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A week ago, 2 of the most gigantic cruise ships in the world docked near my city. If you have seen one, or been on one, you will know how large they are.

Giant Cruise Ship

They look extremely unsafe to me, although obviously safety features must be built into them to stop them listing so far over that they seem to me to be in danger of toppling completely in rough seas or strong sidewinds.

I want to avoid the engineering side and to stick to the physics, which is simply based around the control of the angular momentum of the large vessels.

So my assumptions first, then my question.

I have searched Google for schematics of the design, but nothing jumped out at me, except a greater than expected list of people asking much the same question as this one. Are Cruise Ships Too Top Heavy? and Why Mega Cruise Ships Are Unsafe

They have shallow, but wide, bottom surfaces. Either they have small keel surfaces, or they are able to pull the keel inside the ship, because the draught in my city port is fairly shallow and yet they were able to get well inside the harbor.

I guess they carry a large amount of ballast, they certainly have the room for it.

I also guess that the top floors are made of light material, so as to lower the centre of gravity as much as possible.

They almost certainly have stabilisers, that act to reduce listing and basically do the job that a keel does for sailboats.

My question is, does anybody with experience in fluid dynamics or related areas know what keeps these giant ships stable in roll and reduces their potential to list to large angles in bad weather?

EDIT I am hoping for a physics based answer, but I realise it may be a question for another site, I will migrate no problem if need be END EDIT

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    $\begingroup$ This blog is interesting onlyinamericablogging.blogspot.com/2012/01/… $\endgroup$
    – anna v
    Commented Jul 27, 2015 at 18:39
  • $\begingroup$ @annav thanks Anna, I put it on the post. I saw a few nautical guys comments saying they would not sail on them, but found no schematics or blueprints of the undersea sections. $\endgroup$
    – user81619
    Commented Jul 27, 2015 at 18:56
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    $\begingroup$ Looks can be deceiving. TL;DR: the center-of-mass is nice and low down. $\endgroup$
    – Carl Witthoft
    Commented Jul 27, 2015 at 21:21
  • $\begingroup$ And BTW, all ships do their best to stay perpendicular to storm waves. A large wave which attacks directly from port or starboard will cause much greater roll, with far greater internal damage even if the hull doesn't pass the capsize angle. $\endgroup$
    – Carl Witthoft
    Commented Jul 27, 2015 at 21:23
  • $\begingroup$ @CarlWitthoft this is what you don't want your luxury cruise to turn out like youtu.be/deX7R9RbmX0 $\endgroup$
    – user81619
    Commented Jul 27, 2015 at 22:59

4 Answers 4

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Some dimensions I was able to dig up (mostly from Wikipedia).

Draft of the Allure of the Seas: 31 ft (10 m)
Length: 1181 ft (360 m)
Beam at waterline: 47 m Height: 72 m above waterline

Let's just draw the section based on these simple numbers:

enter image description here

Now if the center of gravity were in the middle of the ship (31 m above the water line), it would indeed not be very stable - any tilt beyond 25° would cause it to tip over:

enter image description here

However, there are several important factors:

  1. The part of the hull below the surface is made of much thicker, stronger, heavier material than the superstructure
  2. The engines etc. are all in the lowest levels
  3. There is an active ballast system that allows pumping fuel and sea water from side to side to help maintain balance

I found one online test of the stability of the hull of this ship in a test facility, where they made a big hole in the side of an accurate model to see how it would fare: https://www.youtube.com/watch?v=Ra4TkHOs4RE . While there are commercial interests at stake, nobody wants a marine disaster on their hands.

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  • $\begingroup$ Thanks very much Floris, I had thought of two modern developments they could take advantage of, the active ballast idea you mention, but I thought the pumps would be too slow to respond, or vertical underwater thrusters, to force the down side back up. $\endgroup$
    – user81619
    Commented Jul 27, 2015 at 19:43
  • $\begingroup$ @AcidJazz most of the active devices are there for comfort. If all engines fail the only thing left is the form stability which requires the low center of gravity. This is where light superstructures come into play. Worth estimating how much the cog shifts when all passengers go to the top deck. 200 thousand tonnes is a lot of mass- 6000 passengers are a few % of that at most. $\endgroup$
    – Floris
    Commented Jul 27, 2015 at 19:46
  • $\begingroup$ "the part of the hull below the surface is made of much heavier material": indeed, they even put a massive metal bit there called keel. See en.m.wikipedia.org/wiki/Keel . Excellent answer btw +1 $\endgroup$
    – Wolpertinger
    Commented Sep 9, 2016 at 5:16
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    $\begingroup$ @Numrok thanks. As far as I know, these large ocean liners often don't have a real keel, just ballast. If you google"cruise ship sectional drawing" you will see this clearly. $\endgroup$
    – Floris
    Commented Sep 9, 2016 at 11:01
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I doubt it is all fluid dynamics. The have to stay upright even with dead engines. If the integral of the lever below the water line is bigger than above then it should stay upright. Ballast at the bottom goes a long way as it has a long lever. Stuff like engines below deck tends to be heavy anyway. Weight is not a big deal as they are not going up hill. They need a low draft to get into port. If you looked at an aircraft carrier out of water it would also appear top heavy.

Let assume a ship with 2/3 above water line. Break the up in 9 units. 6 above and 3 below.
Naturally lightest stuff on top and heavy below. In this example the net lever above is 45 and below 180. 

Height Weight Lever 
6      1       6
5      2      10
4      3      12
3      4      12
2      5      10
1      5       6
net           56
1      10     20
2      20     40
3      40    120
net          180

Imagine that is a teeter-totter with 3 fat kids on one side and 6 skinny on the other. Even if the skinny kids have twice the lever the fat kids still dominate.

Since it pivots on the hull and not the center line the bottom loses leverage as it starts to list. So a narrow hull at the water line is a good thing.

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  • $\begingroup$ Your aircraft carrier point is spot on, I should have thought of that, I saw a quote that said these ships "snap" back to vertical so quickly that passengers complain it's worse than the original listing effect was on their stomachs. Thanks very much for your time. $\endgroup$
    – user81619
    Commented Jul 27, 2015 at 19:48
  • $\begingroup$ Even though fat kids may look twice as heavy as skinny kids, due to their similar bone structure, which has a higher density than fat, they rarely are. Sorta like cruise ships and aircraft carriers, really. ;-) $\endgroup$
    – Mantriur
    Commented Dec 4, 2018 at 12:30
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As I understand it many seagoing vessels also use gyroscopes spinning on a vertical axis attached to the main components of the vessel's structure to null out much of the motion on the vessel's roll axis. Gyroscopes are also popular among motor yachts. I'm not Shure if they're used on giant cruise ships or not but, gyros are one way of providing stability, and safety.

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  • $\begingroup$ well, this does somehow miss the point, gyroscopes may give safety in the navigation-sense, but they definitely do not stabilize in a way meant in the question $\endgroup$
    – Ilja
    Commented Oct 19, 2016 at 16:09
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a large lever attached with a ship will give a huge effort on other side that it can heat the water by driving an engine made of dc motor and emf give to heating element which operate on dc voltage so it may work on either side to produce steam engine can produce electricity at a constant speed to provide for grid operation.

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  • $\begingroup$ I cannot parse this answer and from what I can understand, I don't see how this answers the question. Could you maybe rewrite and indicate how it answers the question? $\endgroup$
    – Martin
    Commented Oct 30, 2016 at 10:31