Cruise Ship Stability

fred henderson
1st June 2006, 20:22
Some Members continue to assert that modern cruise ships are inherently unstable, because of their height. I will try to explain in a simplified way, why this view is unfounded: -

1. Buoyancy
Ships float because the gravitational force that pulls the ship downwards is offset by the upward pressure of the water that is displaced by the hull. The magnitude of the upward force depends on the volume of the ship’s underwater body. All of the watertight volume of the hull and superstructure above the waterline provides reserve buoyancy. In cruise ships this reserve buoyancy is considerable.
A most significant factor is that the upward buoyancy is always exerted vertically, regardless of the attitude of the ship. The relationship between the centre of buoyancy (B) and centre of gravity (G) is the key to stability. When the ship is on an even keel the two are both on the centreline with G usually above B.

2. Intact Stability
As a ship begins to list, say to starboard, the underwater volume to starboard of the centreline increases and the port underwater volume reduces. As a result the centre of buoyancy (B) moves to starboard, although the position of the centre of gravity (G) remains unchanged.
As the upward pressure from B is always vertical, a line can be drawn up from B until it meets the centreline of the ship. This point of intersection is the Metacentre (M). As the angle of heel increases the location of M rises. Even if M is initially negative it will rapidly pass G. After this point, as long as bouyancy remains unchanged (See point 3 below) M continues to rise and the physical forces exerted on the ship increasingly work to return her to an upright position.

3. Depth
A very important factor in these calculations is the depth of the watertight area of the hull. Notice that this is depth, not draught. As the ship rolls, her waterline area increases. This in turn increases the upward water pressure and adds to the stability correction. This benefit is instantly lost if the watertight deck falls below the surface of the sea, which can happen in a low freeboard ship, such as a fully laden tanker. Once the deck on one side of the ship is below the surface, G is moved upwards by the weight of the sea above the deck.
The only condition where draft is important is if the ship rolls sufficiently to lift her bilge out of the sea. Once this happens the waterline area starts to reduce and buoyancy is correspondingly reduced.

4. Hull Shape
Traditional passenger liners had a very fine hull form in the forward half of the ship and a broader hull form aft, although in many, including all P&O and Orient ships, this was negated by very low open decks at the stern. The major part of the above water hull of all modern ships is parallel to the centreline, but below the waterline the shape is very similar to 1960s liners although fuller, with a smaller radius bilge and a flat, or near flat bottom. Modern ships often have a greater beam than their predecessors. These factors provide greater initial stability than earlier designs.

5. Roll Period
The period of roll is most important in relation to comfort. A stiff ship, with a fast roll period is most unpleasant in a seaway. The designer of a passenger ship is aiming for a long, slow, comfortable roll period. The height of a modern cruise ship, with weight well up, is an essential factor in overcoming the excessive buoyancy and stiffness of modern hull shapes.

As a final point, I feel that it is important to remember that most ships are optimised to perform a particular duty. A modern cruise ship is designed to carry its passengers in spacious and luxurious accommodation, in different areas of the world, at those times of the year when the sea will generally be kind to the passengers. The QM2 is designed for the worst North Atlantic conditions, but it would madness to design a standard cruise ship for these conditions. It would be like asking a fast ferry to operate throughout the winter in Baltic ice. Despite these design considerations, if the sea turns unexpectedly nasty, there will be no danger to the ship, but the passengers will experience the same discomfort that was a routine occurrence for liner passengers undertaking winter voyages in the past.

Fred (Thumb)

slick
2nd June 2006, 06:46
As a first trip Apprentice, in the late fifties, returning on the homeward voyage from Geelong with a cargo of Grain, I commented that the ship rolled very slowly I was told by more the knowing Apprentices that it was known as a"Tramp Roll" and that the GM was one foot or there abouts.
I understand that we also used to sail a Composite Great Circle the only time I have heard of its use.
Yours aye

Jan Hendrik
2nd June 2006, 08:16
Hi Fred, Thanks a lot for your technical explanations. Good stuff.
Some passenger vessels have stabilisers which are pulled out during poor weather.
What is your opinion on these?
Are they still being used with the modern cruise ships? I thought not.
Jan

fred henderson
2nd June 2006, 10:30
Hi Fred, Thanks a lot for your technical explanations. Good stuff.
Some passenger vessels have stabilisers which are pulled out during poor weather.
What is your opinion on these?
Are they still being used with the modern cruise ships? I thought not.
Jan

Very much still in use. Many sets are manufactured by Brown Brothers, Edinburgh - now part of Rolls Royce I think.

Fred

flyer682
2nd June 2006, 10:57
The majority of cruise ships which called at Lyttelton over the last season here were fitted with stabilisers.
Very interesting post Fred - thanks for it.

rstimaru
2nd June 2006, 15:26
Can you imagin the chaos if any thing did happen all those passengers and a lot of crew who cannot speak English, I remember that flaged ship of conveniance where the master and crew directed operations from ashore and left the passengers to fend for them selves

fred henderson
2nd June 2006, 15:57
Can you imagin the chaos if any thing did happen all those passengers and a lot of crew who cannot speak English, I remember that flaged ship of conveniance where the master and crew directed operations from ashore and left the passengers to fend for them selves

That is why I would always travel on a big modern cruise ship that is owned by one of the big three groups. No multi-billion dollar, stock exchange quoted corporation is going to spend in excess of $600 million per ship and employ untrained, incompetent crews. It is the old 500 passenger ships that are the safety nightmares. Bought for as little as $3 million, with a dozen previous owners and currently operated by a small company that is just the right side of bankruptcy. These are the owners that have to cut corners to survive.

Fred

Tony Crompton
2nd June 2006, 17:50
500 passenger ships that are the safety nightmares. Bought for as little as $3 million, with a dozen previous owners and currently operated by a small company that is just the right side of bankruptcy. These are the owners that have to cut corners to survive.

Fred

Fred, which ships do yoy mean? Surely you cannot describes ships like
"Marco Polo" "Discovery" the two Saga ships in those terms.
------------------------------
Tony c

newda898
2nd June 2006, 18:08
Very interesting Fred. Thanks for those definitions.

fred henderson
2nd June 2006, 19:01
Fred, which ships do yoy mean? Surely you cannot describes ships like
"Marco Polo" "Discovery" the two Saga ships in those terms.
------------------------------
Tony c

Tony.
Marco Polo belongs to Star Group, the number 3 of the big 3. Saga is a thriving financial services company that still maintains a healthy travel operation, which now has three ships. (Spirit of Adventure, ex Berlin, is their latest). So I do not mean the ships you mention.
I had in mind Eastern Mediterranean owners, many of whom charter their ships out to some of the smaller travel agencies. I saw an advertisment for one such ship 40 year old ship that was described as having "a very professional crew, with Russian officers, Chinese ratings and Philippino stewards." Perhaps it was very professional, but I would not care to put it to the test.

Fred

Tony Crompton
3rd June 2006, 10:46
I see what you mean Fred, but the ships I mentioned are 500ish Passenger ships all with previous owners and by no means new. I believe these ships nearly always are full booked.

I have just booked for another "Maritime Memories" cruise (our third) on "Discovery". The Multinational Crew are absolutely first class,and the ship is maintained to the highest standards.
----------------------------------
Tony C

billyboy
3rd June 2006, 11:09
to get back to stability, the old SS Brighton would take the biscuit for me, nothing for her to have her beam ends wet in a force 8. she had the most awful rolling motion. She would go over a long way and just sit there then flip right over the other way and sit there before returning to the other side. a real roly poly that one was.
Out here in the philippines we have ferries that have changed hands a few times, last one i rode on had three additional decks welded on to her, fortunatly the sea was like a mill pond at the time, she rolled quite a bit in the wash of a passing ship.
I have never experienced a cruise ship, an ambition i wish to fulfil soon i hope.

bob johnston
4th June 2006, 00:40
Some Members continue to assert that modern cruise ships are inherently unstable, because of their height. I will try to explain in a simplified way, why this view is unfounded: -

1. Buoyancy
Ships float because the gravitational force that pulls the ship downwards is offset by the upward pressure of the water that is displaced by the hull. The magnitude of the upward force depends on the volume of the ship’s underwater body. All of the watertight volume of the hull and superstructure above the waterline provides reserve buoyancy. In cruise ships this reserve buoyancy is considerable.
A most significant factor is that the upward buoyancy is always exerted vertically, regardless of the attitude of the ship. The relationship between the centre of buoyancy (B) and centre of gravity (G) is the key to stability. When the ship is on an even keel the two are both on the centreline with G usually above B.

2. Intact Stability
As a ship begins to list, say to starboard, the underwater volume to starboard of the centreline increases and the port underwater volume reduces. As a result the centre of buoyancy (B) moves to starboard, although the position of the centre of gravity (G) remains unchanged.
As the upward pressure from B is always vertical, a line can be drawn up from B until it meets the centreline of the ship. This point of intersection is the Metacentre (M). As the angle of heel increases M rises, rapidly passing G. After this point the physical forces exerted on the ship increasingly work to return her to an upright position.

3. Depth
A very important factor in these calculations is the depth of the watertight area of the hull. Notice that this is depth, not draught. As the ship rolls, her waterline area increases. This in turn increases the upward water pressure and adds to the stability correction. This benefit is instantly lost if the watertight deck falls below the surface of the sea, which can happen in a low freeboard ship, such as a fully laden tanker. Once the deck on one side of the ship is below the surface, G is moved upwards by the weight of the sea above the deck.
The only condition where draft is important is if the ship rolls sufficiently to lift her bilge out of the sea. Once this happens the waterline area starts to reduce and buoyancy is correspondingly reduced.

4. Hull Shape
Traditional passenger liners had a very fine hull form in the forward half of the ship and a broader hull form aft, although in many, including all P&O and Orient ships, this was negated by very low open decks at the stern. The major part of the above water hull of all modern ships is parallel to the centreline, but below the waterline the shape is very similar to 1960s liners although fuller, with a smaller radius bilge and a flat, or near flat bottom. Modern ships often have a greater beam than their predecessors. These factors provide greater initial stability than earlier designs.

5. Roll Period
The period of roll is most important in relation to comfort. A stiff ship, with a fast roll period is most unpleasant in a seaway. The designer of a passenger ship is aiming for a long, slow, comfortable roll period. The height of a modern cruise ship, with weight well up, is an essential factor in overcoming the excessive buoyancy and stiffness of modern hull shapes.

As a final point, I feel that it is important to remember that most ships are optimised to perform a particular duty. A modern cruise ship is designed to carry its passengers in spacious and luxurious accommodation, in different areas of the world, at those times of the year when the sea will generally be kind to the passengers. The QM2 is designed for the worst North Atlantic conditions, but it would madness to design a standard cruise ship for these conditions. It would be like asking a fast ferry to operate throughout the winter in Baltic ice. Despite these design considerations, if the sea turns unexpectedly nasty, there will be no danger to the ship, but the passengers will experience the same discomfort that was a routine occurrence for liner passengers undertaking winter voyages in the past.

Fred (Thumb)

Thanks Fred I was unsure how it all worked I am sure there are many members who do not understand the theory behind stabiliity. Modern ships look unstable and your last comment is very valid, once again thank you.

Bob ( Sydney )

R58484956
4th June 2006, 19:15
On the QE(1) we had 2 sets of stabilisers usually both sets out together, but on occasions only the front set, so you had the front half of the ship being kept upright and the back half rolling in the breeze. In the middle the rivets where grunting and groaning and weeping. When I asked once could we put 2 sets out the answer was " think of my pension laddie" Quite a common saying on the ship or "pass it over laddie" when changing watches.

Pompeyfan
4th June 2006, 22:49
Good posting as usual Fred especially as you are an expert in this field. I have never felt that the modern day cruise ship is unstable or could be in danger due to their height in bad weather. I would not go on them if I were worried about that. My argument is that Oriana for example which I have sailed on a few times now is not so good in similar seas to Canberra, and leans more when turning. I am no design expert like yourself Fred, but I know a ships movement when I feel it?!. In fact, I steered Canberra into port as part of my steering ticket and she did not lean over like Oriana when turning at Cowes. If she did I may have panicked a bit, but Canberra was as steady as a rock. I know that former passenger liners were designed for rough seas, but although I would never be conserned about stability, these tall ships like Oriana certainly roll more in a rough sea than Canberra for example. Oriana was rolling off Ryde. That never happened on Canberra, and I have been on her in all conditions off the Island. Living on the Island meant I always watched with interest when going out from my own deck unless busy in the hospital, and always interested in watching the pilot get off at the Nab Tower especially in rough conditions watching the pilot boat rock up and down which came from Ryde in thoughs days. It always amazed me how they got on and off without injury in rough seas.

So although modern day cruise ships are stable despite their height, and more luxurious than the former liners, I will never be convinced that the modern day cruise ship does not roll as much as ships like Canberra or indeed the old Arcadia which I was on in a force 12, and the Cyclone that destroyed Darwin. The old girl road both seas wonderfully well.

As you rightly say Fred, Queen Mary 2, the first passenger liner built since QE2 was designed for the worst North Atlantic conditions. Stephen Payne OBE the naval architect who designed her, and who you may know well went out of his way to say that she was designed for the North Atlantic as a true ocean liner and not as a cruise ship. However, friends who went on her first trip said she rolled in quite moderate seas whereas QE2 who they travelled on before did not roll in similar seas. So once again it would seem that height does make a difference?. David

David

david
5th June 2006, 01:05
Great to get a simple, non technical and jargon free explaination of a fairly complex matter.
I concur re the oldies still around, but how do you categorize the "Princess Danae", which I believe has had millions spent on her recently, still going strong at 51 years old!!
The centerspread in Ships Monthly Jan 06 shows a splendid small vessel.
Regards,
David D.

Piero43
5th June 2006, 10:08
Fred, a great post indeed, and very comprehensive.
About ship stability and roll period I would add some more consideration.
A slow roll period is index of a poor stability, although most people feels instinctively more safe in a slow rolling ship. As a matter of fact, a fast roll period means a strong righting moment, i.e. the moment that "pulls up" a ship when she lists.
The problem for the designer is to find a compromise between the safety and the comfort of the passengers; a solution is to keep the roll period fast, but to install stabilizers fins to stop the roll at an angle acceptable by the passengers, as happens today. Far from disappear, the stabilizing set are today installed on all passengers ships (two or three sets on bigger ones).
As for the listing of a ship during a turn at high speed, it has nothing to do with the "intrinsecal" stability of the ship, but is just an effect of the centrifugal force, like everyone of us could experience in his car driving fast on a mountain road.
The phenomenon could be unconfortable, but I never heard of a modern ship capsizing for this reason...
Piero

Jeff Egan
5th June 2006, 10:44
If you want to see a ship list when altering course try a Frigate at full speed when the wheel is put hard over as we had to do when they were on acceptance trials.

eldersuk
5th June 2006, 14:12
Do you mean like this?

Derek

spongebob
24th November 2008, 07:42
eldersuk-Derek, Just following through the old thread started by Fred Henderson re Cruise ship stability to find your post of a A/C Carrier doing a rubber burning tight turn.5/6/06.
Brings back memories of when I was a Dockyard apprentice and we went on full power sea trials on the Dido class light Cruiser HMNZS. "Black Prince".
We were thrilled by the 32 knots full speed, the hard helm full turn that virtually tucked the lower rail under then the command for full astern.
As I remember it took a while and about 4 miles before actually moving astern but the following wake caught up and swamped the Quarter deck.
Add to this a broadside or two with her 6 inch guns and it was a day made in heaven for a then teenager

GWB
24th November 2008, 11:44
Derek thats a great shot absolutely brilliant picture.

GWB

Shipbuilder
19th February 2009, 19:35
All that techinical stuff is beyond me and I didn't understand any of it and I certainly don't dispute any of it. Maybe a simpler explanation for similar souls to myself is: Regard a biscuit tin floating on water. It will draw about 0.25 inches, but have maybe three inches above water, but you can't make it capsize very easily! I liken these modern ships to "tin tray" shapes.
Spent several years sailing in WINDSOR CASTLE and always felt that she was safe enough and had a lot of hull under water. Finally, when she was scrapped and I saw pictures of her on the beach at Alang, I could see that most of the hull was actually above water and I was amazed that she could ever have remained upright in some of the weather conditions we experienced in Biscay. I had seen her in drydock of course, but she didn't look as alarmingly top heavy as when sitting on the beach out of the water!
I have no doubt modern designers know what they are about and eveything is as safe as can be, but I simply don't like them, inside or out and certainly have no plans to ever sail in one!
Bob

stoker
2nd October 2009, 14:06
Thanks for the notes on ship stability Fred, could you help me with the following ? when a vessel is rolling are the accelerating forces greater than the decelerating forces, or does it just feel like that ? My most frightening experence was when the Engine Room workshop bench came adrift on my watch one night. These things always happen at night, the vessel was sixteen years old and had made dozens of north atlantic voyages, but the welding had had enough and gave way.

PhilColebrook
15th December 2009, 18:36
Mildly interesting video clip on Youtube of the QM2 in a bit of rough weather, taken from inside. She seemed to be enjoying it. I look forward to crossing on her next year (Thumb)

http://www.youtube.com/watch?v=VpOmTEmX1s8

john strange
1st May 2010, 06:55
Can you imagin the chaos if any thing did happen all those passengers and a lot of crew who cannot speak English, I remember that flaged ship of conveniance where the master and crew directed operations from ashore and left the passengers to fend for them selves

On a Royal Caribbean cruise returning to Sydney. Last day at sea, abit overcast, wind from SE at about 30 knots, white caps but not big ones. Bit of pitch and roll. I sat on deck as far forawrd as I could get to enjoy it all, only person on deck. Paper bags were fitted at all companionways and not only were the bloods talking into them but many of the crew as well.

Pompeyfan
4th May 2010, 20:59
On a Royal Caribbean cruise returning to Sydney. Last day at sea, abit overcast, wind from SE at about 30 knots, white caps but not big ones. Bit of pitch and roll. I sat on deck as far forawrd as I could get to enjoy it all, only person on deck. Paper bags were fitted at all companionways and not only were the bloods talking into them but many of the crew as well.

Great post John. Many of us former crew enjoy seas like that while many passengers especially new cruisers are using the bags provided and many of the crew too, possibly working for some franchise on board while we are sitting eagerly forward with camera hoping for a great shot or just enjoying it as you say. rolling back those years?!. And great to hear words like companionways, few call stairs that these days, and passengers are no longer called bloods. Modern crew have no idea what either word mean. Thanks for the memories.

David

Shipace5
10th May 2010, 19:18
Thank you so much for helpful detailed info.

Shipace5
10th May 2010, 19:27
Tallink have two fast ships, usually ship not sail fast.
GTS Finnjet was really fast ferry. Its top speed ower 50 km/h. The ship not sail yet because it went in scrap metal at 2008. Damage of good ship...

I readed that Queen mary 2 are a big fast ship, It can sail vith speed 30 knots:)-It is admirable speed for so big ship.

Usually sail cruise ships slowly.

CAPTAIN JEREMY
18th October 2010, 11:10
Can you imagin the chaos if any thing did happen all those passengers and a lot of crew who cannot speak English, I remember that flaged ship of conveniance where the master and crew directed operations from ashore and left the passengers to fend for them selves

Technically it was not a flag of convenience. The ship, Oceanos, was Greek flagged and manned with Greek crew members, so by definition it was not a flag of convenience. Shortly after the event I sailed with some of the entertainers who were on board at the time and were instrumental in getting the passengers organised and evacuated after the captain and crew went ashore to "direct" the operation.

However despite all the requirements to have a common working language, and having crew members assigned to passenger evacuation and mustering with the ability to communicate with the passengers, it is still a cause for concern. Sadly it is not only the crew members who are part of the problem. I have seen many examples of officers reverting to their mother tongue in times of crisis, and thus not communicating effectively what is required to the crew members. This is as much a problem, when the senior and junior officers are of different nationality groups, as is prevalent on many ships now.

NoR
18th October 2010, 12:09
I remember being told by an old mate that concentrating cargo in the TD wings can dampen the roll in a stiff ship. I guess that's right because it increases the inertia about the rolling plane apart from reducing GM.

Remember carrying ilmenite from Bunbury to Durban in a standard Tramp (Cape York 1964). The shipper couldn't supply about 30% of the cargo i.e./ the bit that was to be stowed in the tween decks. Boy did she roll! never experienced anything like it since (despite service on ore carriers and supply boats). A few months later she suffered a major fracture in the deep tank bulkhead whilst on a ballast passage. God knows how many tons of ballast flooded into No2 and 3 lower holds (which were common). I wasn't there at the time but it occurred during lunch. The ship suddenly took on a massive list. Anyway it was generally thought to be a result of over stress during the Bunbury Durban trip.

robert the bruce
1st February 2012, 23:16
Thanks to everyone, an interesting read, and a few answers. We, the wife and I decided to take a 6 week voyage on a car carrier, travelling all over the place. The ship was ultra modern but had a horrible roll motion, I was not sure of the build until I studied a diagram of the hull, then I realised. 21ft approx below water, flat bottom, and a long way up, irrespective of the hundreds of vehicles, right up to the 11th deck. Anyway still loved it.

NoR
1st February 2012, 23:34
Fred, Not sure about this bit -
This point of intersection is the Metacentre (M). As the angle of heel increases M rises, rapidly passing G. After this point the physical forces exerted on the ship increasingly work to return her to an upright position.


M is already above G otherwise the ship would 'loll' until it was.

MikeK
2nd February 2012, 09:29
Dragging dim bits from the depths! I wonder if the loss of stability when drydocking, at the moment a ship first touches the blocks, until she takes the blocks F&A , would also come into play when the liner first touched the rocks ?

NoR
2nd February 2012, 09:38
Dragging dim bits from the depths! I wonder if the loss of stability when drydocking, at the moment a ship first touches the blocks, until she takes the blocks F&A , would also come into play when the liner first touched the rocks ?

It seems that is what happened.

PS That beetle thing of yours drives me nuts.

MikeK
2nd February 2012, 12:18
It seems that is what happened.

PS That beetle thing of yours drives me nuts.

Er, don't want to worry you, but WHAT BEETLE ? [=P](Fly)

fred henderson
2nd February 2012, 13:22
Fred, Not sure about this bit -


M is already above G otherwise the ship would 'loll' until it was.

Sorry. I had oversimplified that bit. I have modified the wording.

glojoh
16th September 2012, 18:53
Hi Fred, Thanks a lot for your technical explanations. Good stuff.
Some passenger vessels have stabilisers which are pulled out during poor weather.
What is your opinion on these?
Are they still being used with the modern cruise ships? I thought not.
Jan

Apologies for answering this question but this is a picture of just one of the stabilisers fitted to the latest Queen Elizabeth

kypros
16th September 2012, 21:03
Came through the bay of biscay April this year force 10 never saw a bowl of soup spilt in the main dining salons this was on the Ventura admitted a following sea but I was amazed as have been through that area many times in my MN career in storms could barely stand up never saw any of her 3400 passengers in any discomfort whatsoever. KYPROS