SUBSIM Radio Room Forums - Sub physics

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- Dangerous Waters (https://www.subsim.com/radioroom/forumdisplay.php?f=181)

- - Sub physics - experts needed (https://www.subsim.com/radioroom/showthread.php?t=125372)

There are no legal problems about improving the playable modells ... for example to substitute the actual Akula in DW with the one in the CSS. However there are (to my knowledge) technical problems, mainly I think with the Bridge View and to get all the moving parts displayed and working correctly. Not to mention the fact that you have to export them from 3DS into J3D and the only known exporter works only for 3DSMax 3.1

Just so you guys know ... Dr.Sid released another early version of his subsim

Quote:

New features:
- periscope
- map
- new sky with daytime changes
- better autopilot
- trim tanks
- better terrain (glitches mostly fixed, basics of texturing scheme)
- some other minor invisible stuff
- 3 subs: Akula, 688, S-class.

Other important facts:

The terrain texture is just one tile borrowed from some user mod for Wings over Europe I found on my HD. I will give proper credits soon. I hope I could borrow the whole tileset, or the author, because it's great.

There are 6 tiles of terrain included, covering Gibraltar to Alps. If you zoom the map you will see that resolution of these areas is better. That means terrain is available there. In places not covered by the terrain tiles there is flat sea bottom with depth 100m.

The S-class has no movable parts at the moment. Also the physics modeling is still same for all the boats.

Sun moves on the sky at correct speed. If you want to play with sunsets, you can make it 60 times faster or move it backwards, check included controls.txt how to do it. This combines with normal time compression.
Btw. the screenshot with mountains and a sunset was done in the Alps.

Also note that 688 is right behind the Akula. Here is why: jump into the Akula's scope, turn back the scope, and sail straight away. At some distance you will be able to see 688 vanish behind the horizon, in other words, roundness of the Earth. With increased view distance you can enjoy whole Mallorca hiding under the horizon.

Scope is washed at depth 65 feet or so. 60 is actual recommended PD.

Head over to this thread : http://www.commanders-academy.com/fo...ad.php?t=22809 to discuss and download it. Furthermore, don't forget to take a peek at the CSS Wiki which has some nice screenshots.

I've just posted another version of comsubsim, called Alpha 9.

Get it here:
http://www.commanders-academy.com/comsubsim

Discuss it here:
http://www.commanders-academy.com/fo...ad.php?t=27889

I have a question about typical sub propeller RPM.

I made two calculations .. one based on blades lift, other based on extrapolation of propeller from WW2 sub.

My estimate for 688 would be then 900 RPM for speed 30kts. Am I way too off ?

Um,

You are WAY off.

Things to consider:

1) the props are MUCH larger than the WWII boats.

2) More blades typically than the diesel boats

3) Slower RPM is quieter

I would LOVE to give you a roundabout figure, but this is a big HUGE no-no for me. I can but give you points to ponder.

but it is less than 900 though..:p

Great .. yesterday I just realized that 900 RPM would translate to TPK 7.5 (turns per knot per 15 seconds) and in DW many subs have TPK 7, while I'm not sure if LA too. So I was happy my estimate has another backing.

Ok .. so then I'll think more about it.

Btw. Xabba .. have you seen this ? Might come handy for modeling.

http://maps.live.com/default.aspx?v=...2732&encType=1

It's been there for some time as I understood from the page which referenced it. I found this yesterday when looking for info about props.

So are you saying 900 is correct by your assumptions?

By my assumption it is correct. But it seems my assumption is wrong.
I will slow it down somehow in next release and see what will BubbleheadNuke say :|\\

I also made some pessimistic estimates for slowest possible RPM, based on relative blade angle. It is clear then blades need positive angle of attack. Blade angle of attack is defined by combination of sub forward speed and shaft rotation.
That means that if for example sub is moving 5kts and blades have angle 30 degrees to blade-plane, the slowest possible rotation speed would be about 0.4 rotation per seconds, which is 24 RPM, otherwise the blades will have negative angle of attack, thus negative thrust. For 30 kts this makes 6 times more, 144 RPM.
But mind this is pessimistic estimate. Also those 30 degrees of blade angle are just wild guess.

I don't know any stuff on the net that explains prop physics...I though the angle of attack was based purely on physical properties, ie a blade will have a fixed pitch of 10 degrees and that as soon as it starts moving its AoA will be positive.

Here is some lame picture I made:

http://roger.questions.cz/other/blade.gif

Red arrow shows blade speed component given by prop rotation.
Blue shows speed component from ship movement.
Green shows resulting blade speed, which defines angle of attack.

So for every forward speed there is some RPM at which the AOA is zero (I mean at which blade lift is zero, it's not that simple). At RPM slower then this limit RPM the blade would produce negative lift.

Time to wade into this discussion in a little more detail.

There seems to be some basic misconceptions here.

A screw is NOT a propeller. You are moving water and not air. You are not trying to create lift.

Here is the simplest way to think of how it works:

A screw, when turning, creates a low pressure are in FRONT of the screw. The difference in pressure from the FRONT of the screw and the BACK of the screw impart a thrust vector that moves the ship. The greater pressure difference imparts a greater amount of thrust.

What causes cavitation is the transition of bubbles from the lower pressure front (where the water can actually BOIL due to the lower pressure) to the higher pressure BEHIND the screw that collapses these bubbles.

This is why deeper and or colder water reduces cavitation. You are changing the pressure/temperature relationships.

Okay, with this in mind, a screw is constructed in such a manner as to create the greatest amount of lower pressure in front of the screw and then have a controlled transition to the higher(sea pressure) that is behind the screw.

If you look at the picture referenced earlier in this thread, you will notice that the screw has a particular curvilinear shape. The blades, if you look carefully, do not have a constant AOA.

Check out http://www.ussalbacore.org/img/img_v...tour/04-lg.jpg. the screw on the far left was off the USS Jack and while it is an older design you can see what I am talking about.

Now, with all that said, there IS some displacement of water by sheer movement at lower RPM's but you are dealing with a MUCH dense medium than AIR. A movement of the screw in water, just like a wood screw in a chunk of wood, if going to cause motion. AOA does not have SQUAT to do with RPM and speed. It has to do with controlling cavitation and blade efficiency.

The screws on submarines are as big as they can make them. If they could get away with one twice the diameter of the hull they would do it. The larger the screw, the slower it can turn. They have to take into consideration sitting next to a pier, streaming a towed array, and other things. Could you imagine watching the blades coming out of the water when surfaced?? Each time it smacked back into the water the stress on the propulsion train would be HUGE. If you notice, the waterline of just about ANY single screw boat is about 1 foot above the top of the screw. That is one of the limiting factors in screw design and size. They want to keep the screw under the surface interface at all times.

Please excuse my spelling and tone as I have worked the 30 of the last 48 hours (not including the 2 hours of travel time) so I am a bit punchy and tired. I will clean this up later if needed.

That's what I thought, I tried to find some stuff online about prop design but a lot of it was too technical for me....

I just wanted something simple.

I totally agree. I just don't see how difference in pressure on front and back side is different from lift. Sure airplane prop can't cavitate, It has other problem (like supersonic tip speed), but in general the principles are very same.