Naval Architecture
ciechanow.ski
ciechanow.ski
How did I not know this? It's so counter intuitive that a thin column of water can cause the same pressure as a wide one.
The video they link shows this in action: https://www.youtube.com/watch?v=EJHrr21UvY8
One mind bending fact she shares in the video is that a thin layer of water, touching the damn wall, is the same pressure as an entire lake.
Parent was simply commenting how that math was not intuitive (and so repeating that it was just math doesn't do much).
If you see pressure as coming from the weight of the fluid above and now replace fluids with solids for a mental model, it obviously doesn’t work: imagine two parallel rigid plates connected by some rigid structure. The top plate will represent the (fluid) boundary between the top of the barrel and the tube and the bottom plate the bottom of the barrel. If you put a narrow column of metal on the top of the top plate, then the pressure exerted by the bottom plate is, say, p. If you make that column much wider, increasing the weight above, the pressure is say 10p.
I think the problem is that this intuitive model of pressure is just wrong but if it doesn’t come from the weight of the water above, it is hard to intuitively see where it does come from.
My intuition (wrong here) is that the extra surface not beneath the stack of pennies (your analogy) would in fact distribute the pressure and therefore represent a lower PSI on all sides of the jar.
And so the bottom bag's "pushing" outward from compression would be affected only by those above it.
Yet you can distribute the weight of a structure on larger bases. If you put two columns on pennies on a steel plate, the pressure under the plate will be higher than if you put only one column.
It is easy to imagine the bottom vessel as similar to such a plate.
I think the key to make this intuitive is to realize a few things:
- pressure in liquids are transmitted differently - water is actually very slightly compressible - atmospheric pressure is also an important part of the system
It is counter intuitive though precisely because the pennies analogy doesn’t work.
If I put a 1m stack of quarters on a pressure gauge, then I put one quarter on the gauge and a stack of pennies on top of it up to 1m high, I get two different readings. Conversely if I measure the pressure at the bottom of two bottles the same shape as the stacks of coins, I get the same readings.
In case it was not clear in my analogy a "penny" (it can be anything solid and incompressible) is representative of a water molecule. If you want to compare the pressures of stacks of pennies and quarters because for some reason you are fixated in these specific physical coins rather than what they represent then I will make it explicit. Imagine pennies represent water and quarters represent mercury. Two different liquids, two different coins, two different pressures. I don't get what you guys don't get.
Can you explain to me why you think it is intuitive that the downward pressure of a liquid at the bottom of a container should vary for equal depths but different volumes? E.g. you seem to intuitively think that if you have a big pan filled with 1 inch of water the water pressure at the bottom of the pan is greater than the pressure at the bottom of a 50 ml flask also filled with one inch of water. This is false but I am interested as to why you and the others think it is "intuitive?" Where does the additional force come from to increase the pressure in the bigger pan? Gravity pulls straight down.
If you have a bottle of 1 kg of water and put it on a plank between two stools, what matters to know if the plank will break is the total weight (and torque) of the bottle and the surface of contact. The shape of the bottle is irrelevant.
When you have that image in mind and someone suddenly tells you that actually no, 1 kg of water on a 50 meters high column can actually break things 1kg of water in a bucket can't, it is very counterintuitive.
And the stack of pennies is really unhelpful I feel. Make an inverse pyramid with 1000 pennies, all of them resting on one at the bottom tip, or make a column of these, the force exerted on the bottom one will be the same. The force per area as well. Not so with water.
The difference is that the water molecules move until they find an equilibrium in which there is a gradient of pressure and where molecules push back in every direction equally. Pennies do not, they are content exerting "pressure" in a single direction
In the case of solid objects, indeed. This is because in solids atoms are bonded together and so the weight can distribute. An easy and extreme case of this last statement is to imagine standing on a bridge. Your weight is supported not just by the part of the bridge underneath you, but, via transmission, by the two end points attached to land.
In liquids, the atoms are not bonded, so the same distribution cannot happen.
https://open.umn.edu/opentextbooks/textbooks/85
http://civilcafe.weebly.com/uploads/2/8/9/8/28985467/fluid_m...
Are there particular aspects you are interested in?
E.g. Computational methods
Turbulence
Topology / geometry
Shocks and expansions
Superposition, radiation/diffraction, added mass
Analogues with other systems
Some of this stuff involves simplifying assumptions. Sometimes those reveal neat corners of behavior. (E.g. the superposition business)
Some of it might be better introduced qualitatively at first, like say turbulence. It all depends where you are.
By the way, I don’t know some great amount of this stuff at some expert level. I may be able to guide you in some direction of interest is all.
I wish I knew of an overview which scanned across all this funny behavior but some is pretty widely separated.
Naval architecture text books focus on the linear aspects, and the free surface of course.
Cfd (computational fluid dynamics) just about has to deal with “everything else” except the topology and “geometry of” (physics, mechanics, fluids, etc) which kind of lives the hermit life, like the quaternions or geometric algebra or something.
There are additional aspects that may or may not be lumped in somebody’s exposition - such as the transport of heat and coupling of the energy equation in the compressible Navier-Stokes equations.
Some of my friends did like it though (one got to explain how he spent his summer in the maths department learning about fingering.)
After I graduated, I returned to visit some friends doing master’s degrees and it was only at this point that I learned there was a moderately large laboratory under the maths department doing real physical experiments with actual fluids.
Both in terms of understanding the physics (weight of water above the column divided by the area of that column, and then any water around the column just has to have the same pressure to contain that column) and just plain practical experience from e.g. dipping underwater in the ocean and not getting crushed like a bug.
You have to visualize the atmospheric air pressure to reconcile the result with intuition.
Quality sensors cost a lot - too much for domestic purposes. Much cheaper ones can be bought from China, so I've been looking for some way to test them, without actually altering the level in a gigantic water tank.
It occurred to me I should be able to just use a thin vertical pipe. But as you say, this seems counter intuitive, especially if the pipe is barely wider than the sensor itself. Just doesn't... Feel right.
You could also use a small pressure vessel/sealed tank, and pump in water with a hand pump. You could simulate nearly any sized tank that way too.
Maybe a flow meter is a better choice for that. (Bonus, you can use that to cross-calibrate with the pressure/level sensor)
I've been using ohm/square for decades. I know the math. I've measured it. It works. I know it's true. But my mind refuses to accept that ohms/square can possibly be a unit. Every single time my mind is like, "ohms per square what?"
Pressure is force per area, the area doesn't matter by definition. Similarly to how we measure rainfall in millimetres: volume / area = length.
Whereas if you were to place a bucket of water on your head, the area of the bucket would surely make a big difference to the force you feel, all else being equal.
There was no theory, ship's screws were designed by trial and error.
So the Wrights invented the first propeller mathematical theory. It produced propellers that were 90% efficient, about double the efficiency of other experimenters' ad hoc propellers.
Double the efficiency meant the Wrights needed half the horsepower to get into the air.
All famous hull designers draw their curves by hand.
As an interesting anecdote, when I was still working for the (Dutch) navy they had a project going on to use constraint solvers to generate new submarine designs. The design team would generate 10 designs every week, take them to the sub guys who would spot new problems ("there is no bathroom close to the command deck" for example) and then go back and translate all the problems into new constraints for the solver. Later iterations even had VR models so they could "walk" through the virtual ship.
I find that much easier than using a pencil.
The early marine propeller designs consisted of an Archimedes-type screw with multiple full turns. During tests of one such design on a small boat in the Paddington Canal in London, half of the propeller broke off. The broken propeller (with only one turn) turned out to be able to propel the boat twice as fast. [0]
The inventor, Francis Smith, amended the patent to describe either a single-turn screw propeller or one with two screw threads each describing half a turn (essentially a two-bladed propeller).
[0]https://www.bluebird-electric.net/boats_images/propellers-fr...
Attempts to build flying replicas of the other claimants' machines don't impress me because they don't address the power needed to get those contraptions into the air with the engines available at the time. (The Wrights couldn't find an engine with the power/weight needed, and had to design/build their own powerplant.)
It's too bad there aren't many naval architecture careers in the US. We hardly design or build any ships here anymore. The one exception is military ships. So if you have a naval architecture degree your main employer options are a) government or b) government contractor.
Source: Naval architecture degree.
I'd love to design ships as a career, but as you said, there isn't much work, but why not learn for the sake of learning?
Also, aeronautical engineers, I'd love to learn that too. How to go about it?
If you want to try and pick it up on your own, start with the book "Introduction to Naval Architecture" by Thomas Gillmer and Bruce Johnson, from the US Naval Institute. From there, if you're still interested, probably "Applied Naval Architecture" by Robert Zubaly or something from SNAME (Society for Naval Architects and Marine Engineers).
If you want to go to school and you don't want to get a degree in it, you can study something similar; but, related. (I majored in Ocean Engineering, which included a number of naval architecture courses.)
Loosely related: here is a video of the German Maritime Search and Rescue Service (DGzRS) trying to 'sink' one of their (then new) smaller rescue lifeboats which has self-righting capabilities:
https://www.youtube.com/watch?v=dz_N6MG5tt0
(Ofcourse it was a test if it does have these capabilities, not an attempt at actually sinking it.)
In solo around the world races like Vendee Globe, the boats are required to be fully buoyant and self righting no matter how they end up. The most common approach to achieving this is to rig a canting keel with a device that when the boat capsizes, lets the keel swing to one side, creating a weight imbalance that rights the boat. They're quite serious about it too: you don't get to race the boat unless you demonstrate it works that way at the pier.
https://www.dco.uscg.mil/Portals/9/DCO%20Documents/5p/CG-5PC...
https://ciechanow.ski/internal-combustion-engine/
https://ciechanow.ski/cameras-and-lenses/
They do fantastic work.
Speaking (indirectly) of the equations of motion, I didn’t see added-mass as I scanned through. Could be fun to talk about as well as diffraction radiation.
Somehow the above are more fun sounding to me than Navier Stokes. I dunno. My burnout shifts with time.
And:
https://hn.algolia.com/?query=Naval%20Architecture&type=stor...
What are those?
{ // Start of block statement.
const foo = "a" + 2; // Only available inside block statement.
console.log(foo); // "a2"
} // End of block statement.
console.log(foo); // Uncaught ReferenceError: foo is not definedThe reason submarines can be neutrally buoyant at specific depths is because water is compressible, and water's density changes with depth. Adjust the submarine's density to match the water's density at a certain depth, and the sub will be neutrally buoyant at that depth.
Edit: Ah I think you just worded it badly and we're agreeing.
Well done!
The discussion about propeller design is also very similar to aircraft as well - not just aircraft propellers but also compressors in turbofan engines.
The fact that there's a ton of similarity between the disciplines isn't too surprising, but the great visuals in this blog post made that connection seem particularly satisfying.
https://mitpress.mit.edu/books/men-machines-and-modern-times...
Apparently the guy who took my job when I left had a nervous breakdown. I feel a little bad about that, but not really. He should have paid attention in class.
I already knew some about it since I already liked playing Naval games, but it took at least 12 hours or learning and experimenting before I could design a hull design which didnt sink or roll over, or other odd behavior. This is all despite the game not modeling some of the other aspects like water pressure, and being simpler compared to real life.
Basically, there is a lot more to designing ships that meets the eye.
However, I guess, those adhering to said fancy model must not be bothered by such complexity of thought…
Excellent. Bookmarked.
No. It is about the engineering of all sorts of things. Ships are a subset. I'd say that it covers all things that float, but that wouldn't include docks, cranes and other things that integrate with ships.
>>As containers are added the ship will sink a little and increase its draft – the distance between the bottom of the hull and the waterline.
This is the wikipedia answer. In the real world "draft" is the lowest part of the ship, which might be something other than the hull. Sailboats especially measure draft from the bottom of their keel, a thing lower than the hull. The "hull" is the watertight body and doesn't include things like keels and rudders which, while uncommon on large vessels, normally extend well below the hull's depth.
Admittedly "beaching" a nuclear air craft carrier is more important to the USA than a local bubba beaching his fish trawler on a sandbar; but to bubba as an individual, its more important not to beach his fishing boat as avoidance of beaching his fishing boat is actionable for bubba, whereas watching TV reports of a naval accident are not.