Why would you launch a large ship by dropping it sideways?

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I'm referring to the process shown in this video:

https://youtu.be/Quyr5R1Rbfw?t=20

Or this image from Wikipedia:

enter image description here

In it, a large warship is launched into the water by essentially dropping it sideways down some ramps and off of a pier. The ship rolls hard to one side, and then oscillates back to the other, making the process seem like a fairly risky one. For instance, if it rolls back towards the pier too aggressively it might strike the structure and cause damage both to the pier and the shiny new warship. Or if it rolls too far on the initial drop, the ship might capsize.

So my question is, what are the advantages of launching a large ship sideways like this, as opposed to, say, dropping or gently lowering it vertically into the water?

aroth

Posted 2016-02-21T00:46:55.283

Reputation: 208

8If the ship capsizes from such a launching, it's probably not going to do too well out in rough waves in the middle of the Atlantic in a storm.ceejayoz 2016-02-21T19:01:33.203

Answers

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A specific reason for doing this is simply when there isn't enough room to do a bow or stern first launch. This is often the case when a ship or boat is built in a yard on a river or canal either because the hull is especially long or the channel it is being launched into is narrow. There is also the consideration that a sideways launch can be done from any quay that will take the weight but a bow first launch requires a specially constructed slipway.

There is also the consideration that if you go in sideways the load is distributed along the whole length of the hull whereas bow first has the potential to put a big bending load on the hull.

In terms of risk that sort of roll should pose no problem at all for an ocean going ship and it is likely to be fairly heavily ballasted to compensate as well.

Small boats can be craned into the water but with larger tonnage hulls this just isn't practical due to their sheer weight.

Chris Johns

Posted 2016-02-21T00:46:55.283

Reputation: 12 837

3I don't have enough to add to this explanation to merit another answer. It is, however, worth noting that this lateral slipway pier's toppling hull support brackets hinged into a protective position immediately as the ship left them. From that position, those (probably intentionally energy absorbing) brackets were fairly well placed to protect both the ship & the pier from all but the most vicious of plausible launching impacts.Robherc KV5ROB 2016-02-21T06:21:24.093

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End to end, it would bend and snap in half under its own weight. I know it's just a movie, but watch "Titanic" for a visual. The support required otherwise would be huge and expensive. A boat/ship will naturally rock side to side without breaking.

Impact with the water will act as a brake and keep it from rocking too far. The weight of the ship will cause it to sink more and more, adding to friction with the water. The water does not want to compress downward so it surges upward into a wall. Gravity pulls the wall back down, even though only part of it is still pressing backwards on the ship.

Physics will also fight it rocking back against the dock, inertia and gravity. Think about a bolt in your ceiling attached to a weight, like a bowling ball with a rope. Pull it back from the center where it hangs, to your face, and let it go. It swings out away from you, and then swings back towards your face. It will not swing back far enough to hit you because gravity is pulling it down, and friction with the air slows it down. (Water has even more friction than air!) Internal friction of the individual strands of the rope against each other also comes into play, but does not illustrate the point enough to mention in great detail.

Ronk

Posted 2016-02-21T00:46:55.283

Reputation: 151

1I don't think your bowling ball analogy works. The ball starts displaced but with zero velocity, whereas the ship starts vertical but with non-zero velocity. The analogous question for the bowling ball is, "Ronk has a bowling ball hanging from the ceiling. He pushes it sideways. It's going to swing back towards him. How does he know it won't hit him in the face?" The answer to that question has to be based on properties of the bowling ball and how far the ball would swing back: it can't be answers just by saying "it's a pendulum" because energy was added to the system.David Richerby 2016-02-21T17:37:02.913