Luxury Liner SS United States Cannot Be Put Back In Service (miamiherald.com)

tomhath writes: Once the fastest ocean liner ever built, the SS United States has been mothballed for almost 50 years. An ambitious project to refurbish the SS United States as a luxury liner has been abandoned due to insurmountable technical and commercial obstacles. Plan B, to turn it into a floating hotel/convention center, might go forward. Miami Herald provides some history of the SS United States in its report: "The iconic 1950s vessel, which was bigger than the Titanic and once carried celebrities across the Atlantic Ocean, was set for a $700 million overhaul by the Los Angeles-based luxury line, which also has offices in Miami. The SS United States was decommissioned in 1969 and has been gutted and docked in Philadelphia for two decades on the Delaware River. On its maiden voyage in 1952, the ship traversed the Atlantic in three days, 10 hours and 42 minutes -- a record it held until 1990."

Comments

Re:Actually, in this case...

[rmdingler]

At least one.

Re:Actually, in this case...

[daremonai]

Also the above is not helpful at all: 32 knots is about 60 km/h. That's a quite low cruise speed for a car, for example.

But a lot faster than the average car's cruise speed on water, though.

Re:Actually, in this case...

[Solandri]

I've got a small speedboat that will do slightly over 40 mph on a calm inland lake, the notion that this can cruise at a similar speed is astonishing giving its size and open ocean conditions.

It's not that astonishing. The main speed constraint on a displacement hull like an ocean liner's is the bow wave. As a ship moves forward, the water it pushes aside has its pressure increased slightly, so it bulges upward at the bow. What goes up must come down, so this bulge eventually drops down to sea level, then overshoots and drops below sea level. This is called a bow wave. The key here is that this motion of this wave is dictated purely by the physics of the water (and the water depth, but that effect is small enough it can be ignored in the ocean). And that the front of this induced pressure wave is stuck to the bow of your ship (it's a standing wave when viewed from the ship), hence why it's called a bow wave.

I'll skip the math, but the net effect is that at slow speeds, your ship is moving through multiple waves of its own creation and stays relatively level. But at a certain speed called the hull speed, the wavelength almost exactly matches the length of the ship, and the bulk of the ship's mass sinks down into the trough of its self-induced bow wave. At that point, your ship is basically trying to power itself "uphill" through the water (opposite of surfing), and the energy required to move faster increases dramatically.

There are two ways to bypass this problem.

• Stop displacing water. That's what your speedboat does. At speeds above about 20 knots, it starts planing on top of the water, instead of forcing its way through it. This lifts the hull out of the water, and thus no more standing wave problem.
• Make the ship longer. The longer the ship is, the faster it can go before this standing bow wave lengthens to match the length of your ship. This is how displacement ships like ocean liners, cargo ships, and navy ships get around the problem. (Actually the nuclear powered navy ships can just increase energy output to power through this - it's not an absolute limit like the speed of light; and if you go fast enough the back of your ship climbs higher out of the trough so the energy requirement decreases).

This is also the rationale for the bulb underneath the bow of large oil tankers and cargo ships. It's location underneath the water slightly forward of the ship makes the water act as if the ship is slightly longer (the bow wave starts earlier), allowing it to eek out a tiny bit more speed at the same amount of wave resistance.