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Discussion Starter #1 (Edited)
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)
 

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Period of Roll

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
 

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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
 

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Discussion Starter #4
Jan Hendrik said:
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
 

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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.
 

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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
 

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rstimaru said:
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
 

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fred henderson said:
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.
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Tony c
 

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

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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.
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Tony C
 

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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.
 

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fred henderson said:
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 )
 

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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.
 

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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

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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.
 

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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
 

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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.
 

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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
 
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