Design your own Ship

[AbhiJ]

A small Pack of 8 could be Accommodated. IMHO, This Ship could be completely be a One Man Ship capable of doing all the Missions on its own.

[Manish_Sharma]

The idea is to make our own indigenous submarine : Petrol Electric !


Just an idea of designing our own electric submarine with Kaveri Gas Turbine to recharge batteries :

Displacement: 4500 tons

Engine : Kaveri Gas Turbine (Plus points its fully our own, Second like in car petrol engine makes less noise than diesel so will Kaveri than foreign diesel engines)

Kavery GT is mounted on rubber rafts of course.

Another special feature is that engine section is in two layered capsules, outer capsule is made of wood and all the air from between the space is sucked out, thus making engine part much more quieter.

Whole Engine section can be slided 1 meter to the back so engine shaft gets attached to propeller shaft and propeller moves superfast rpm like Kaveri GT, giving the submarine superfast speed in case it is found and in danger.

Mostly engine needs to be switched on only to charge the batteries, only in desperate trapped situation the engine engages propeller shaft to make a super fast escape.

Batteries : Lithium-ion batteries from Germany or Japan

Turbine shaft : composite if possible

Torpedo : Shakti/Takshak + Seahawke 4

Propellers : Germany (Extra Big and more blades; as more blades even if propellers move slowly the speed doesn't suffer, also slow moving propeller doesn't make much bubbles and since the bubbles are reason for noise the lesser they are quieter is the sub)

Two mini propellers in the belly just like ocean smx sub.

Roof of the submarine is slanted like cottage :
http://www.manalitreehousecottages.com/ ... ertree.jpg
As slated roof will be able to take more pressure of water hence more diving depth.

Nose is pointed like jet fighter so as to help in speed as it mostly runs on batteries so more speed on less power.
=========================================================

Hope I am not making a complete fool of myself, its just that during the BRF was down I had nothing to do so watched lots of youtube videos and designed the sub on photoshop, I was hesitant to upload on BRF, then thought wth.... at the most people get good laugh !!!

[shiv]

In general submarines cannot rely on speed to make a getaway. The faster they go the more noise they make and they can never go faster than the slowest helicopter hovering over the sea with a dunking sonar. Better to go totally silent.

I am not gas turbine expert, but I think they are noisier than internal combustion diesel engines, consume more air weight for weight and need to run at constant high speeds requiring some serious gearbox speed reduction (noisemaking) to get it down to "slow propeller" speeds. That aside, jet/gas turbines cannot simply be started in a few seconds and revved up to max power. they have to be kept running and that will be a serious problem for stealth.

Why not nuclear power?

[Manish_Sharma]

Thanks Shiv ji,

I was designing P 75I submarine with as much indigenous stuff as possible, hence didn't go nuclear.

My idea of Kaveri GT was a bit simpleton thinking 'cause generally it is said that diesel engine makes more noise than petrol... so in addition to the fact that we don't have diesel engine of our own and get either from ukraine, usa or france. But your explanations have removed the possibility so guess along with propellers, batteries we'll need foreign engine too.

[kit]

Some one please think up a nice heavy duty battery ( maybe tesla types ) and electric propulsion for the pumpjets or a cross between a small reactor /battery with pumpjet .. you can have stealth and speed as the need arises .. ask uncle for non magnetic steel , and DCNS for non hull penetrating sensor masts and pumpjet tech

[kit]

btw making the nose pointy means you will need to put your sonar elsewhere .. keep in mind the more T/R modules in your sonar the bigger it will be (and probably better .. the le triumphants sonar has ranges exceeding 1000 nautical miles ! ) ..and putting it in the side means you might be looking at a single hull instead of a double hull ( more survival) ..

[Manish_Sharma]

Thanks kit ji, so it seems every tech I have proposed is turning to be just fantasy, thanks for pointing out these faults.

[kit]

just my views .. you could be right if you can factor in the design requirement for the purpose ..are you trying to make a SSK or a hybrid ? .. What are you designing it for ? How much tech access do you have ? and certainly what are you prepared to spend

[Manish_Sharma]

Loosely the idea was based on certain things mentioned in many discussions, like :

1.) SSK is more difficult to make, as they are small and a lot needs to be fitted in small frame opposed to nuke subs.

2.) To make indigenous SSK first an indigenous diesel propulsion is needed and also the propeller blade tech is only with few nations.

3.) Noise levels need to be kept down as much as possible.

4.) Parrikar's statement on mdl's new line opening occastion. That he wanted next subs to have more indigenous tech as current scorpenes are less than 35% desi.

Tried to address point 1.) by going like japanese or shortfin having big size, like scorpene is only 1500 - 1800 tons. I went 4500 tons. In addition I took the cue from japanese on doing away with AIP sort and filling submarine up with "Lithium-ion" batteries, this way sub runs faster on batteries and silent too. Big size running on batteries I tried to give it pointy nose like jet fighters so it cuts through water fast and makes do with less power.
So Having big space we don't have to minitiarise things, and we can do things with what capabilities we have.

Point 2.) since we don't have any diesel propulsion of our own I thought why not do with what I have that is Kaveri GT, here i confess that maybe to rationalise indigenous propulsion I may have dreamed up its advantages over diesel, which Shiv ji corrected.

Point 3.) Some years back I talked to an engineer who placed generators in Osho Ashram and to silence it they made the wall double and sucked all the air our out from between inner and outerwall, killing all sound. So I used that tech for encapsulating the engine and making the gap between walls airless. Plus of course mounting the engines on rafting.

Point 4.) was to make as much as indigenous but propeller blades etc. may not be possible with certain other optronics etc. Here Germans can help, every deal Bharatvarsh has gone through whether french or russians or usa or hawk deal with uk has had complaints regarding foreign companies being cheaters. But never ever with Germans, whether Arjun Tank Engine or Shishumar Submarines they honestly Tot and support whatever they provide. So we can use composite properller shaft , propellers , optronics and Seahawke 4 104 kilometer range torpedo from them.

I was just playing around and was hesitating putting up on BRF, but this scorpene scandal gave me energy and go ahead to up my design here.

[JayS]

I have zero knowledge of Submarines. But from my domain expertise I can offer following comments.

- I don't think pointy nose is good idea. It will give rise to separation and cavitation e.g. in changing directions. Water is 1000 times more dense than Air. So aerodynamics is not exactly same as hydrodynamics.
- We cannot run propellers at high speed due to cavitation limits. One needs wide chord blades. But they are noisy so newer designs use shorter chord blades but then to keep propulsive force same you need more blades. So its trade-off (Solidity is the technical parameter). Faster you turn them more will be the cavitation and after a certain rpm it will stall giving very small propulsive force. So super-fast speed is no-no. Anyway as Shiv pointed out you cannot outrun the danger, better go silent.
- The slanted roof - I didn't quite understand, but sub is basically a pressure vessel, and one moving inside water. Cylinder is ideal shape for both requirements. Pressure inside water is omnidirectional - that is its trying to squeeze the sub from all the sides equally.
- Movable engine room could be a problematic for CG management - particularly by being so far from the CG. CG and Buoyance force should be collinear IIRC, buoyancy being volumetric force is fixed at centroid of the volume. So if you move CG you will need to counter the couple created using some control system. (Actually you don't need to move it anywhere just decouple from prop shaft when not needed using clutch, but I see lot of complexity where the jet engine needs to be coupled with both prop shaft as well as electric generator and prop shaft coupled with jet engine and electric motor. But there could be some interesting config where a jet engine, electric motor/generator and prop are connected in series. The electric motor/generator can be used as motor or generator and jet engine can be coupled/decoupled as per need. Just running wild with some thoughts ).
- I am not sure the assumption that - since petrol engine makes less noise than diesel, a jet engine will be less noisy - holds true.
- I think Diesel engine would be more agile in start-stop than Jet engine. Technically you can go to full power from zero under less than a minute(for example RM12 in Gripen can TO in 45sec from fully cold soaked condition - they call is TIger start - but this takes huge toll on engine life and is only used for warlike situations), but Diesel engine should be much faster than jet engine in practical life.

Just my 2 futi kaudis...

[Yagnasri]

Sharmaji, I remember the allegations that Germans gave intel on subs sold to us to South Africa. So they also may be doing some cheating. Am I wrong?

[Manish_Sharma]

- I don't think pointy nose is good idea. It will give rise to separation and cavitation e.g. in changing directions. Water is 1000 times more dense than Air. So aerodynamics is not exactly same as hydrodynamics.

I think being a non-science person that's the mistake I made, I tried to take the pointy missile shape and tried to bring it in water submerged platform.



Basically I thought if I made my submarine shaped exactly like this above parker pen it will cut through water faster using up less energy, but not only it doesn't work but kills the space for bigger sonar.

Thanks to you all I have learnt this.
---------------------------------------------------------------------------------------

Yagnasri ji my point was regarding manufacturing and ToT never did we read any reports of germans reneging cheating not-fulfilling the contract in Shishumar subs unlike russian, french, british cheats.

Not that germans will keep our secrets from fellow whites or if under pressure with chinees of porkis.

[JayS]

^^Technically speaking, you can make that pen-shaped sub, going at very high speeds, using super-cavitation. But its not practical for current technology I suppose. Only 2-3 torpedo projects based on the principle exist e.g. Barracuda..

[vina]

So aerodynamics is not exactly same as hydrodynamics

Beg to differ. Mathematically, as long as the air flow is sub sonic, they are exactly the same, (sure, density and viscosity are different), but math wise, the equations are exactly the same and can be and are applied!

It is only when compressibility effects start becoming important (think trans sonic and supersonic), they start diverging fundamentally.

- I don't think pointy nose is good idea. It will give rise to separation and cavitation e.g. in changing directions. Water is 1000 times more dense than Air

In the nose, section, it is accelerating flow and the pressure gradient is favourable ( you are talking sub sonic flows here and not the very high Reynolds no flows) and will follow the contour of the nose without any of the stuff you talked about. It is only where the nose meets the side of the boat, you tend to see separated flow. Pointy nose is not needed because a blunt nose is optimal for a sub as there is no wave making resistance (unlike a surface ship, as there is an air-water boundary, in this a sub is more like an airship /blimp) when submerged, and it follows from the optimal shapes for that kind of thing (first used in aero stuff). However if you see WWII subs, the bow will be shaped like conventional boats because they spent a huge amount of time on surface. But after the research on the "GUPPY" boats, everything is optimised now for underwater performance.

We cannot run propellers at high speed due to cavitation limits. One needs wide chord blades. But they are noisy so newer designs use shorter chord blades but then to keep propulsive force same you need more blades. So its trade-off (Solidity is the technical parameter). Faster you turn them more will be the cavitation and after a certain rpm it will stall giving very small propulsive force. So super-fast speed is no-no. Anyway as Shiv pointed out you cannot outrun the danger, better go silent.

It is like this. The power & torque absorbed by a given blade (for a given dia) if cavitation is of utmost importance , is limited. So to do that, you increase the number of blades and increase the E.A.R (expanded area ratio -- that is the correct term) . However notice, what you are doing is trading off efficiency for quietness . A blade with a lower EAR will be more efficient (less metal area sweeping the water , less friction loss) than one with higher. So, taking to the extreme, while a modern pump jet is probably better in shielding radiated noise and indeed , you can channel all the noise astern of the boat , it will be less efficient than a large slow spinning propellor , even one with a larger than usual EAR for cavitation performance.

[Manish_Sharma]

This video from 40:49 mins onwards gives explanation of bubbles cavitation and propeller fan numbers etc. in great detail :

[JayS]

So aerodynamics is not exactly same as hydrodynamics

Beg to differ. Mathematically, as long as the air flow is sub sonic, they are exactly the same, (sure, density and viscosity are different), but math wise, the equations are exactly the same and can be and are applied!

It is only when compressibility effects start becoming important (think trans sonic and supersonic), they start diverging fundamentally.

Saar, I wasn't being pedantic there, true. Just was being layman - by "not exactly" I meant "you cannot copy paste just like that"

- I don't think pointy nose is good idea. It will give rise to separation and cavitation e.g. in changing directions. Water is 1000 times more dense than Air

In the nose, section, it is accelerating flow and the pressure gradient is favourable ( you are talking sub sonic flows here and not the very high Reynolds no flows) and will follow the contour of the nose without any of the stuff you talked about. It is only where the nose meets the side of the boat, you tend to see separated flow. Pointy nose is not needed because a blunt nose is optimal for a sub as there is no wave making resistance (unlike a surface ship, as there is an air-water boundary, in this a sub is more like an airship /blimp) when submerged, and it follows from the optimal shapes for that kind of thing (first used in aero stuff). However if you see WWII subs, the bow will be shaped like conventional boats because they spent a huge amount of time on surface. But after the research on the "GUPPY" boats, everything is optimised now for underwater performance.

Saar, rule of thumb - no pointy thing for subsonic flow - least of all in water. A pointy thing is infinite curvature (ideally speaking), fluid cannot take that sharp turn and separates. For subsonic flow, as such best shape is blunt shape. Since water has far larger inertia than air, its even more crucial. With a pointy nose you will see flow separation right at the sharp point/edge and cavitation in the separated zone is very likely. This has nothing to do with favourable pressure gradient, Re number etc. I cannot explain nor I could find a video or something, but you imagine yourself as a pointy nose trying to wade through water at high speed you will know what I am talking about. Actually Sub can have a more slender nose shape such as ogive or parabolic with a blunt front end (you can have pointy front with these shapes as well), but the very large blunt nose is due to volume requirements and/or for large sonar (same way why Polaris or our K4 has highly blunt nose shape). Its a compromise in short.

It is like this. The power & torque absorbed by a given blade (for a given dia) if cavitation is of utmost importance , is limited. So to do that, you increase the number of blades and increase the E.A.R (expanded area ratio -- that is the correct term) . However notice, what you are doing is trading off efficiency for quietness . A blade with a lower EAR will be more efficient (less metal area sweeping the water , less friction loss) than one with higher. So, taking to the extreme, while a modern pump jet is probably better in shielding radiated noise and indeed , you can channel all the noise astern of the boat , it will be less efficient than a large slow spinning propellor , even one with a larger than usual EAR for cavitation performance.

EAR and Solidity are the same things conceptually (I shouldn't have copy-pasted Aerodynamic term here, This is why i said not "exactly same"), essentially telling the same thing in a different manner. Thrust is the area under pressure loading curve. Now since you cannot load the hydrofoil too much since you want to avoid cavitation, you need larger chord for higher thrust requirement. But now if I want to reduce chord for some reason (noise) I cannot extract same amount thrust since I still cannot go below vapour pressure but the now the chord is small, so single blade will have less thrust. So to compensate I increase number of blades. Now I am not aware of efficiency or how small chord props are more silent. I couldn't find some easy stuff to get a primer in this subject but this explanation regarding chord length and no of blades is correct. I can keep EAR same by reducing chord but increasing number of blades.

[vina]

Saar, rule of thumb - no pointy thing for subsonic flow - least of all in water. A pointy thing is infinite curvature (ideally speaking), fluid cannot take that sharp turn and separates. For subsonic flow, as such best shape is blunt shape. Since water has far larger inertia than air, its even more crucial. With a pointy nose you will see flow separation right at the sharp point/edge and cavitation in the separated zone is very likely. This has nothing to do with favourable pressure gradient, Re number etc.

It is like this. If you put the same kid of sections you have in the nose of a plane /bow of a ship/sub (top sectional view of airplane hull, also called waterplane sections in boats) , in the stern, the flow will separate out in the rear, even if it is attached and follows the contours in the nose. The flow in the nose is accelerating and has a favourable pressure gradient and will be attached in those Reynolds number regime (which after all is a measure of inertial and viscous forces). After the max cross section, the flow is decelerating and has adverse pressure gradients .That is why the rear sections taper far more gradually and gently.

I cannot explain nor I could find a video or something, but you imagine yourself as a pointy nose trying to wade through water at high speed you will know what I am talking about.

Actually , for high(er) speed boats like destroyers ( considering only displacement hulls), the top view sections will be a sharp pointy tip, with the max sections , well aft of middle, and tapering off to the stern. And of course they will have much higher length to width ratios.Google around for pics of such boats from the air and it will be quite visible. In a surface ship,in addition to skin friction (governed by Reynolds number), the wave making resistance (governed by Froude number) present and more important. But in a submerged sub, there is no wave making resistance , so the optimal shape is actually that like what you get in wind tunnels for blimps / airplane fuselages etc in low subsonic regimes.

Actually Sub can have a more slender nose shape such as ogive or parabolic with a blunt front end (you can have pointy front with these shapes as well), but the very large blunt nose is due to volume requirements and/or for large sonar (same way why Polaris or our K4 has highly blunt nose shape). Its a compromise in short.

The sub nose is outside the pressure hull, is just a cover (like the radome in an aircraft nose), theoretically, you can have any optimal shape you want that enclose given the dimensions of the sonar (analogous to radar antenna). If you see photos of Nuke boats (like the US ones) which can have high sustained underwater speeds, I would think the nose would be different from that of diesel electric boats which cannot do such sustained speeds and will anyway have less top speeds. The hull shape will depend on design speed.

EAR and Solidity are the same things conceptually (I shouldn't have copy-pasted Aerodynamic term here, This is why i said not "exactly same"), essentially telling the same thing in a different manner. Thrust is the area under pressure loading curve. Now since you cannot load the hydrofoil too much since you want to avoid cavitation, you need larger chord for higher thrust requirement. But now if I want to reduce chord for some reason (noise) I cannot extract same amount thrust since I still cannot go below vapour pressure but the now the chord is small, so single blade will have less thrust. So to compensate I increase number of blades.

The prop blades (unless you are doing some super dooper stuff, you probably will for subs, where you will custom design airfoil sections using 3D CFD codes) are just bog standard, "Me Ol Daddy" - NACA profiles and those design tables have been around forever (for planes and ships).

Again, going to old-old Ideal Disk - Momentum Theory, the ideal propeller would be a LARGE (as large as practical) rotor disk, spinning at the slowest possible speed . That is the first design consideration (whether ship or plane) . Then you would choose the LEAST number of blades (why?) (one would be ideal, but not practical, because it would be unbalanced and rip apart), so if you can do with two ( many planes , esp small ones have just two) , see if you can plug that into the NACA tables and pull out a prop , do the smell test, and if fine, you are done. For most cargo ships, it will be 3 blades at most.

For subs, you cant have a large prop (it will hit the bottom when it is operating close to the bottom or lying in the bottom in shallow waters, or if it is operating on surface, a large prop will partly be out of water and if it operates and hits the water from the air water boundary, the shock loading will rip it top pieces), so you are forced to go for a smaller than optimal disk diameter, so you have to go for higher number of blades as is. In addition, if you want very low blade loading because of cavitation, you will increase the number of blades

Now I am not aware of efficiency or how small chord props are more silent. I couldn't find some easy stuff to get a primer in this subject but this explanation regarding chord length and no of blades is correct. I can keep EAR same by reducing chord but increasing number of blades.

The more solid you make the rotor disk(as per momentum theory), the higher is the area of blade in contact with water, and hence higher frictional losses and power required. Consider an extreme case, you basically fill the entire pitch thread path of the prop with metal , for say one whole revolution (like sort of like a metal screw that you use to join things together, but shortened to one pitch thread, used instead of a prop Archimedes Screw fitted instead of a conventional prop) . Sure , that will have brilliant cavitation performance , but piss poor propulsive efficiency.

Talk of efficiency, this is one of the standard questions that are always asked, so I think I will ask here (No googling please, sometimes, I wish Chaccha didnt exist).

For an airplane, you normally put the prop in the nose . However for a ship, would you do the same ? Why not put the prop in the nose like in an airplane? Why do you put it in the stern?

[shiv]

For an airplane, you normally put the prop in the nose . However for a ship, would you do the same ? Why not put the prop in the nose like in an airplane? Why do you put it in the stern?

Har de harhar

Love questions like these because teachar told me not to leave out any qweschuns even if I did not know the ansar.

I can think of two reasons

1. Fluid from puller will only add to resistance from body skin of plane/ship. In plane it matters less because airflow on the body/wings can be used for lift, and also, (I guess) using props of large diameter ensures that a huge volume of air bypasses the aircraft body and wings because the cross sectional area of prop is large compared to the axial sectional area of fuselage/wings. Not so for ships where a huge mass of water will flow along the body causing resistance.

2. For manoeuvring in small spaces, like parking a ship in shipping mall, pusher is better, just like reverj parking is better in car

[Yagnasri]

One super duper Mango man idea - Why not use fuel cells that convert petrol into electricity and instead of shaft use power supply cable to the propellers. It may be a stupid idea but as a mango man, it is my right to have such ideas.

[JayS]

It is like this. If you put the same kid of sections you have in the nose of a plane /bow of a ship/sub (top sectional view of airplane hull, also called waterplane sections in boats) , in the stern, the flow will separate out in the rear, even if it is attached and follows the contours in the nose. The flow in the nose is accelerating and has a favourable pressure gradient and will be attached in those Reynolds number regime (which after all is a measure of inertial and viscous forces). After the max cross section, the flow is decelerating and has adverse pressure gradients .That is why the rear sections taper far more gradually and gently.

Saar, I am afraid you are going slightly tangent here. Why bring in anything aft of the leading part of the Nose?? My comment was only limited to that part. Sharp nose tip isn't a good idea and even high finess ratio nose that Manish had shown in his diagram, will hamper steering ability. Smaller radius at nose tip means it will lose effectiveness even at small angle of attack. That's all I wanted to say. Also keep in mind it was with reference to high speeds, not the normal ~30kt speed thats subs have typically.

The prop blades (unless you are doing some super dooper stuff, you probably will for subs, where you will custom design airfoil sections using 3D CFD codes) are just bog standard, "Me Ol Daddy" - NACA profiles and those design tables have been around forever (for planes and ships).

Again, going to old-old Ideal Disk - Momentum Theory, the ideal propeller would be a LARGE (as large as practical) rotor disk, spinning at the slowest possible speed . That is the first design consideration (whether ship or plane) . Then you would choose the LEAST number of blades (why?) (one would be ideal, but not practical, because it would be unbalanced and rip apart), so if you can do with two ( many planes , esp small ones have just two) , see if you can plug that into the NACA tables and pull out a prop , do the smell test, and if fine, you are done. For most cargo ships, it will be 3 blades at most.

For subs, you cant have a large prop (it will hit the bottom when it is operating close to the bottom or lying in the bottom in shallow waters, or if it is operating on surface, a large prop will partly be out of water and if it operates and hits the water from the air water boundary, the shock loading will rip it top pieces), so you are forced to go for a smaller than optimal disk diameter, so you have to go for higher number of blades as is. In addition, if you want very low blade loading because of cavitation, you will increase the number of blades

This is all absolutely correct. Again my comment was keeping in mind that diameter, rpm, thrust are the same. Then if you want to reduce chord length you have to increase blade numbers to compensate for that and vice versa.
[/quote]

For an airplane, you normally put the prop in the nose . However for a ship, would you do the same ? Why not put the prop in the nose like in an airplane? Why do you put it in the stern?

Let me take a dig at this one. For aircraft there is always a debate between the pusher and tractor config. Some like one, others like the other one. There are a number of reasons why for aircrafts tractor config is preferred over pusher one. Actually pusher config is a better option aerodynamically speaking, since it does not spoil the flow over the wing and fuselage. But then the propeller itself can get affected badly due to the fact that its in the turbulence/wake of wing/fuseage now. The overall efficiency of the configuration would be dependent of many parameters. But there are other nagging issues with pusher config like FOD damage, less clearance to ground while take off rotation, engine cooling etc. For single engine general aviation places tractor config is great for CG management since the most heavy components engine/gearbox seat in the nose easily making it a static stability. I would say final choice should be dependant on specific layout of the aircraft.

I don't know what would happen if you put propeller in front of a ship exactly. But it would definitely be a hindrance to stuff like Sonar, torpedoes if you are planning to put them in the nose. Also the wake hitting the submarine surface would make more noise, cavitation damage to outer skin is possibility. Also i think it will be difficult for the tractor prop to move the heavy load. Pushing heavy load is generally preferred. Drag on the submarine would increase, since now the propeller wake is hitting the nose which is higher stagnation flow - higher pressure drag on the sub as well as higher skin friction drag which scales with velocity squared. No propeller noise shielding while in approach is one more issue I can think of.

[vina]

.... I don't know what would happen if you put propeller in front of a ship exactly. .

Fluid from puller will only add to resistance from body skin of plane/ship. In plane it matters less because airflow on the body/wings can be used for lift, and also..

Well yes. Shiv has sort of got the answer. The first highlighted part is the reason. The reason why you put the prop in the rear of the ship is because it increases the propeller's efficiency! .How ? The propeller operates in the ship's wake and the water that is flowing into the prop (because it has been slowed down by the friction from ship's body) is actually a little bit slower than the open water /free stream velocity. So, the Prop actually delivers a little bit higher thrust than if it were operating in open water , like it would if fitted on the nose of a shop. Remember, props deliver max thrust at zero speed and that thrust decreases as the craft's speed increases (i.e. free stream velocity increases if you consider the prop as fixed and fluid moving) . This shows up as increased efficiency.

Google for terms.. Hull Effect, Wake Fraction and Quasi Propulsive Coefficient (QPC) and you can find a wealth of details. Even the wikipedia entry on Propeller has a brief section titled "Actual Performance" , but doesn't explain it in any detail.

But the question remains, okay, it makes sense to put the prop in the back of the ship, but shouldn't this apply to airplanes as well ,since the props delivery higher propulsive efficiency if in the rear? SO why do you put it in the nose in a plane ?

That part is answered by the 2nd highlighted part of Shiv's answer. For a plane, unlike a ship, the bulk of the significant flow (and most of resistance, due to lift induced drag) is due to the flow over the wings , not around the fuselage ! So it makes sense to put the prop in the airplane's nose for well known advantages (such as CG management, structural design, need for tail and empennage etc) in the nose!

[shiv]

MBBS exams can also be passed by guesswork- just sayin..

[kit]

no negative marking

[prasannasimha]

This sharp nose thing is simple - a sharper nose indeed does slice through any fluid faster when it is subsonic which is why fish have sharp noses. Saying fluid dynamics is significantly different from gasflow is not true in the subsonic regime.It all depends on the type of fluid Newtonian or non Newtonion and the viscidity and wether you are crossing the Reynolds number leading to turbulent flows.essential principles continue to be the same. Saying a submarine has a bulbous nose helps cut through water is not true the reasons are mor to accomodate the sonar but if you see smaller underwater submarines they do have pointed noses.

[Indranil]

That's not correct sir. If you are going to be fully submerged, the teardrop is the most efficient shape. Let's get back to subsonic air flows.

What is the shape of the nose of planes which are designed for most efficient subsonic cruise?



What is the shape of subsonic external fuel tanks?


Why is the leading edge of an subsonic aerofoil not pointed?

Getting back to water, it makes all the more reason to make the nose even more blunt (yes, it is in accordance to laws of fluid dynamics). That is why submarines and torpedoes (they are much smaller in size) employ the teardrop shape.

The shape of fishes is not just determined by hydrodynamics alone. Often it's the only business end of the fish. The fastest fish have the most pointy nose, but they are not the most efficiently shaped. They have large slender and muscular bodies to propel them.

[Manish_Sharma]

I thought this swordfish the fastest in world, would be best shape for speed.

[shiv]

The marine animal-ship comparison is interesting and while I can see that there is "something there" I am not sure it is relevant.
Dolphins that can do some insane speeds have a stupid looking shape for speed. It probably has nothing to do with appearance.

Rather than absolute speeds that fish reach, it may be relevant to compare how many fish lengths can be covered per second. For example - if a Goldfish can swim 4 times its own length in 1 second and a nuclear powered aircraft carrier can do only 20% (0.2) of its own length in 1 second - we have a more relevant comparison of speed/power versus length

While I am no fluid dynamics guru, I am certain that the fact that fish swim by swinging their tails and bending from side to side is relevant. That seems to be doing something about friction in water. Another relevant comparison might be mass per unit volume. Ships have low mass per unit volume and that means that the surface area offering resistance is higher. I think fish, like all animal tissues, probably have a mass per unit volume similar to water itself.

[vina]

The marine animal-ship comparison is interesting and while I can see that there is "something there" I am not sure it is relevant.
Dolphins that can do some insane speeds have a stupid looking shape for speed. It probably has nothing to do with appearance.

Rather than absolute speeds that fish reach, it may be relevant to compare how many fish lengths can be covered per second. For example - if a Goldfish can swim 4 times its own length in 1 second and a nuclear powered aircraft carrier can do only 20% (0.2) of its own length in 1 second - we have a more relevant comparison of speed/power versus length

While I am no fluid dynamics guru, I am certain that the fact that fish swim by swinging their tails and bending from side to side is relevant. That seems to be doing something about friction in water. Another relevant comparison might be mass per unit volume. Ships have low mass per unit volume and that means that the surface area offering resistance is higher. I think fish, like all animal tissues, probably have a mass per unit volume similar to water itself.

Well, fundamentally it is this. Nature is FAR FAR more efficient than anything man made and that efficiency lead is vast.

Coming to fishes /marine mammals, it is a very well known fact, that fishes expend far less energy (calculated by marine biologists) than than equivalent boat or sub. A Tuna (a high speed fish) spends far less energy , while swimming at it's top speed ,than a boat or a sub at that speed (ie. the Tuna spends far less energy per unit mass , ie specific energy, than a boat or a sub). How do they say this ?. Well, they took frozen Tuna/ Stunned Tuna or other high speed fish in a towing tank and towed it it's max speed and discovered that the strain gauges showed far higher towing force required and energy expended than what the Fish's physiology is capable of expending per what biologists calculate !.

How can that be ? The same fish, which is dead and pulled through water requires more force, than a live one swimming under it's own power? It is postulated that the fish are such brilliant swimmers that they are able to recover energy from the eddies and vortxes that are shed from the fluid flow and and are more efficient than a dead fish being pulled through water!

Same thing with birds. A humming bird or an eagle can perform maneuvers that will be impossible for an F22 or a SU-30MKI with the latest thrust vectoring! How ? well, the birds have far more control surfaces than a plane, are able to vector thurst shape their wings and tail , and basically out fly anything man has made..

Talk about sensors. An eagle / hawks eye , weighing a few grams at best is able to spot a brown /white coloured hare against a desert or snow background from kilometers up in the sky. Seriously doubt that a FLIR pod, even the best ones out there, will be able to have that kind of resolution , not to talk that a FLIR pod weighs thouands of times more than a hawk's eye.

As for flight and mission computer, a "bird brain" (setting aside the pejorative connotation), is able to do that incredible flying by controlling it's flying surfaces, does mission planning , controls it's sensors and hunts prey and chases down a mate and it weighs a few grams, while a mission computer on a modern plane will be closer to a 100 Kg!

What man has been able to do (very successfully), is able to use the brute force method of harnessing energy to power through everything (sort of like an energy unconstrained existence). Nature makes the absolute best use of constrained energy.

For eg, a bird will migrate from Arctic to Tropics over a few weeks, eating a few bugs and possibly fishes along the way, while an airplane will gulp Jet Fuel like a Paki taking Baksheesh from his Phour Fathers, and remember, the specific energy in Petrol/Diesel/Jet Fuel is HIGHER than gun powder! The specfic energy expended in both cases is simply not comparable.

Check out this article Efficient Swimmig by MIT and I am quoting from the article titled "Delphine Mystery"

While planning our machines, we availed ourselves of the long trail of theoretical /experimental and biological studies of how fish swim. In 1936 the British zoologist James Gray created a stir by calculating the power that a dolphin would need to move at 20 knots, as some were reported to do. Gray assumed that the resistance of the moving dolphin was the same as that of a rigid model and estimated the power that the muscles of the dolphin could deliver. His conclusion, known as Gray’s paradox, was that the dolphin was too weak, by a factor of about seven, to attain such speeds. The inescapable implication is that there are flow mechanisms at work around the body of the moving dolphin that lower its drag by a factor of seven.

Think of this. For a Diesel Electric SSK moving underwater at 20 Knots, an equivalent Dolphin /Orca / Sperm Whale is possibly spending 7 TIMES LESS energy !

[Yagnasri]

Replacement of Ohio class sub is to have electric drive propulsion as per the news reports. Can we look into that for our subs?

[shiv]

^^What is electric drive propulsion?

[JayS]

I got this one paper from 1950 which is considered first such kind of attempt to systematically study submarine shapes. One doc mentions that this study laid foundation for all future US submarines' shapes starting from USS Albacore in 1951. This is the paper:

http://oai.dtic.mil/oai/oai?verb=getRec ... =ADA800144

It experimentally evaluated 24 various shapes and concluded 6 parameters influence the overall resistance the most and gave some optimum numbers for them solely based on minimum resistance considerations. Among them is Nose radius and the researcher gives non-dimensionalized value (defined as (r/D)/(D/L) where r = nose radius in m, D = dia of Sub in m, L = length of Sub in m) of "0.5". He has considered shapes with sharp nose as well.

I just did rough calculation for Ohio class with numbers from Wiki and I get nose radius of about 6.5m for 0.5 non-dimensionalised value.

But the fineness ratio seems to be more important parameter and a good value suggested in that papers is L/D = 7.

Anyway, while comparing fishes with submarines, care must be taken as pointed out by Shiv. Just like care must be taken to apply Aerodynamics principles from Aeroplanes to insect/bird flight. They have different Re number regimes. Animals have lot of tricks other than shape for example sword fish has a oil layer on its skin while shark has small small scales inducing micro-vortices reducing skin friction drastically. And since fishes operate in lower Re regimes that ships/Subs skin friction is more dominant there relatively speaking. Another factor is Animals use shape in dynamic ways. So is fish does not dash like a frozen shape like a sub does. So shapes does not tell us the whole story in general.

[deejay]

...

Think of this. For a Diesel Electric SSK moving underwater at 20 Knots, an equivalent Dolphin /Orca / Sperm Whale is possibly spending 7 TIMES LESS energy !

Vina Ji brilliant explanations. Loved the post.

[JayS]

A nice article on various factors possibly contributing to efficient swimming of Dolphins..

http://phys.org/news/2006-06-marine-tec ... phins.html

According to this Gray's estimation of 7times less energy is a gross underestimation due to various assumptions he took. Nonetheless Dolphins are way more efficient than anything that man can make in that size even with same power input assumed.

Another paper I found which tries to find optimum axi-symmetric minimum drag shape for varied Re range. Main take away for me from this paper was - Each Re number has a different shape for minimum drag under similar conditions and as you increase Re the shape moves from being slender to become fatter (i.e. decreasing fineness ratio). Now this particular result is limited to consideration that Laminar flow is maintained throughout, so it has limited practical value. The writer also notes thats if the flow is assumed fully turbulent then the shape has not much effect on the drag.
http://www.dtic.mil/dtic/tr/fulltext/u2/744314.pdf

[Manish_Sharma]

^^What is electric drive propulsion?

The way I see it is like the fan whirling above my head. The fan has a motor in the middle "tikki" part and electric wires go out of it to the regulater+switch. The electric whirls the motor...

fan is wired through the ceiling and connected to the wire that controls either a remote box or a switch on the wall for operating. The ceiling fan capacitor torques up the electric motor, allowing it to start and run. An electrical current reaches the motor and then enters coils of wire that are wrapped around a metal base. As this current passes through the wire, a magnetic field is caused that expends force in a clockwise motion that actually changes the electric energy into mechanical energy. This action causes the motor coils to spin. As the coils are spinning, the fan captures this spinning motion, transferring it to the fan blades. The slicing of the air caused by the fan blades is what pushes the air downward, causing the breeze created by the ceiling fan. http://modern-electronics.weebly.com/ceiling-fan.html

Without studying that's how it pictures in my mind that "electric propulsion" would work, that charged up batteries give current to a motor connected to propeller shaft and moves it this moving the propeller like fan.

[shiv]

He he. Thanks Manish. My question was on whether there are any new sources of energy to power submarine motors. You must have read the news item that Samsung S7 deliveries were stopped because a battery exploded. Actually, although we don't think of them that way, all batteries are bombs. Bombs are just concentrations of energy stored in a small space. If that energy is released in a very short time you get a bomb. Slow release is what we are looking for. Slow release from a cooking gas cylinder is cooking for many days. Rapid release followed by ignition of gas+air makes a bomb

Our subs use car battery-type lead-acid batteries. AIP uses stuff that I will have to Google again to find out. But I think someone suggested "fuel cells" for subs. The idea sounds good but the point is how will the fuel cell get recharged? After one discharge where will the Hydrogen come from?