Is speed of a ship limited by its propellers or the engine

Question

For both nuclear and diesel powered ship, an aircraft carrier for example ,

1. Is it the engine that can’t produce enough power to drive the propellers to get as much as speed you can get within structural limitations of given set of propellers
2. Or, Engine are fully capable,but propellers can’t handle such power without mechanical breakdown due to the structural limitations of real-world materials, i.e. Steel is that tough to handle that much load
3. Both are capable,but are toned down by us for economical reasons

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Context : The radius earth in this world is 3 to 4 times ours (9x-16x surface area). It’s moon a donut planet so there’s a gravity gradient along latitudes.

There are monsters upto 300-800m long, and some serpentile one go upto 1200m in low gravity equatorial regions.

Magic exist, but it requires conscious input of every little effect, like how voltage your lightning strike has (no automations like spells). So, not very reliable for continuous prolonged use even for expert. Materials like admantite are available. Civilisation like more tech advanced than us

So, this aircraft carrier needs to very huge to handle huge guns and tank giant monsters attacks. But fast to reach far away emergency quickly especially considering giant size of planet as fighter plane can’t go that far across the ocean on their own

Answer 1

You should read up on a concept called "hull speed".

For a vessel with a displacement hull (that is, anything that doesn't use dynamic force of motion through water to lift part of the hull out of the water -- like the planing hull of a ski boat or a hydrofoil) there's a maximum speed above which the power required to go faster increases very sharply, with most of the additional power going to plow a huge wake (relative to the hull size) rather than just go faster.

This speed is related primarily to hull length, at least for hulls that aren't grossly wrong (like the squared off hull of a barge). This speed is that at which the first wake produced by the bow just reaches the stern before dropping below ambient water surface height. For a modern American supercarrier, with a waterline length of around 300 meters, the hull speed is around forty knots (~ 68 km/h). To go faster, a Nimitz or Abraham Lincoln would need to be longer; even doubling the power of reactor and turbines wouldn't gain more than another knot or so.

There are online calculators that will approximate the hull speed for a given waterline length (this is an important consideration for yachts, freighters, tankers, etc.) as well as what power is required to reach that speed.

Worth noting, however, that many ships operate well below hull speed. A supertanker or container ship, for instance, has installed power sufficient to accelerate to cruise speed in a reasonable time (under an hour?), but will shut down some fraction of the engine (in some cases I'm aware of, three of the four huge cylinders will be closed off, both valves and injectors stopped so the air compression acts like a monstrous spring) in order to maintain cruise on minimum fuel consumption -- because it takes far more power to accelerate a huge mass of ship and cargo (or ship and ammunition and aircraft and their fuel and crew and provisions and so forth) than it does to maintain an efficient speed.

Answer 2

Both. For best efficiency, the screw must be designed to fit the speed and power of the engine and the shape of the hull. Overly fast propeller rotation can lead to cavitation, which was a problem especially with early turbines before there was gearing.

An oil-fired warship would be designed to cruise at much less than top speed to get a good range/endurance. A nuclear (war)ship or a modern oil-fired freighter might be designed with cruise speed close to the practical top speed -- the nuclear ship because it has the power, and the freighter because the routes and fuel consumption are very predictable and it makes no sense to design them with much 'sprint' capacity.

Longer distances on your planet might lead to more nuclear ships, even civilian ones.

Answer 3

I think this is an interesting question because the size of the ship would cause all sorts of issues logistically, how you would support it, crew it and support it. How deep the water would need to be, how maneuverable it could be and if there would be special ports to berth it. Just building and launching it would pose all sorts of story building issues. The question of power and speed seems relevant to all of the above.

There are several things to consider.

1. Drag from the water. This is typically dependent on hull area in contact with the water. This increases with velocity.
2. The length of the ship. This is because you lose energy generating a bow wave pushing water out in front of the ship and also a wake behind the ship. In the worse case the bow wave and wake constructively interfere for a certain speed and the energy loss is maximized at that speed. For heavy ships with limited propulsion this is the limiting factor.
3. The number of screws and how fast you can turn them. The number of blades and and shape of the blades matter some but ultimately cavitation becomes limiting (and can also erode the material of the propeller). Up to a point, you can increase the number of screws and increase the speed. A larger screw generates more thrust, but the larger radius means the linear velocity of the tip is moving faster and result in earlier cavitation.

The good news is that longer ships have higher top speeds, and that this effect dominates over the drag. Although having a clean hull or a dirty hull can still make a big difference of several knots. A 1 km long ship is about 3 times as long as a modern aircraft carrier, but you may want to think carefully about the width.

I think with point number 3, if you keep the aspect ratio the same you can have more screws for more propulsion. Although this might also be limited by the size and number of nuclear reactors you want to have. One per screw? So then you have all kinds of engineering complexity as to how you would coordinate the different reactors, what you would do if one reactor magical or nuclear had a problem etc. Other aspects of engineering plant could also be more limiting than just raw power. The shaft has to withstand the torque, usually there are reduction gears etc.

If you make the vessel more needle like then there may be some structural issues with what happens in heavy weather. Since depending you could have several wavelengths of wave under the ship. That would stress the hull. For a short boat you can ride the wave. For the very long boat you could be supported by two or more waves with sections of the boat being under a lot of compressive stress or tension depending on if supported at the ends or in the middle. This would ultimately become a strength of materials issue.

The choice of nuclear power or similar power density is probably necessary. The speed you want to achieve of 45 knots is higher than the published top speeds of US aircraft carriers. In general though as long as a carrier can move fast enough to launch airplanes, about 30 knots, there is not a lot of incentive for them to go that much faster.

The simplest calculation you can do, is to calculate the hull speed. The hull speed formula is the velocity in knots is 1.34 times the square root the length of the waterline. If you use that formula and the published length of a modern carrier (longer than the waterline length) you get about 44 knots. A more accurate way would be to calculate the Froude number.

So if you are worried just about speed and you can hand wave enough power, and perhaps some cavitation suppressors, I think you can probably get to your 45 knots. You can also play around with some different ways to minimize drag with anti fouling paint or special magical coatings.

Answer 4

It sounds like you are going several different ways here.

1. The above (excellent) answers seem to be speaking to non-fiction
   physics, which is what the first part of your questions seems to ask.
2. The second seems to be saying "how can I make a carrier such that one could water-ski (racing) behind, think 112 knots.

I will try to engage the second part of the question.

Options:

• Like Zeiss Ikon obliquely points out, a hydrofoil can bypass some of the hull-speed limit by lifting part of the hull out of the water. There are limits to this, as the US Navy's largest was only about 300 tons link which is nothing compared to the 100,000 tons of an aircraft carrier.
• The Russian Shkval allows a torpedo-missile to move about 4x to 5x faster than the fastest conventional torpedo by putting the bulk of the body in a vapor instead of a liquid. If heat or steam exhaust could be supplied in the opposite sense of boundary layer suction then you could give your max speed (assuming lots, but not infinite) power about a 5x boost, moving it from 33-ish knots to something between 132 and 165 knots.
• On a water-world there is a lot of water. If your technology has bottled very-warm-but-not-too-hot fusion, then an old-school Orion could be used to just give it stupidly massive thrust. It might be terrible on the wildlife even if it didn't use fissionables. To my mind it smells like a very big cousin of a Reaver-style ship.
• If the water could be frozen in front of the ship, then instead of hydrofoils, ice skates could be used. Ice skates have very low drag, but the load would be immense, and moving that kind of heat around is very challenging.
• Tuna are investigated because they use their wake for thrust. If the body of the vessel could vertically undulate, though possibly requiring catamaran form, it could potentially recover a significant portion of the energy spent moving the water out of the way of the ship.
• If you had a train of ships, then you could have one that was small but paid the majority of the wake-cost in energy, like a Vee of flying geese. This would depart from the perpendicularity of the catamaran, and have staggering of the tandem hulls. You would have to TRIZ-like separate it into bits in a spatial sense.
• I liked Buckaroo Bonzai and his oscillation overthruster, but I don't see how he was able to go through matter horizontally, but be supported by it vertically; they could call it a flashlight but how do you align the road to your tires or turn off the effect after the end of your car has gone through it. If you become invisible to electromagnetic fields acting in a particular directions, that means drag wouldn't really exist. Partial invisibility would mean lots less drag. A funny piece of this is called "optically induced transparency" so you can use light to bork up how matter interacts with light. That thing, just a bit better than today, might make partial invisibility, that is to say much less drag by making the fluid act much less dense or less viscous, because even though the matter is there, it becomes non-interacting. If you apply this only within the stream-tube, especially if you apply it near the boundary layer, then you might have something like the big blinky triangular modern UFO's, except for water. ;)

Best of luck in your creative work.