The largest and most profitable vessels can transit neither canal
No there are plenty of ships that can’t go through the Suez or Panama canals. The largest size of bulkers are known as capesize vessels because they have to go around the Cape of Good Hope and Cape Horn. In the tanker trades there are ULCC’s and VLCC’s which are too large for the canals (Ultra and Very Large Crude Carriers respectively)
The Seawise Giant was a ULCC not a container vessel.
But your mention of that vessel is a good illustration of the key point which is:
The Seawise Giant was close to a white elephant. It made few voyages and spent much of it’s time as a floating storage facility. There wasn’t a market for cargoes that large.
Kinda, but…
Container vessels are already divided horizontally. The containers in the holds do not support the containers on the hatches so there are effectively already two tiers. A third tier would avoid the first problem you mention but it isn’t practical for other reasons.
Crane height is a real issue but cranes could be made taller if that’s what ships required.
I’m not saying you are wrong but I’ve never heard that before. Wouldn’t it rather depends on the notoriously volatile charter rate market at any given time?
I don’t think this is correct. Surface area increases with the square of length, whereas volume - and thus the space available for propulsion equipment - increases with the cube.
Further, there is no static component of friction to resist the initial motion of a floating vessel. So even very modest thrust will “make it go”. (Obviously, if it’s huge and needs to go at a practical speed, the thrust will need to be well above modest.)
My comment about the curvature as a limiting factor was intended to be hyperbolic; as @msmith537 observed, the practical limitations of being able to enter harbors, transit passageways and canals, et cetera, is really the practical limiting factor.
I don’t know why there would be an inherent limit to the size of a spacecraft beyond being strong enough to not collapse under its own gravitational field or strong enough to resist whatever tidal forces it might encounter around a star or large planetary body, and these are limits you only start approaching at megastructure-scale vessels. If you had a vehicle so large that the applied torque of rotating it by thrusting transverse to a long moment arm from the center of mass would break it, you’d just distribute thrusters along its length to distribute the load an minimize banding and torsional loading. You would want to do that anyway because a large spacecraft is not going to behave like a rigid body and will display many complex modal responses (vibrations) that would make it nearly impossible to control without active damping distributed throughout its structure.
I suspect the actual limits to the dimensions of a spacecraft would be thermodynamic, i.e. having enough outward facing radiating surface to reject the waste heat produce by the propulsion and other power-generating systems. Something like the Death Star from Star Wars should probably be radiating like a small sun and would be unmistakable as an artificial object. It is a problem that is virtually ignored by nearly all science fiction but is actually one of the biggest system engineering and design challenges in even modest size real world spacecraft and satellites.
The issue isn’t the available volume of the craft but the area upon which a propulsion system could act, e.g. the width of the stern. Of course, you can always make a ship wider at the stern to accommodate more propellers or water jet outlets but that has other design consequences for maneuverability and performance as well as the aforementioned practical considerations of being able to pass through waterways.
If you treat water as a Newtonian fluid this is true; in reality, there are viscus and inertial effects that, while not significant on the scale of smaller craft would come to predominate with a very large hull. Notice that boats generally taper back from the beam (widest point) to the stern to minimize losses where the laminar flow breaks from the hull but with a very wide stern you would have a huge vacuum radiating from the centerline of the boat at the stern ‘sucking’ the boat backward as it tried to go forward. This is one of the reasons that multihull boats are faster despite technically having more linear waterline than a single hull. It should also be noted that while you can get a heavy boat moving slightly with even a small amount of thrust in still water, boats frequently operate in moving water due to currents and tidal flows, and so the propulsion system has to be capable of generating sufficient speed in excess of that expected flow in order to achieve net forward movement and maintain steerage.
Stranger
Well, the Death Star does have a 2 meter wide thermal exhaust port. Shooting a missile into it seems to cause the Death Star to radiate exactly like a small sun.
Think of the superluminal scissors. Consider some scissors that are a light year long and you snap them closed. Do the tips close at the same instant? No. They do not. The scissors will bend as the wave travels down their length.
The important part is any material MUST bend…even hypothetical-pretend material. Real world stuff absolutely bends in such conditions.
So, if something is long enough, it must bend.
On earth, big ships bend too. Heck, skyscrapers bend.
Sure, any real world structure flexes. Your car chassis has measurable flexure every time you make a sharp turn or go over a bump too aggressive for your suspension to completely absorb, and the vehicle structure and tolerances of things like doors and windows are all designed to accommodate that movement. It’s not a fundamental limitation, though; as previously stated, if you had a structure that was so long that applying thrust at one end would cause enough bending to exceed the material limits at another you’d just distribute the thrust along its length.
You would probably want to do that, anyway, because the flexure makes it difficult to predict the response and your autopilot would erroneously take the modal response as rigid body motion and try to correct for it; if the rate of that correction matched the frequency of the response it would create a positive feedback that would end up with the vehicle spinning out of control or tearing itself apart. If you had a vessel long enough that you actually had inertial effects resulting from hypothetical relativistic effects a la your superluminal scissors example then you’d almost certainly be exceeding the fundamental tensile strength limits of any real material anyway.
Stranger
No need to make it artificially wide - the width of the stern will scale with the other dimensions of the ship. Nor is the area for propellers limited to the area of the stern: check this view of USS Nimitz in drydock.
It’s already true that the largest ships can’t pass through any restricted waterway. Here, we’re discussing theoretical ships that are very much larger, and whether they can be made to move at all.
And yet existing very large ships move cargo more efficiently than their smaller sisters.
Yet the largest ships are essentially all monohulls. (Has there ever been a multihull large enough to put among the top 1000 ships, ranked by length or displacement?)
The area for propulsion is still essentially limited by the beam of the ship even if the props aren’t mounted at the stern; you couldn’t put a bunch of propellers or impellers in line with one another and get a linear increase in propulsive capability, and of course where you mount the propellers affects the maneuverability of the vessel.
Larger monohull bulk haulage and container ships are cost effective because it takes essentially the same crew to operate them and efficiencies of scale. Shipping is far more about cost control and efficiency than speed and in fact shipping operators often limit the speed at which their vessels operate to ensure that they don’t get racked up awaiting to get into port or too many empty vessels returning at once.
Stranger
But that means all of the ship faces drag from the water, where current ships have water drag on the bottom and air resistance on the top – overall, much less static resistance.
So wouldn’t a very long submarine face greater sailing inefficiency, thus increasing the fuel operating cost? Which is another limit on ship size – it has to be economically viable.
Another consideration about the dimensions of this monster ship. There is a formula between with and length that will determine the speed the ship can travel.
Unloading a monster ship will also be a problem. At one time In the SF Bay area when a jumbo tanker arrived in SF it could not unload at any of the refineries because a jumbo only go in to the SF Bay where it would have to anchor. While anchored it would unload cargo onto regular or Baby super tankers. And the smaller tankers would take the oil to the refineries.
Ironic given its silliness in every other respect, but this is one thing that Avatar got right. The ISV Venture Star dedicated a significant portion of the vehicle to radiators, which were run red-hot (which is bad for energy efficiency [acceptable given the antimatter fuel], but great for radiative efficiency due to T^4 scaling).
I hope I don’t need to say this… but this wouldn’t work. The same tides that pull the oceans would also pull the cargo.
Actually you’d probably want the radiators to be white hot, perhaps even molten droplets to maximize temperature and radiative surface. I don’t really know anything about the spacecraft in Avatar but if it is powered by antimatter there will inevitably be a vast amount of waste heat. How they actually convert the intense gamma rays from matter-antimatter annihilation
is another question entirely; most practical schemes involve using the radiation pressure to ‘catalyze’ (drive) high performance nuclear fusion or heat some kind of unobtainium material to provide both high thrust and high specific impulse, but it is pretty easy to poke holes in those proposed technologies.
Put the cargo on frictionless rails and give it a huge shove when the tides are starting to draw on it. Or…just have a big sealed annular vacuum tube around the Earth and chuck stuff in at orbital speed. When it gets to its destination it just gets shunted off to a linear decelerator that converts the kinetic energy back to electricity stored in giant ocean water superconductors to be used to accelerate the next shipment. I’ll have my team work up a presentation so I can take credit for the idea and promise to save the world in between tweeting about how unfair it is to tax billionaires and impose regulations on manipulating the stock market for my own benefit.
Stranger
It’s proton-antiproton annihilation. A very low fraction of the energy will end up in gamma rays. Most of it ends up in neutrinos, so the real trick is figuring out how to do it so they get a net direction preference.
Well, also figuring out how to get the antiprotons to begin with.
Sure, the hotter the better–my point is just that if you compare to, say, the ISS radiators at ~300 K, radiators running at ~1200 K would radiate at 256x the rate per square meter. That’s only reasonable if your energy source is so far above that temperature that Carnot efficiency does not kill you (i.e., nuclear or antimatter). And while I don’t know if they did any back-of-the-envelope calculations when sketching out the ship, it’s at least past the “obviously impossible” level in terms of heat rejection. They didn’t completely ignore the problem, unlike–as you noted–virtually all other sci-fi.
Bringing this back to the OP though, I don’t think heat would be an issue. The big ship has no obvious limits to length or breadth, but does have a limit on height for buoyancy reasons (assuming our ship has a relatively conventional structure and carries cargo). So we can assume an upper bound on the volume of ship per unit surface area (say, a few hundred cubic meters per square meter). Each square meter of the ship is sitting on its own column of water, and that water can be used for active cooling. Water has a tremendous heat capacity and so has an excellent cooling effect. You could even pump the water into an evaporative cooler for even more capacity.
I would assume that any scheme using matter-antimatter annihilation would use the eletron-positron reaction for the reason reason that you note, since the mesons and ultimately neutrinos will be emitted in some random direction. (If you can figure out how to emit uncharged mesons in a specific direction and control their decay you’ll have an unstoppable weapon that can sploosh people from the inside out.)
I don’t know where we find a source of bulk positrons ready for use other than making a bunch of 11C and waiting for β+ decay. Probably not a practical enterprise at scale of interstellar propulsion but then, what is?
Stranger
Fair point - yeah, substitute any rigid material - it isn’t really rigid unless the speed of sound in that material is infinite (which is impossible)