Out of curiosity I looked up some cruise lines that still offer transatlantic crossings. Obviously nowdays nobody who picks an ocean liner over flying is concerned about speed or cost. The crossings I found took about a week or more. On her maiden voyage in 1952 the SS United States made the trip in 3 and a half days. If for some reason airtravel was taken out of the picture and speed actually mattered for ocean liners how fast an ocean liner could we build?
With a standard hull, not much faster. Any traditonally-hulled ship has an optimum speed, called its ‘hull speed’, which is related to its length. After that speed, drag starts to build up rapidly and it becomes very energy-expensive to go faster.
Modern hull designs have improved this somewhat, but there are still limits: The Titanic had a top speed of 23 knots. The Queen Mary 2, a completely modern design with a wave-piercing bulb keel and computer-designed hull, has a top speed of about 30 knots.
Even modern warships aren’t much faster, because of the limit of the displacement hull design. A nuclear powered aircraft carrier like the Enterprise might be able to do about 35-40 knots flat out, but that’s about it.
So if you want to go faster, you have to look at non-traditional hulls - catamarans, hydrofoils, etc. There are a few catamaran luxury yachts.
The Hales Trophy is awarded to the fastest ship crossing the atlantic.
The United States (the ship) held it from 1952 to 1990 when its 3d 10h crossing was beaten (just) by an English Channel catamaran ferry with 3d 8h.
Current record is held by the HSC Fjord Cat, an ocean-going catamaran, with 2d 20h.
The Unites States never made the trip full out with all the nozzles open on the main engines. In theroy her top speed should be in the range of 42 knotts. Until 1970 her hull and engine room was classified.
But when you chock & chain a few F-14s at the stern of the flight deck, and they go to afterburner …
… they supply a lot of power for a short period of time, before running out of fuel. By which time the Enterprise has sped up by an amount barely noticeable. Is my guess.
Enterprise carries 8,500 tons of aviation fuel (12 days flight operations).
The ending smiley indicated I wasn’t serious about the scenario increasing the speed.
But it would look cool at night 
Sooooo, which one of us is hooking up the refuelling hose, while the plane is standing on deck with the afterburner running?
I vote you 
Is there anything active that can be done to surfaces to reduce their drag? - I don’t mean just making them slippery, but stuff like covering with microscopic vibrating tiles, or jetting out a froth of tiny bubbles (appreciate that could affect buoyancy).
There are things that could be done and are easy but gain only some knots. If you double the engine power of a ship going 25kn you get to maybe 30kn. Also, much of the hull design is concentrated on building fast and cheap because the extra knot does not bring revenue the same way getting your ship cheaper and faster.
This is a rough sea area so the optimization can not be made for pure speed. Competing with speed would be quite a gamechanger for cruiseline industry. Nowadays the idea is to give people a lot of time to dine and buy drinks.
Is the question purely theoretical, or are we concerned with practicalities/economics? If the latter, well, the operators of current ocean liners seem to have decided that a practical/economical cruise speed for transatlantic service is about 30 knots.
Clearly faster speeds are possible; it’s simply a question of how much power you want to throw at the problem. For form drag, power requirement scales with the cube of speed: double the speed, you need 8X the power.
So how many nukes and screws can you add to the QM2? Right now they have only about 60% of the power generation capability of a Nimitz-class aircraft carrier, and it doesn’t all go to the screws. And even a Nimitz probably has room for extra nukes. Turn the lights out on the ship, reduce the passenger/luggage capacity so you have room for extra power plants, line the hull with big props tuned for fast running, and I would guess you probably could double the top speed.
Passengers would need a home equity line of credit to be able to afford the ticket though.
I am not a marine engineer, but I was in the U.S. Navy.
There are a few things that can be done to speed up ships that haven’t been mentioned. Most of these aren’t really applicable to something the size of a cruise ship, though.
One is to use a catamaran hull instead of a monohull. There have been some mega catamarans built that can reach speeds of up to 50 knots or so. The HSV-X1 Joint Venture was a high speed catamaran that operated as a troop transport. It had a displacement of 1,668 tons and could operate at a speed exceeding 40 knots. It was ocean-going and could ferry up to 325 combat personnel and 400 tons of cargo up to 3,000 miles.
Another option is to use a planing hull instead of a displacement hull. A planing hull uses the speed of the ship to lift it so that the draft of the ship decreases as the speed of the ship increases. While this is often used with small motorboats, it’s not very practical for a cruise ship.
Another option is to use a hovercraft (air-cushion vehicle). Some of these can cruise at speeds up to 60-70 knots, but they max out at a few hundred tons. Here’s an example of one. Not particularly useful for an ocean liner.
The last alternative that comes to mind is a submarine. Modern submarines with teardrop-shaped hulls do not have to deal with surface drag, and are capable of very high speed submerged operation. The unclassified maximum speed of U.S. submarines is currently stated as “greater than 25 knots.” Some of the larger military submarines that have been built are comparable in size to an ocean liner, but there’s not much of a view, and there aren’t any balconies or even portholes. 
A hovercraft could do it, if it weren’t full of eels.
Hey! Let me do it! Sounds like fun!
Is there anything I need to know?
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The Russian “Alfa” class nuclear submarine could reach 42-45 knots, so they say.
The hull design efforts for cruise ships today is focused on a smooth, stable ride. When you’re spending about $1,500-2,000 a person to go nowhere fast, you don’t really care how fast the ship can go - you just care that you get to Aruba and Half Moon Cay, or wherever it is the travel agent said you were going to, and that the ship doesn’t list so badly that you spill a drink.
To that end, they’re using things like bulbous bows and active stabilizers. To help keep things calm and quiet in the dining rooms, which are often on the aft lower decks, some ships are using air bubble curtains along the hull surface to absorb noise and vibration from the propellers. (More likely, they’re using Azipods instead of props and rudders.)
The ship I was on last year had all of these new tech goodies, and it was indeed, a very genteel ride. Actually, the trip was fantastic, but the ride itself was a bit boring, but then, I’m one of those weird people who hope for a bit of turbulence when they fly. Nothing drastic - just enough to remind you that you’re moving.
Yes, by reportedly omitting “unnecessary” heavy components such as reactor shielding. :dubious:
They also had very small crews, which complicates maintenance and damage control.
Since we are stuck with “Ocean Liners” as part of our requirements, we have to assume we are talking about cruise ships.
We can look at modern cruise ships, not think outside the box, and determine that we are somewhere near 40 knots… give or take?
The only way to plane a cruise ship is to get it a flat-bottom planing hull, but that’d be big and it would be slamming everyone. It might plane though! SLAM SLAM SLAM over waves, since there is no V to cut through 'em. The crystal would last five minutes on a planing hull like that. 
Forget a V-shaped planing hull. Would require far too much power to lift the bow, then the stern, and climb over the bow wave.
Think like this: Cat style, with a dynamic balanced passenger deck with something along the lines of high-tech stability control that counters against the sudden up/downs.
Could we build it now? Nope.
What are the possibilities of an ocean going hydrofoil?
30 years ago when I was in Cozumel, they had a wicked big hydrofoil ferry making the run between there and Cancun. Maybe half as big as a smallish cruise ship of that era (I saw them together)
I think we could build it today if someone really wanted to. They have gun stabilizers on tanks that hold the gun rock steady while the tank is bouncing all over the place. I see no reason this couldn’t be done on the ship as you describe.