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Deathblow
Joined: 11 Sep 2005
Posts: 392
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 Posted: Sun Apr 09, 2006 10:15 am Post subject: What determines turn radius? |
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If my understanding is correct, the turn radius of a ship is determined by two factors: the mass of the ship, and the "turning power" of the aft control surfaces....
And if that is the case, then why are the turn radii of the Oscar and Akula, both monsters of mass at 20ktons and 10kton respectively, 20% *less* than that of the LA at 7ktons? The akula and oscar rudders are not *that* much more powerful.... are they?
I think this is a inaccuracy in the performance modeling. |
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GunnersMate
Joined: 17 Feb 2006
Posts: 225
Location: Boston, MA
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| Posted: Sun Apr 09, 2006 10:47 am Post subject: |
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Turning radii are more dependant on speed and rudder angle than anything else (possibly rudder size as well)Perhaps 688's have smaller rudder angles as well. Comments anyone?  |
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Deathblow
Joined: 11 Sep 2005
Posts: 392
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| Posted: Sun Apr 09, 2006 12:04 pm Post subject: |
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Just to clarify. I'm not referring to "turning speed" (how fast the sub turns), but to "turn radius" (how tight the turn is made) which are not neccessariliy the same thing.
The issue comes up when one looks at the turning radius stats for the Akula/Oscar vs the LA. Akula/Oscar Turn radius is set at 500m where the LA is set at 600m. Though the Akula is 3000 tons larger than the LA and the Oscar 10,000 tons larger than the Akula. Just doesn't seem right. Though there may be a good reason that I'm not considering... |
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LuftWolf
Joined: 09 May 2005
Posts: 1872
Location: Free New York
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| Posted: Sun Apr 09, 2006 1:19 pm Post subject: |
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The database doesn't necessarily have a reason...
In this case, the 688i has a larger turn radius because it is faster...
You've got a case of Western Superiority today haven't you DB? |
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Kapitan
Joined: 10 Mar 2005
Posts: 5385
Location: essex england also st petersburg russia
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| Posted: Sun Apr 09, 2006 1:27 pm Post subject: |
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| Depends on the ship, My ship dart 8 turned in less than 500 yards knock nevis turns in around 30 miles. |
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Deathblow
Joined: 11 Sep 2005
Posts: 392
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| Posted: Sun Apr 09, 2006 1:38 pm Post subject: |
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| LuftWolf wrote: | The database doesn't necessarily have a reason...
In this case, the 688i has a larger turn radius because it is faster... |
?  The akula and LA have the same speed.... and even if they didn't, turn speed and turn radius aren't necessarily the same thing. The LA is lighter by almost 30%. 
| LuftWolf wrote: | | You've got a case of Western Superiority today haven't you DB? |
Yep! Sure do!
Last edited by Deathblow on Sun Apr 09, 2006 1:46 pm; edited 1 time in total |
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Kapitan
Joined: 10 Mar 2005
Posts: 5385
Location: essex england also st petersburg russia
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| Posted: Sun Apr 09, 2006 1:43 pm Post subject: |
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| Even if it is western superiority, the akula still gives the seawolf a run for its money. |
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LuftWolf
Joined: 09 May 2005
Posts: 1872
Location: Free New York
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| Posted: Sun Apr 09, 2006 1:47 pm Post subject: |
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Well, they're the same speed if you compare Akula II to 688i, which I suppose is good enough to shoot down that argument... so I'm left with: the turn radii are different because that's how it is in the database.  |
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Deathblow
Joined: 11 Sep 2005
Posts: 392
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| Posted: Sun Apr 09, 2006 2:03 pm Post subject: |
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I'm not trying to argue that I'm right.... I already know I'm right ....
Hmm.... lets fix that database then    |
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LuftWolf
Joined: 09 May 2005
Posts: 1872
Location: Free New York
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| Posted: Sun Apr 09, 2006 2:34 pm Post subject: |
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I've never been inclined to go through the database and change things just because it doesn't look right... that's how you take a database and turn it into a mess.
There is going to be a full review of individual platform values... that might make it, it might not. The criterion have yet to be set, but we will consider it. |
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Kazuaki Shimazaki II
Joined: 03 Jan 2006
Posts: 146
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| Posted: Sun Apr 09, 2006 11:39 pm Post subject: Re: What determines turn radius? |
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| Deathblow wrote: | If my understanding is correct, the turn radius of a ship is determined by two factors: the mass of the ship, and the "turning power" of the aft control surfaces....
And if that is the case, then why are the turn radii of the Oscar and Akula, both monsters of mass at 20ktons and 10kton respectively, 20% *less* than that of the LA at 7ktons? The akula and oscar rudders are not *that* much more powerful.... are they? |
How would you know? Have you studied hydrodynamics? I understand such things are based on many subtleties and we are talking 100m.
The Oscar, in any case, also has the advantage that it can attempt to reverse one of the screws. Real noisy, but if you are shooting for minimum turn radius at flank....
Besides, should I, for example, justify that Akula should have a much better climb rate than the 688 because it has 30% reserve buoyancy (which I can use) compared to 10?
To be fair, the more subtle areas of the DB tend to be rather crappy. Look up all the missiles and find some stunning, uh, revelations as to their performance as defined by Sonalysts |
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Deathblow
Joined: 11 Sep 2005
Posts: 392
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| Posted: Mon Apr 10, 2006 5:24 pm Post subject: Re: What determines turn radius? |
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| Kazuaki Shimazaki II wrote: | | How would you know? Have you studied hydrodynamics? |
Yes I have actually, but that was a while back though and only in an introductory fashion. How about his, lets try some rough estimates over what type of factors effect the turning radius...
... we can *roughly* estimate the bulk of a sub as a cylinder, and view the turning force of the aft sail as a force acting at the cylinder end to enact a turning moment. That being said, the rotation of a solid object is governed by the angular motion equation...
| Quote: | Alpha = I * T
where Alpha is the angular acceleration
I is the moment of inertia about the turning axis
And T is the torque being applied to the object. |
In the case of a cylinder the moment of inertia about the turning axis, iirc can be estimated as a function of the mass, radius and length in the equation
| Quote: | | I = Mass*(1/12*length^2 + 1/4*radius^2) |
combining with e angular acceleration equation to become
| Quote: | | Alpha = T*Mass*(1/12*length^2 + 1/4*radius^2) |
So a cursory look at most basic factors effecting turning ability displays a dependence on the mass, length, height, and turning force of the aft sail. Now granted, the additional complexity of the hydrodynamics of fluid flow over our cylindrical body introduces infinite complexities to any model, however, given that the bulk shape and profile of the various submarines is roughly equilavent (all cylindrical shaped, with blunted noses, and taped ends) one could presume the various contributions of dynamic fluid flow over the sub surfaces, while adding numerous complexities, will ultimately roughly equilvalent in each sub... leading to the difference in maneuvering more or less a result of mass and dimension differences. Someone please correct any mistakes noticed.
So the question remains.... how can a sub that is 25% more mass turn in 2/3rds the radius. And one that is 200% larger still turn in 2/3rd the radius. Nothing really supports this as true.... just seems contradictory to me. :hmm: :nope:
| Kazuaki Shimazaki II wrote: | | Besides, should I, for example, justify that Akula should have a much better climb rate than the 688 because it has 30% reserve buoyancy (which I can use) compared to 10? |
Well, IIRC, reserve bouyancy doesn't effect the dive or climb rates of a sub while submerged, because while submerged, even if changing depths, the sub will maintain more of less neutral boyancy with the water around them, and it is only the dive plan angles that provide depth changes... i.e. the subs are not "floating" to a new depth, they are "flying" to a new depth and maintain more or less neutrality with the water at all times (with the exception of shallow depth maneuvers where subs might make themselves slightly heavy on purpose, only using the dive planes to maintaing depth, in order to prevent an accidental surface breach... meaning the reserve boyancy isn't really a factor when submerged.
When reserve boyancy is a factor is when the subs try to emergency surface and blow all ballast tanks. In that instance the larger reserve will provide a superior upward bubble and skyrocket the sub to the surface faster. Large RB gives a sub a better shot at making it to the service after sustaining a torp hit. IIRC.
But anyway..... can anyonep justify the turn radii of the current database? |
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LuftWolf
Joined: 09 May 2005
Posts: 1872
Location: Free New York
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| Posted: Mon Apr 10, 2006 7:44 pm Post subject: |
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US submarines have a much lower length to width ratio than the Russians? Rounder submarines tend to turn faster than longer submarines?
It also doesn't matter that much... PM me if you want the REALLY dirty secrets about the database. Actually don't... |
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Deathblow
Joined: 11 Sep 2005
Posts: 392
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| Posted: Tue Apr 11, 2006 12:20 am Post subject: |
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| LuftWolf wrote: | | It also doesn't matter that much... PM me if you want the REALLY dirty secrets about the database. Actually don't... |
:hmm: ooooooo.... do tell... I promise I won't make a fuss... well, sorta promise 
ps) my apologies for that incrediable, longwinded and unneccessarily physi-o-tastic soapbox lecture. Sometimes equation crunching helps me to make more sense of things when in doubt. |
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Kazuaki Shimazaki II
Joined: 03 Jan 2006
Posts: 146
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| Posted: Tue Apr 11, 2006 12:24 am Post subject: |
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| Ditto. I know about the missiles, but not a lot about the rest. |
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