10k limit strikes again...
Displacement is to ships what weight is to land vehicles or maximum take-off weight for aircraft. Ships are the intermediate between a land vehicle and an aircraft, they seem to work like land vehicles because water seems to provide stable support but in reality they are constantly "flying" in water.
Displacement is how much weight the ship carries while maintaining equilibrium in water. The confusion comes from the fact that displacement is measured as if it was mass which is a scalar, which both weight and displacement are not (entirely), even though they are measured in scalar (kg) rather than vector (kN) units. That's because it's a simplification of the model where weight/displacement/MTOW are really mass * g. The same thing in orbital dynamics is never treated in this fashion for the very reason.
Water can have different physical properties due to salinity which changes its density and therefore ships with displacement of 1000t in freshwater can technically have, say 1050t displacement in the Baltic Sea, 1100t displacement in the average ocean and 1200t in the Dead Sea.
Displacement is also influenced by hull shape and mass distribution which determine ship stability. Ship stability is more important than mass/weight because ships move in two ways:
Ships are also vessels with dynamic load because as ships go to the sea they are closer to full displacement while when they return to port they are closer to empty displacement. If the ship is deformed at the latter stage it may not remain stable at full displacement once it is fully loaded again and travels in high sea states.
Sea states are another thing - all hulls are designed for specific sea states. Look up what a dynamic model for ship buoyancy looks like. You'd be surprised how unstable these things are when waves grow only a little bit.
The reason why 054B has 7000t has to do with whatever has been done inside the ship above the waterline that likely affected stability more than weight, and required adding more mass as ballast. Increase in displacement is caused by the additional mass at the bottom balancing additional mass at the top. Displacement is all about center of mass and stability.
This is for example why may ships carry ballast near the keel. Broader hull with lower center of gravity will carry greater weight and therefore that ship will have greater displacement. The same hull that has approx. 4000t can be increased to say 8000t or perhaps more simply by concentrating mass within the boundary of geometrically optimal distribution which in a section would look something like a bowl which incidentally is a bowl because it has to be stable while partly resting on a table and partly "floating in air" while mass and momentum is being put inside it. This is why bowl is heavy everywhere but, heaviest at the bottom.
The new radar which is at approx 25m above waterline is heavier than the old radar. AESA has greater mass per element because every element is a transmitter and a receiver and needs individual controller unit. Because of the energy it also needs cooling which usually means some combination of air and liquid cooling to all elements e.g. air cooled array and liquid cooled processing units. Another issue is information transmission - while the computers in CIC may use optic fibre the radar likely does not and latency is an issue, so the shortest possible routing requires cooling close to the array so as not to drag slow cables into more convenient location. And then there's the issue of maintenance and repair access. For example AN/SPY-6 can be fixed from within the ship allowing repairs at sea while engaged in combat. I expect 052D and 055 to be similar. 054B radar as well as say Type 45 radar are not viable for repairs at sea or in combat because the array is difficult to access but that's why 054B is not an air defense ship. It can lose its air radar. A modern AAW ship can't. Ever. For those reasons high performance AESA systems as a functional design element - which is how they are treated in the design process - are significantly heavier than traditional radar like the one on 054A.
Another reason for greater displacement may be simply greater mass of fuel that was not necessary for 054A mission profile due to other limitations. Ship autonomy is always limited by the crew first and foremost. There's no need to waste space for fuel is your crew can't operate efficiently past X day limit. Fuel can be easily replaced. Human resources can't. So as automation and better accommodation are introduced so the autonomy can extend from the usual 30 days for frigates to 45 or 60 or more, provided replenishment at sea. A ship can be designed to maintain indefinite function with sufficient living space and only basic resource replenishment. When autonomy is increased then more fuel is useful for greater speed or longer range.
Another reason is silencing. There are many ways to silence a ship, but one of the most fundamental ones is placing vibrating elements directly onto high density/mass elements to dampen the vibration via momentum conservation.
Think of a heavy hammer hitting the ground vs the anvil. Ground will carry a low "thud" very far signifying a low frequency matter wave while the anvil will barely move producing high "ring". High frequency waves are easily dissipated in any medium so placing noise source on heavy supports will transfer that energy into high frequency noise which is better absorbed in water. And since engines are low inside the hull it also improves stability.
Rafting helps only so much and it has more to do with shifting/spreading the frequency of noise or shielding from random high noise like a breakdown than silencing. Submarines use rafting because more mass means going down, and subs don't have the option of making larger hulls to stay up because of pressure at depth. Surface ships have no such problem. If you need to add mass you add hull because it's the easiest and cheapest solution.
In ship design ultimately everything you do has to fit within the dynamic buoyancy model. If it does it's good. If it doesn't it's bad. This is why ships are designed "inefficiently" in terms of our land-based common sense. The very first thing that every ship designer does is trying to replicate "land" at sea. As I was once told by a naval architect:
Weapon systems are one of the easiest things to be added provided they don't change stability, and it is fairly difficult to affect a stability of a frigate with tens of tonnes of mass just above waterline and along the main axis of the ship. Weapons are a huge change in FAC and other small ships because they sit at edges of low profile/low draught hulls where tipping over at high speed is very likely. E.g. Tarantuls had absolutely awful stability at low speeds because of twin Termits on each side and radars higher up but at 30+ knots they had excellent stability.
All in all naval warfare is much more about keeping several thousand tonnes of steel with humans in it out at sea which is an extremely hostile environment to all things come of land than it is about shooting missiles and detecting targets. And this is why there's an entire culture developed by people who spend long time at sea. Sea is more dangerous than air. The only thing that kills you in air is the land coming too fast at you. There are many things that can kill you at sea and the further and longer you're at it the more you're exposed to all of them.
I used this image, in higher resolution, for quick reference:
Differences are visible but they mostly affect volume and volume is deceptive. Remember that 1 m3 of steel weighs close to 8 000t and a sphere with radius r=2 has 8 times the volume of a sphere with radius r=1. If the hull can displace 8x the volume of water you can put a lot of additional mass on your ship and that will still be a fraction of the mass of that displaced water.
So why do people think big ships are bad and/or expensive? Because big ships used to be expensive.
In the past all ships had to be built in situ because precision of design and construction was as good as your next slip in measurement. Try to build a 10m fence in your garden and move the angle of the fence one degree and see what happens. Without computers everywhere a mistake anywhere meant all the preparation was in vain because nothing matched. Now with computers as the standard tool we have precise design, precise construction elements, precise modules and precise machinery and quality control and that saves time which is money. Large ships were difficult to build because they required a lot of measurement and correction by humans and thus cost of ship size scaled geometrically. Now cost of size scales arithmetically or sometimes even less than that because if you have to include other design elements like signature reduction, acoustic silencing, future-proofing or serviceability then building a hull that is larger ends up reducing the overall cost of the ship because that volume allows for cheaper solutions in other areas.
Displacement is to ships what weight is to land vehicles or maximum take-off weight for aircraft. Ships are the intermediate between a land vehicle and an aircraft, they seem to work like land vehicles because water seems to provide stable support but in reality they are constantly "flying" in water.
Displacement is how much weight the ship carries while maintaining equilibrium in water. The confusion comes from the fact that displacement is measured as if it was mass which is a scalar, which both weight and displacement are not (entirely), even though they are measured in scalar (kg) rather than vector (kN) units. That's because it's a simplification of the model where weight/displacement/MTOW are really mass * g. The same thing in orbital dynamics is never treated in this fashion for the very reason.
Water can have different physical properties due to salinity which changes its density and therefore ships with displacement of 1000t in freshwater can technically have, say 1050t displacement in the Baltic Sea, 1100t displacement in the average ocean and 1200t in the Dead Sea.
Displacement is also influenced by hull shape and mass distribution which determine ship stability. Ship stability is more important than mass/weight because ships move in two ways:
- they move over/in a moving medium which applies dynamic loads to the hull in terms of buoyancy
- they move within their own structure which applies dynamic loads in terms of rigidity and load-bearing
Ships are also vessels with dynamic load because as ships go to the sea they are closer to full displacement while when they return to port they are closer to empty displacement. If the ship is deformed at the latter stage it may not remain stable at full displacement once it is fully loaded again and travels in high sea states.
Sea states are another thing - all hulls are designed for specific sea states. Look up what a dynamic model for ship buoyancy looks like. You'd be surprised how unstable these things are when waves grow only a little bit.
The reason why 054B has 7000t has to do with whatever has been done inside the ship above the waterline that likely affected stability more than weight, and required adding more mass as ballast. Increase in displacement is caused by the additional mass at the bottom balancing additional mass at the top. Displacement is all about center of mass and stability.
This is for example why may ships carry ballast near the keel. Broader hull with lower center of gravity will carry greater weight and therefore that ship will have greater displacement. The same hull that has approx. 4000t can be increased to say 8000t or perhaps more simply by concentrating mass within the boundary of geometrically optimal distribution which in a section would look something like a bowl which incidentally is a bowl because it has to be stable while partly resting on a table and partly "floating in air" while mass and momentum is being put inside it. This is why bowl is heavy everywhere but, heaviest at the bottom.
The new radar which is at approx 25m above waterline is heavier than the old radar. AESA has greater mass per element because every element is a transmitter and a receiver and needs individual controller unit. Because of the energy it also needs cooling which usually means some combination of air and liquid cooling to all elements e.g. air cooled array and liquid cooled processing units. Another issue is information transmission - while the computers in CIC may use optic fibre the radar likely does not and latency is an issue, so the shortest possible routing requires cooling close to the array so as not to drag slow cables into more convenient location. And then there's the issue of maintenance and repair access. For example AN/SPY-6 can be fixed from within the ship allowing repairs at sea while engaged in combat. I expect 052D and 055 to be similar. 054B radar as well as say Type 45 radar are not viable for repairs at sea or in combat because the array is difficult to access but that's why 054B is not an air defense ship. It can lose its air radar. A modern AAW ship can't. Ever. For those reasons high performance AESA systems as a functional design element - which is how they are treated in the design process - are significantly heavier than traditional radar like the one on 054A.
Another reason for greater displacement may be simply greater mass of fuel that was not necessary for 054A mission profile due to other limitations. Ship autonomy is always limited by the crew first and foremost. There's no need to waste space for fuel is your crew can't operate efficiently past X day limit. Fuel can be easily replaced. Human resources can't. So as automation and better accommodation are introduced so the autonomy can extend from the usual 30 days for frigates to 45 or 60 or more, provided replenishment at sea. A ship can be designed to maintain indefinite function with sufficient living space and only basic resource replenishment. When autonomy is increased then more fuel is useful for greater speed or longer range.
Another reason is silencing. There are many ways to silence a ship, but one of the most fundamental ones is placing vibrating elements directly onto high density/mass elements to dampen the vibration via momentum conservation.
Think of a heavy hammer hitting the ground vs the anvil. Ground will carry a low "thud" very far signifying a low frequency matter wave while the anvil will barely move producing high "ring". High frequency waves are easily dissipated in any medium so placing noise source on heavy supports will transfer that energy into high frequency noise which is better absorbed in water. And since engines are low inside the hull it also improves stability.
Rafting helps only so much and it has more to do with shifting/spreading the frequency of noise or shielding from random high noise like a breakdown than silencing. Submarines use rafting because more mass means going down, and subs don't have the option of making larger hulls to stay up because of pressure at depth. Surface ships have no such problem. If you need to add mass you add hull because it's the easiest and cheapest solution.
In ship design ultimately everything you do has to fit within the dynamic buoyancy model. If it does it's good. If it doesn't it's bad. This is why ships are designed "inefficiently" in terms of our land-based common sense. The very first thing that every ship designer does is trying to replicate "land" at sea. As I was once told by a naval architect:
Weapon systems are one of the easiest things to be added provided they don't change stability, and it is fairly difficult to affect a stability of a frigate with tens of tonnes of mass just above waterline and along the main axis of the ship. Weapons are a huge change in FAC and other small ships because they sit at edges of low profile/low draught hulls where tipping over at high speed is very likely. E.g. Tarantuls had absolutely awful stability at low speeds because of twin Termits on each side and radars higher up but at 30+ knots they had excellent stability.
All in all naval warfare is much more about keeping several thousand tonnes of steel with humans in it out at sea which is an extremely hostile environment to all things come of land than it is about shooting missiles and detecting targets. And this is why there's an entire culture developed by people who spend long time at sea. Sea is more dangerous than air. The only thing that kills you in air is the land coming too fast at you. There are many things that can kill you at sea and the further and longer you're at it the more you're exposed to all of them.
I used this image, in higher resolution, for quick reference:
Differences are visible but they mostly affect volume and volume is deceptive. Remember that 1 m3 of steel weighs close to 8 000t and a sphere with radius r=2 has 8 times the volume of a sphere with radius r=1. If the hull can displace 8x the volume of water you can put a lot of additional mass on your ship and that will still be a fraction of the mass of that displaced water.
So why do people think big ships are bad and/or expensive? Because big ships used to be expensive.
In the past all ships had to be built in situ because precision of design and construction was as good as your next slip in measurement. Try to build a 10m fence in your garden and move the angle of the fence one degree and see what happens. Without computers everywhere a mistake anywhere meant all the preparation was in vain because nothing matched. Now with computers as the standard tool we have precise design, precise construction elements, precise modules and precise machinery and quality control and that saves time which is money. Large ships were difficult to build because they required a lot of measurement and correction by humans and thus cost of ship size scaled geometrically. Now cost of size scales arithmetically or sometimes even less than that because if you have to include other design elements like signature reduction, acoustic silencing, future-proofing or serviceability then building a hull that is larger ends up reducing the overall cost of the ship because that volume allows for cheaper solutions in other areas.
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