The Ship Model Forum

The Ship Modelers Source
It is currently Sat Nov 16, 2019 11:21 pm

All times are UTC - 6 hours [ DST ]




Post a reply
Username:
Subject:
Message body:
Enter your message here, it may contain no more than 60000 characters. 

Options:
BBCode is OFF
Smilies are OFF
Do not automatically parse URLs
Question
What is the name in the logo in the top left? (hint it's something dot com):
This question is a means of preventing automated form submissions by spambots.
   

Topic review - "Advance" before turning distance for ships
Author Message
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
The Iowa class BB's, the Alaska BB's and the Fletcher DD's all had very similar tactical Radiuses, a bit over 800 yds. For the Alaska's this was somewhat better than the turning performance predicted in the original model basin tests.

As with everything in ship design, a matter of some compromise.
Post Posted: Fri Aug 17, 2018 10:56 am
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
Thoddy wrote:
Quote:
say
ship wit one screw and 50000 PS will do 30 kn
how much PS will be required to achieve the same speed with two screws
exact values are not required but some kind of a approximative value would be fine.


I found a german statement of th K-Amt wich state -according to own experiments-
that-
Propellerefficiency of a 3-screws ships was 0.48
and in the case of a 4-screws ship it was only 0.42

so the 3-screws ships requires less HP (about 15 percent) to achieve a given speed

they found no way to increase the efficiency of a 4 screw ship significantly.
Post Posted: Mon Jul 02, 2018 8:07 am
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
And, in case the above was not simple enough.

The motion of a fluid around an object can have the fluid moving in ways the fixed object cannot.

The fluid can undulate, twist, contort, etc. (turbulent flow).

While the "object," say a "rudder" cannot. It is simply a solid object.

The fluid and rubber affect each other, but in very different ways.

The fluid affects the rudder through what is basically a simple force acting on the rudder in a direction and magnitude.

But the rudder's affect on the fluid is going to be on each particle in the fluid, which then each affect the particles around them. This produces a multitude of varying motions from what looks like a single force.

Obviously this is a subject which, while I am capable of doing the math, is something that is far more involved than I had previously considered (due to having an education that applies the math I know to more stable systems). The physics I have did not go into Fluid Dynamics beyond very simplified models of idealized systems (perfect point-spheres moving through uniform temperature and density fluids of finite volume).

And without current access to the Library Database I usually used, finding useful materials on the subject is difficult.

But I would still like to find something that is capable of giving an estimate of the advance and tactical radius of turn.

If nothing else, it should be possible to just get a statistical measure of the various advances and radii from known ships, and extrapolate a generalized estimate (which I recognize would not be very precise. But it would be more precise than simply pulling a number out of the air).

Mb
Post Posted: Mon Sep 28, 2015 1:13 am
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
It's much more that a frame of reference issue. For example, to calculate the trajectory of a shell you can assume an initial velocity and direction and you calculate the path by taking the following into account: mass and inertia of the projectile, rotation and position wrt to its path, drag coefficient and other effects of wind force and so on: you know the forces on the shell and where it will end up. Meanwhile, you have 'summarized' the effect of wind into a few simple coefficients like so

Image

If you want to determine this drag coefficient you can do various tests, or, you try to simulate the flow as accurately as possible. That means that would you've just done for the shell you now need to repeat for each molecule in the flow. Now, this is a bit too much, so you use the Navier-Stokes equations that apply to a small volume of the flow. Basically, the Navier-Stokes equation are nothing more than Newtons second law: Force equals mass times acceleration, applied to a flow. The tricky part is that, compared to a single shell, there isn't really a single mass to deal with. Now, acceleration is the change of speed, so Newton actually said that force equals mass times the change in velocity. For Navier-Stokes you first need to go to force equals the change in mass times the change in velocity. or

F= m*a --> F=m*change(v) --> F=change (m*v)

Speed times mass is called momentum, so basically newtons second law is a more primitive version of force = change in momentum.

Modern calculation methods rely on subdividing the flow into many small volumes, taking into account that fluid that flows from one volume to the next is equal, that forces equal out and flow through each volume. So, you know how the fluid flows and forces interact. The problem is that solving the flow motion in each small flow volume is a massive undertaking. Ships are hundreds of meters long while the flow near the hull consists of small vortices much smaller than millimeters: tracking them all is currently impossible because you hardly have the memory to subdivide the flow around the hull in all these volumes that all capture the smallest vortices. This is where the turbulence modeling comes in: you assume the flow doesn't really have these vortices but that the flow is more viscous than it really is in real life because of these vortices. This works pretty well but you need to be really, really careful how you go about it. Near the hull our calculation volumes are very very thin, more so than a sheet of paper; further away from the hull the volumes become more cube-like and larger. Using these approaches makes the problem solvable (we use several 10s of millions of volumes for normal calculations).

And while you do this, you STILL need to take the mass and velocity of the ship or shell into account to predict its motions. And if you're really precise, you need to determine how much the ship will deform under the fluid loading and predict the influence of ship deformation back onto the fluid. At the moment this is all way too much for our computers to handle all at the same time but progress is very fast :) When we calculate the self-propulsion of a ship at one speed, we use 256 processors and calculate for 1 week for one answer...
Post Posted: Sun Sep 27, 2015 7:16 am
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
MatthewB wrote:
But the issue here is that the Navier-Stokes equations really deal with the movement of the FLUID around or through a body, rather than the movement of the BODY through the Fluid


Could you expand on this?

On first (and second!) reading there's no obvious difference - it's just a frame-of-reference issue.

A.
Post Posted: Sun Sep 27, 2015 6:24 am
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
The computer that can solve the full NS for a ship at speed hasn't been built yet. The closest thing so far is applying models with advanced turbulence models running on 200,000 cores in parallel, i.e., several orders or magnitude more computer power than industrial calculation clusters. Resolving the full NS has been obtained for flow structures of about 1m length scale (100,000 cores).

For at-home solutions with modest computer power (I know GPUs can be very very fast but you need a few more) you start using Reynolds-Averaged NS equations, i.e., using turbulence models to resolve the boundary layer growth. The choice of model, the topology of your calculation grids plus many of the finer details determine how accurate your prediction will be. This will include how you will handle the predicting of the free surface and trim & sinkage of the ship. Getting these issues right is currently what the industry is aiming at and many generalized codes not aimed specifically at resolved ship flows fail with these details. We are developing such a code at work, as are many others, and we continuously compare the results of all these codes among each other.

Moving on to simulating a turning ship requires your first resolve the time-accurate self-propulsion and then the rudder execute. It can be done, of course, but it is currently a difficult job. I've seen a few presentations of what Mathematica can do (which is a lot, really), but I have not seen anyone in our field use it to calculate ship flows and do not know if people managed to get a good prediction at high Reynolds numbers (why not, should be possible, and the trend is working towards generalized codes).

For nice estimates of ship resistance you end up with methods like Holtrop & Mennen's; also good for a first estimate but it cannot capture the finer physics for hull flows that can be very sensitive to minor shape variations.
Post Posted: Sun Sep 27, 2015 4:23 am
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
Thoddy wrote:
Quote:
We do maneuvering calculations and it's usually complicated (not really my subject); we model the ship and a few maneuvring characteristics and let the simulation run.

Maybe the following question is somwhat OT, but i dont want to open a new thread-
to what extent is the drag of a ship hull is increased by increassing count of screws?
Is there any rule of thumb?

say
ship wit one screw and 50000 PS will do 30 kn
how much PS will be required to achieve the same speed with two screws
exact values are not required but some kind of a approximative value would be fine.


It is things like this I am trying to find a way of estimating.

There are a LOT of variables that go into the drag coefficient of an object (in 2007, our Engineering Practical Lab had an actual boat project where we needed to figure out the drag created by various choices of props we had to choose from, and the various hulls. The shape of the hull can radically alter how the prop creates or effects drag.

BUT, since WWII warships all use the same design model (the props are not in recessed channels, they all tend to be two, three, or four prop ships, and they all seem to use three or four-bladed props, the outer-two which clear the sides of the hull), we should be able to find a generalization for the ships.

MB
Post Posted: Sat Sep 26, 2015 8:31 pm
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
EJFoeth wrote:
Solving the NS equations with sufficient accuracy for ship maneuvering is something that goes far beyond a simple implementation in Mathematica and Matlab (and the capabilities of your computer). In fact, if you were given a NS solver and access to a computational cluster then I'd say setting up such a calculation is easily worthy of an MSc assignment. What would be more to the point are maneuvering models such as the Nomoto model, basically eqs for simple body movement and coefficients to estimate the various forces on the model. The FREDYN link I posted earlier is such a model. However, getting the right coefficients is again very tricky. We use either advanced CFD or model tests to train these models and getting 10-25% accuracy for all tested cases is already very difficult. You'd end up with models like these http://www.diva-portal.org/smash/get/di ... TEXT01.pdf (lots of info and references and leads to further study there). The question you posted is actually quite challenging.

Answering the question regarding the number of screws: there really isn't a rule of thumb as you go from a single-screw hull shape to a twin-shaft hull shape making a 'simple' comparison very difficult. Typically single-shafted systems are more efficient but there are so many effects at play at the same time.


Mathematica has a module for Fluid Dynamics, which can be used for precision NS-Modeling, or simply a precision or estimate "good enough modeling."

The module does list explicit computational requirements for the levels of specificity required. And, Mathematica has automatic multi-threading to make use of parallel processing. So, even if you connect to multi-core laptops via a USB Cable, Mathematica can Coopt the external CPUs.

HOWEVER... Better than CPUs are GPUs (Graphic Processors), a few of which are worth four to 128x their number of CPUs for doing pure math.

Again, Mathematica has built in GPU Multi-threading optimization to detect and utilize the graphic cards in a computer.

A 64GB 12-core Mac Pro with 4 NVidia 4-core to 16-core GPUs is more than enough for full NS Modeling with Mathematica. That is what they use at NASA Ames, where I interned in 2009 to to simulations for ducted-turbine vectored thrust for an experimental airship (which had the full NS modeling module for Mathematica).

You might be stunned to learn some of the applications to which Mathematica is applied these days.

Since around 2006, it has included functions and modules which used to require supercomputers to implement. And, it still has advanced functions which require a supercomputer to make use of. Stephan Wolfram usually has a panel on this at the yearly Mathematica Conference and workshop, where they show-off that year's release.

MB
Post Posted: Sat Sep 26, 2015 8:26 pm
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
Solving the NS equations with sufficient accuracy for ship maneuvering is something that goes far beyond a simple implementation in Mathematica and Matlab (and the capabilities of your computer). In fact, if you were given a NS solver and access to a computational cluster then I'd say setting up such a calculation is easily worthy of an MSc assignment. What would be more to the point are maneuvering models such as the Nomoto model, basically eqs for simple body movement and coefficients to estimate the various forces on the model. The FREDYN link I posted earlier is such a model. However, getting the right coefficients is again very tricky. We use either advanced CFD or model tests to train these models and getting 10-25% accuracy for all tested cases is already very difficult. You'd end up with models like these http://www.diva-portal.org/smash/get/di ... TEXT01.pdf (lots of info and references and leads to further study there). The question you posted is actually quite challenging.

Answering the question regarding the number of screws: there really isn't a rule of thumb as you go from a single-screw hull shape to a twin-shaft hull shape making a 'simple' comparison very difficult. Typically single-shafted systems are more efficient but there are so many effects at play at the same time.
Post Posted: Sat Sep 26, 2015 9:44 am
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
Quote:
We do maneuvering calculations and it's usually complicated (not really my subject); we model the ship and a few maneuvring characteristics and let the simulation run.

Maybe the following question is somwhat OT, but i dont want to open a new thread-
to what extent is the drag of a ship hull is increased by increassing count of screws?
Is there any rule of thumb?

say
ship wit one screw and 50000 PS will do 30 kn
how much PS will be required to achieve the same speed with two screws
exact values are not required but some kind of a approximative value would be fine.
Post Posted: Sat Sep 26, 2015 7:34 am
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
I am just trying to find out what is involved in the models.

I have everything I need to make simplistic models that would provide rough estimates that are "good enough" for the purposes for which I need them.

I happen to be an accomplished Mathematica and MatLAB user/coder, and I know enough advanced calculus to be able to set up the equations (I cannot imagine that the calculus goes much beyond differential equations typical of fluid dynamics, like Navier-Stokes Equations.

While I cannot solve the Navier-Stokes myself without difficulty, I do know how to set-up the equations in Mathematica to feed it the requisite variables for the Cauchy Stress Matrix, and the different values that are a part of a fluid system.

But the issue here is that the Navier-Stokes equations really deal with the movement of the FLUID around or through a body, rather than the movement of the BODY through the Fluid (although they remain a part of that solution/problem - the Navier-Stokes equations are used to predict the lift generated by a wing, as an example).

So if I could just find a freaking source that described what the variables and equation models are that are themselves involved in finding things like the Advance, Transfer at the new heading, Pivot-Point at speed N, Heel at speed N, Turning Radius at speed N, and loss of velocity through a non-powered turn.... Then I could produce my own simplistic models that just needed to be supplied the correct information about the ships in question.

I don't need a model that will produce a precision with an estimated error < .05%.

I only need to find a model that produces a prediction that is within ±10% to ±25%.

MB
Post Posted: Sat Sep 26, 2015 12:13 am
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
.

Unfortunately I read this in the days before the NMM let cameras into their archive.
Post Posted: Fri Sep 25, 2015 2:24 pm
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
Quote:
SIMPLISTICALLY at higher speeds and smaller rudder angles the two classes weren't too different. But at bigger angles of rudder whilst the instantaneous response of the KGV was better the SD soon overtook and for large changes of course they were much faster turning with a smaller radius (and consequent larger loss of speed)


phil does the report provide some turning data wich you can post? thank you
Post Posted: Fri Sep 25, 2015 1:25 pm
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
MatthewB wrote:
KGV class?

MB

King George V
Post Posted: Fri Sep 04, 2015 1:42 pm
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
KGV class?

MB
Post Posted: Fri Sep 04, 2015 7:14 am
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
.

After criticism of KGV class turning versus South Dakota class DNC got some information on the South Dakota turning characteristics at various speeds from BuShips and did a report comparing the two classes. They also did model testing of both hulls.

Due to the very different hull forms and one versus two rudders the ships had very different characteristics and the speed at which the turn took place AND the amount of rudder applied was very important.

SIMPLISTICALLY at higher speeds and smaller rudder angles the two classes weren't too different. But at bigger angles of rudder whilst the instantaneous response of the KGV was better the SD soon overtook and for large changes of course they were much faster turning with a smaller radius (and consequent larger loss of speed)

The report emphasised the KGV's better instantaneous response and INITIAL better response as a consequence, but the report then went on to admit the much better SD's performance. Basically they tried to paint as good a picture as they could, but had to admit the SD's better turning performance.
Post Posted: Fri Sep 04, 2015 5:41 am
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
EJFoeth wrote:
I wasn't trying to sell you the software but we can also calculate all these ships for you ;)

I can ask our maneuvering guys if we have a few simple rule-of-thumb kind of calculations for a first rough estimate, that might be a better idea.


It would be nice to get accurate Advance and Turning Radii for the Fleets of WWII (or best guesses/simulations in the place of historical record).

What would it take to calculate these things? How would I go about getting the data?

MB
Post Posted: Thu Sep 03, 2015 12:26 pm
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
I wasn't trying to sell you the software but we can also calculate all these ships for you ;)

I can ask our maneuvering guys if we have a few simple rule-of-thumb kind of calculations for a first rough estimate, that might be a better idea.
Post Posted: Thu Sep 03, 2015 11:34 am
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
EJFoeth wrote:
It's a model and depends on some coefficients, so you need to determine those in advance, but otherwise it performs well. Here's a description:

http://www.marin.nl/web/Facilities-Tool ... lation.htm

It's not the do-it-at-home-type of calculation ;)


That is an expensive piece of software.

I have already paid enough for Mathematica and all of my CGI software (Maya licenses aren't cheap - neither is a Full License for Mathematica - although I a one release behind, I think).

As a matter of simplifying things, do we know what these characteristics were for the various ships in WWII?

I know, for instance, based upon the little math that I have seen, that Destroyers had a very short advance, compared to a BB, but that they had a much larger turning radius that many of the larger ships, due to their high Length/Beam Ratio compared to a larger ship.

I know that as speed increases, the pivot-point of a ship moves forward, which affects the turning radius.

I may be looking for something too complex for what I want to do, and I may have to set some arbitrary values for sets of typical conditions (Ship type, wind and direction, and sea conditions or currents). There may be a few ships that have atypical advances or turning radii. but overall, I would like to find out how different the advances and turning radii for each class of ship (Class as in "Benson/Gleaves-class," "Benham-class," "Brooklyn-class," "Fubuki-class," or "Agano-class." not simply "Destroyer" or "Cruiser" or "Battleship").

MB
Post Posted: Thu Sep 03, 2015 10:39 am
  Post subject:  Re: "Advance" before turning distance for ships  Reply with quote
It's a model and depends on some coefficients, so you need to determine those in advance, but otherwise it performs well. Here's a description:

http://www.marin.nl/web/Facilities-Tool ... lation.htm

It's not the do-it-at-home-type of calculation ;)
Post Posted: Thu Sep 03, 2015 6:23 am

All times are UTC - 6 hours [ DST ]


Jump to:  
Powered by phpBB® Forum Software © phpBB Group