, 45° port rudder.
The result was also very different from 2006, indeed the speed loss was only 1.7kts, and the resulting tactical diameter was 8 lengths. The angle of drift (Diff between heading and CoG) was 1.4 (mean), very low, but of course there can be lag in the readings of the display, so that this result is questionable.
But Agile Solution doesnt satisfy IMO manoeuvrability rules, far from that.
The rudder is perhaps defective, but I saw or heard no alarm?
Regards,
Luc
-
Hi Luc :)
With the latest beta version I have measured the tactical diameter to 0.2 nm ~ 3,5 ship lengths.
IMO Criteria
.1 Turning ability
The advance should not exceed 4.5 ship lengths (L) and the tactical diameter should not
exceed 5 ship lengths in the turning circle manoeuvre
Regards
groennegaard
-
Hello groennegaard,
Well, then the beta is back to the 2006 characteristics? It is good to know they went to the shipyard to fix this defective rudder. Is there a date fixed to get her out of the repair dock? ;)
Regards,
Luc
-
Well, then the beta is back to the 2006 characteristics? It is good to know they went to the shipyard to fix this defective rudder. Is there a date fixed to get her out of the repair dock? ;)
I don't think it is back to the 2006 characteristics. I believe SS08 uses a different dynamics engine and I guess they started from scratch when setting the dynamics for the SS08 ships. I think it is far better now than it was in SS06.
I don't when the 'stand alone add-on'/update will be ready for release. I think VSTEP has learned the lesson from the v1.1: Not to mention even an estimated release date... ;)
Regards
groennegaard
-
The tactical diameter of Agile Solution SS06 is 3.38 ship lengths, the speed falling from 16.5kts to 8kts, so that it was rather good, independently of any other characteristic. Hopefully, there will indeed be a hydrodynamics engine incorporated in Quest3D, so that the efforts will concentrate on the true ships parameters, instead of hunting look alikes of forces.
Very good news, then, and I understand very well they are reluctant about any release date publication! I had to try...
Regards,
Luc