Super Ship-Carrying Ship

[lpetrich]

What cruise ships do when they need to hitch a ride | CNN Travel

Constructed in 2012, the 275-meter-long Boka Vanguard is the largest ship of its kind. Typically it transports large offshore oil and gas structures, but occasionally it will be called on to to carry other vessels -- such as happened with the Carnival Vista back before the pandemic.

The Carnival Vista is the second largest in Carnival's fleet, with capacity for 4,000 passengers and 1,500 crew, and in July 2019 it needed rescuing after experiencing a malfunction in Caribbean waters.

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This peculiar-looking vessel has a flat, open central deck -- completely empty -- with five large vertical wall-like structures protruding from its sides. The control bridge and the crew's living quarters are nested in one of them, which is considerably larger and taller than the other four.

Then about how it works. It has several ballast tanks which can be filled with water and later emptied.

In the float-on stage, the ship's ballast tanks are filled, lowering its main deck for its cargo. Then its cargo is moved onto its main deck. When that is done, the ballast tanks are emptied, raising its cargo for transport.

In the float-off stage, the procedure is repeated, this time with its cargo removed from its main deck.

Ship-carrying isn't among the most common assignments for this type of ships. The bread-and-butter work is moving platforms and heavy equipment around the world's major offshore oil and gas fields. But you never know.

In the case of the Carnival Vista, the need to call in the Boka Vanguard arose when the ship's propulsion system malfunctioned and the only dry dock in the region wasn't available. An alternative solution had to be found.

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The Boka Vanguard is not only the largest ship of its kind, making it possible for it to comfortably contain the 325-meter-long Carnival Vista within its structure, but it can also be used as a moving dry-dock, making it possible to conduct maintenance on the ship while underway or in port.

This was the case with the Carnival Vista, a clockwork operation assisted by multiple smaller vessels, that has provided us with mesmerizing footage of the whole sequence.

The article has a picture of the Carnival Vista atop the Boka Vanguard. This was for maintenance for the cruise ship's azimuth thrusters, which had mechanical trouble.

 Carnival Vista
 BOKA Vanguard

 

[lpetrich]

One has to watch out for something weird in ship statistics: the "tonnage" is not a measure of the ship's mass but of its internal volume.

A ship's mass is often measured with the volume of water it pushes aside when it floats, something called the "displacement". From the water's density, one gets the ship's mass.

I will estimate the Carnival Vista's displacement. Its length is 323.63 meters, its beam or width is 37.2 meters at the waterline, and its draft or depth below the water is 8.55 meters. That gives a below-water bounding-box volume of about 109,000 cubic meters. The actual below-water volume of the ship is typically less than that, and I once estimated about 60% from calculations from various other ships. That makes about 60,000 cubic meters, giving about 60,000 metric tons or 60 gigagrams.

That's actually close to the stats of the RMS  Titanic, that ocean liner that caught people's imaginations by running into an iceberg and sinking in its first voyage in service. It had length 269.m, beam 28.2 m, draft 10.5 m, displacement about 53,000 metric tons, and it was the largest ship of its day. But while the Titanic had 8 decks, the Carnival Vista has 15, and while the Titanic could carry 3327 people, the Carnival Vista can carry 6427 people, both numbers including the ships' crews. The Carnival Vista's top deck is roughly the height of the tops of the Titanic's funnels.