Cruise Ship Stability

The Star Princess is due in Sydney later today and then here in my home port a few days after that. At 293 metres long she's the largest Cruise ship to ever visit Hobart - but it's her other dimensions that have me amazed.
65 metres from the water to the top - and 73 metres high from the keel. That means there's just 8 metres underwater. There must be some serious ballast and form stability involved here - why don't these things simply fall over!?!
I can only assume that the forces required to heel the vessel to the point of no return - what 40 - 50 degrees? - are so extreme that they are never likely to be encountered (hopefully... )

 

What's even scarier, Will, is that the metavcentric height of those things is no more than on most small boats. IIRC, the QE2 had just about 1 metre of GM. This is what makes the things possible, as the motion on a big mother like that would be horrible if it were any higher.

 

They have roll damping tanks.

 

Some of the older ships had a problem and required modification when the IMO made use of the same bottom tanks for fuel and water ballast illegal. The practice of using the same tanks for both had resulted in residual fuel oil being pumped into harbors with the ballast water just before ships were refueled. Each bottom tank must now be designated either seawater ballast or fuel oil. As fuel is used the ship still takes on seawater ballast, but in a tank designated for that purpose. Each passenger vessel must have a new stability study completed every 5 years. I've been involved in four or five of these studies. I'd be interesting if you asked the ship's officers to let you see the stability book to see if they let you.

Steve Baker (SailDesign) is right about GM req'd being largely independent of size. Weird, huh.

 

Steve D,
I used to work for a company called Keel Marine in England, and our bread and butter at the time was stability booklets. Everything from Grain ships to Crane Barges and trawlers.
Some of them were pretty scary, especially when you hit on a new breed of boat, like the converted VLCCs that carry sheep from Australia to Arabia, where you have to account for feed going "upstairs" while dung goes "downstairs" and some sheep don't make it while others gain weight and they all try to leave the ship at the same time (causing one to capsize with a massive loss of (sheep) life at the dockside.)

Happy memories....... I don't touch ships now if I can avoid it.

Steve

 

What is a VLCC?

 

VLCC = Very Large Crude Carrier. i.e. a smaller SuperTanker (a giant tanker would be a ULCC for "ultra" large...)
Frikkin' acronyms - too many of them around these days.

Steve

 

Seems they need to be loaded verrrrry carefully. This one wasn’t.
http://albany.bizjournals.com/albany/stories/2003/12/08/daily31.html

Gary

 

I agree that a cruise ship must have incredible ballast beyond water.
The density of ballast tank water would be the same. I would suspect that lead or other high density solids are below. You don't hear much about what is below. We know that the turbines and other generators are placed low. There is a lot of use of light fiberglass and wood panels (cabins) on the superstructure. The superstructure is also largely hollow, empty space.

The hallmark of most modern cruise ships is their shallow draft when compared to the older liners.

 

Dont forget active stabiliser systems. A lot of modern high sided vessels would be sunk without them. Literally.

 

Stabiliser systems are used on ships because it pays to use them. For cruise ships it means happy passengers spending money inthe shops and bars. For Container ships it means not having to slow down or alter course to reduce rolling and G forces on their high value and highly stacked cargos.

Now if only they had had them in the Posieden Adventure. I don't recall seeing any on the upturned hull.

Cheers
David Gray

PS How do I get some of that stability book work from Keel Marine?