Pools and stability of the ship.

Someone who knows things said that pools on a ship are bad for stability, especially when located where they usually are, on the highest decks.

I can see how a big weight on top of a 10-storey building makes it easier to make the building topple. But if you are convinced that it won't actually topple, a huge weight seems to be actually a good thing for stabilizing, like this one. To have a stable ship, any weight that doesn't sink it should help by making it harder for waves to move the ship.

Maybe the water in the pool would sync with the waves and add to the movements. In that case, simply emptying the pool could be done within minutes.

So, am I wrong to think pools are actually providing more stability?

Someone who knows things said that pools on a ship are bad for stability, especially when located where they usually are, on the highest decks.

 

I can see how a big weight on top of a 10-storey building makes it easier to make the building topple. But if you are convinced that it won't actually topple, a huge weight seems to be actually a good thing for stabilizing, like this one. To have a stable ship, any weight that doesn't sink it should help by making it harder for waves to move the ship.

 

Maybe the water in the pool would sync with the waves and add to the movements. In that case, simply emptying the pool could be done within minutes.

 

So, am I wrong to think pools are actually providing more stability?

The weight of the water in the pools fairly minor compared the weight of the mechanical equipment and material stored below the water line of a ship. The ship is designed so that about 80% of the weight is below the water line.

Sorry, but if I put this question next to the one about palm readers, I can only assume that you're pulling our legs.

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Buildings are 'anchored' firmly to the ground. Those movable weights on the top of building are meant to counter the effect of earthquakes. Ships are not anchored to the ground and aren't bothered by earthquakes (except for any resultant tsunamis). However ships can roll over, and are more likely if they are too top heavy.

Think of standing in a canoe as an example of having too much weight too high up. Yes you can do it but you're pushing your luck. Ships are designed to not have a problem with the pools where they are, but conceivably too much weight on top could lead to instability.

Most cruise ships today are built of steel - at least up to the top of the hull. That is normally where the promenade deck and the life boats are located. Above that the ship's structure is mostly aluminum - in order to keep most of the weight at the bottom of the ship for greater stability.

The swimming pools - normally on top - carry quite a bit of weight. Although most everyone complains that the pools are too small, they still hold a great deal of water. The typical ship swimming pool usually has around 80 to 100 tons of water inside it. And they are intentionally small because ship designers cannot have too much weight at the top of the ship if they want it to be stable.

In order to understand ship stability, one needs to understand kinetic energy.

While most buildings rarely move at 40 kilometers per hour, ships do it all the time.

This creates kinetic energy.

Heavy objects that are moving in one direction have absorbed that kinetic energy, and resist moving in another direction.

If your ship has 3 swimming pools like mine does, containing about 280 tons of water, and the ship tries to change direction, the kinetic energy in that 280 tons of water resists that change of direction. This diminishes the stability of the ship. Instead of just turning, it begins to heel - or lean over - as a result of that kinetic energy resisting the change in direction. The faster you are moving, the more kinetic energy you have in the water, and the greater the instability in the turn.

Go fast enough - or turn fast enough - and the kinetic energy capsizes the ship.

The rolling of the ship is another matter - but still subject to kinetic energy.

If most of the weight is at the bottom of the ship, it rolls slightly and quickly and then quickly recovers and rights itself.

If too much weight is at the top of the ship, it rolls much more slowly - but much farther off center - and it recovers more and more slowly each time. If there is enough kinetic energy in all that weight (Water or otherwise), it pushes the ship past a safe return. The ship is unable to recover from the roll - and capsizes.

Sorry, but if I put this question next to the one about palm readers, I can only assume that you're pulling our legs.

 

 

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Nah, just got bored with the palm reader thing after getting bored with using cruise ships to actually get somewhere and before that, drones. Definitely a different line of questions concerning cruising.:D:D:D

....Those movable weights on the top of building are meant to counter the effect of earthquakes.

And also movement caused by strong winds. At certain wind speeds, all tall buildings will begin to sway and oscillate. Like a tuning fork, a specific wind speed can cause the building to sway back and forth at a certain resonance frequency, which can severely stress the structural components, possibly causing them to fail. The large weights located at carefully positioned elevations counteract that oscillation. Called tuned mass dampers, by using hydraulic dampening methods these counterweights absorb some of the energy created by the wind and minimizes the swaying.

Cruise ships use extendable fins on each side of the ship for the same reason. They act as controllable underwater wings that pivot to counteract the roll created by waves, much like the ailerons on the wings of a plane pivot to stabilize the plane.

 

So, am I wrong to think pools are actually providing more stability?

Yep, Wrong on every point. :p

Buildings are anchored to mother earth. Skyscrapers will be rocked by winds or earthquakes, but massive weight at the top makes it more difficult to start a sway and slows down that momentum.

Ships aren't anchored, so like any other loose cannons such as trucks and buses and trains and race cars (.....errr, and loose cannons ;)) they're designed with the weight as low down as possible and with as wide a footprint as is practical. That's why a ship or a London bus can roll at an incredible angle before it reaches the point of capsize.

Swimming pools and any other liquids (and even loose material like grain or gravel) are a particular problem, as are heavy solid things that aren't nailed-down. Unrestrained, they will move with the sway of whatever is carrying them and their momentum dramatically increases the chance of a capsize - just ask Mr Newton.

But sometimes, practicalities outweigh stability concerns.

A classic example is the roll-on / roll off ferry, which has (had) no watertight compartments on the vehicle decks, and carries that cargo above the waterline.

Some years ago, in calm seas & with secured loads, a North Sea ferry set sail with its bow door open. The amount of seawater that entered the vehicle deck was quite small, only a matter of inches. But it set up a motion flowing from side to side & that momentum capsized the ferry with a high loss of life.

Watch a ship's swimming pool in heavy seas, the effect (either fore & aft or side to side) is the same, and the reason pools are usually closed in high seas or even heavy swells.

Liquid containers have either separate compartments or baffles, to prevent the liquid from building that momentum.

Neither option is well-suited to swimming pools. Hence if a ship wants more swimming space on an upper deck, they'll add more pools rather than increase the size of a pool. Which gives them other advantages such as separating out the kids.

So weight up high, and a large volume of unrestrained water up high, are both bad for stability. All taken into account when designing a cruise ship.

JB :)

unrestrained water

in ship damage control this is referred to as the 'free surface effect'

Have you ever been on a cruise ship in bad weather and seen the pool begin to slosh from side to side ... what's happening from a stability sense is the weight ... the center of gravity of this body of water is shifting ... this is bad. You'll typically see ships empty the pool in this condition, eliminating the wave pool . . .

In the same way this condition can come into play in partially full fuel or water tanks. This is why a ship does not have one BIG fuel tank, but many small tanks. The best conditions are for the tanks to be empty, or 'pressed up' so the contents can't shift due to ship motion.

I believe this even was a factor in the Concordia .... even wonder why the ship settled with the big hole out of the water? the flooded engine room was a large body of water unrestricted from side to side movement. It shifted to the undamaged side of the ship ... and caused the ship to roll to the undamaged side. Perhaps by the last turn which caused a list to that direction . . . had the ship been kept going straight rather than making an abrupt turn, the ship may have stayed on an even keel lessening the evacuation issues.

In Captain school they taught us no abrupt turns if you have lot's of 'unrestrained water' places it does not belong

Most cruise ships today are built of steel - at least up to the top of the hull. That is normally where the promenade deck and the life boats are located. Above that the ship's structure is mostly aluminum - in order to keep most of the weight at the bottom of the ship for greater stability.

 

The swimming pools - normally on top - carry quite a bit of weight. Although most everyone complains that the pools are too small, they still hold a great deal of water. The typical ship swimming pool usually has around 80 to 100 tons of water inside it. And they are intentionally small because ship designers cannot have too much weight at the top of the ship if they want it to be stable.

 

In order to understand ship stability, one needs to understand kinetic energy.

While most buildings rarely move at 40 kilometers per hour, ships do it all the time.

This creates kinetic energy.

Heavy objects that are moving in one direction have absorbed that kinetic energy, and resist moving in another direction.

If your ship has 3 swimming pools like mine does, containing about 280 tons of water, and the ship tries to change direction, the kinetic energy in that 280 tons of water resists that change of direction. This diminishes the stability of the ship. Instead of just turning, it begins to heel - or lean over - as a result of that kinetic energy resisting the change in direction. The faster you are moving, the more kinetic energy you have in the water, and the greater the instability in the turn.

Go fast enough - or turn fast enough - and the kinetic energy capsizes the ship.

 

The rolling of the ship is another matter - but still subject to kinetic energy.

If most of the weight is at the bottom of the ship, it rolls slightly and quickly and then quickly recovers and rights itself.

If too much weight is at the top of the ship, it rolls much more slowly - but much farther off center - and it recovers more and more slowly each time. If there is enough kinetic energy in all that weight (Water or otherwise), it pushes the ship past a safe return. The ship is unable to recover from the roll - and capsizes.

The only ship that was built with an aluminum super structure was the QE2. They maintenance was problematic due to corrosion of the aluminum in a salty environment. I have a DVD of the construction of one the Voyager Class ships. They mention the issue of aluminum coming in contact with steel st sea.

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Someone who knows things said that pools on a ship are bad for stability, especially when located where they usually are, on the highest decks.

 

I can see how a big weight on top of a 10-storey building makes it easier to make the building topple. But if you are convinced that it won't actually topple, a huge weight seems to be actually a good thing for stabilizing, like this one. To have a stable ship, any weight that doesn't sink it should help by making it harder for waves to move the ship.

 

Maybe the water in the pool would sync with the waves and add to the movements. In that case, simply emptying the pool could be done within minutes.

 

So, am I wrong to think pools are actually providing more stability?

As far as stability goes, the pools are a "drop in the bucket".

That said, as far as the movement of the water in the pools being in sync withe the waves, It is! There is a reason why the pools have those large apron areas around them-that is to collect the water that spills out of them so that it can go back into the pools.

I have seen the pools really moving in moderate seas. You do not want to be in them when the ship is really rocking and rolling....

The only ship that was built with an aluminum super structure was the QE2. They maintenance was problematic due to corrosion of the aluminum in a salty environment. I have a DVD of the construction of one the Voyager Class ships. They mention the issue of aluminum coming in contact with steel st sea.

 

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The SS United states, launched in June 1051, used aluminum extensively in her superstructure. Here are two paragraphs copied from this web site describing her design and construction: http://www.ss-united-states.net/SSUnitedStatesWebpageFiles/WebPages/PagesConstruction.htm

"There were five primarily design principles that went into the SS United States. Deviation from these principles was not tolerated by the Gibbs brothers. First, the ship had to be fireproof. There was a saying that the only wood to be found onboard was in the galley chopping block and in the ship's pianos. During construction, if William Francis Gibbs saw wood incorporated anywhere onboard the ship, he would personally remove it and substitute it with an aluminum part. In fact, Gibbs tried unsuccessfully to persuade Theodore Steinway to build an aluminum piano to be used onboard the ship. Aluminum could be found extensively in the superstructure, which was the single largest object at that time constructed entirely of aluminum alloys. In addition to the superstructure, aluminum was found everywhere onboard the ship, including the lifeboats, davits, launching equipment, oars, railings, interior decor, furniture, deck chairs, and even in passenger keychains."

"Finally, she was to be capable of operating at a high rate of speed to transport thousands of troops thousands of miles in the minimum amount of time possible. In addition to being fireproof, the aluminum used in the construction of the SS United States was lighter than steel, which increased the power-to-weight ratio and contributed to the ship's extraordinary speed."

Military has learned some lessons about aluminum in ships too. Remember the Navy Frigate USS STARK, hit by two exocet missiles in the Persian Gulf in 87? The resulting fire was so hot the aluminum superstructure (the stuff above the main deck) began to melt . . .

Military has learned some lessons about aluminum in ships too. Remember the Navy Frigate USS STARK, hit by two exocet missiles in the Persian Gulf in 87? The resulting fire was so hot the aluminum superstructure (the stuff above the main deck) began to melt . . .

Not sure about you, but I have never been on a cruise ship that was cruising an "Adventures of Combat" itinerary. :D

Princess . . .

Princess . . .

 

Cruise ship fires like this are extremely rare. Not sure what the point of your photo is? :confused:

I love being in the pool when the water is rocking. When the waves get too high, they will empty the pools.

Sorry, but if I put this question next to the one about palm readers, I can only assume that you're pulling our legs.

I don't like having to defend myself, but here's a post especially for you.

The palm reader was meant to be a far fetched example of a possible service on a ship. I did make it clear that I wouldn't be a customer and didn't even think a palm reader would actually work. Trust me, I will not use a palm reader as an example again.

I'm sincerely very interested (yes, bordering on being obsessed) in how ships work, how the industry works etc. If "trolling" (the official word for pulling legs on a forum) was my objective I would have chosen another forum than CC.

Nah, just got bored with the palm reader thing after getting bored with using cruise ships to actually get somewhere and before that, drones. Definitely a different line of questions concerning cruising.:D:D:D

"Bored" is not the right word. People are so nice to answer my questions and followup questions, and I try to stop asking questions and pitching palm readers when knowledgeably people tell me it's just a stupid idea. Btw, in February I will have to show up in Spain and still like the idea of getting there by ship, flying is simply not an option.

Besides the questions I ask here, there are many more Google has answered. If you really think I'm asking questions for the sake of asking questions, I've attached proof that I bought a quite expensive book just to understand how waves actually work.

....in February I will have to show up in Spain and still like the idea of getting there by ship, flying is simply not an option.

No one is stopping you from using a cruise ship to get from one place to another. People do this all the time on transatlantic cruises. Our last cruise started in Barcelona and ended in Venice. It is perfectly fine as long as the passengers stays with the ship the entire cruise, from originating port to final destination port.

But that isn't the main purpose of a cruise ship. If it was, it would only make short stops at each port to drop off and pick up passengers, and make the full voyage in half the time a cruise ship does. If you don't mind it taking much longer than it needs to be, and at a higher cost than what a ferry might cost, then go ahead. But don't expect to be able to get on at any port other than the one the cruise starts at, or get off at a port other than where the cruise ends. A cruise is not designed to be just transportation.

You have me curious - why is flying not an option? That leaves you very few options when traveling to overseas locations.

..............

 

 

"Bored" is not the right word. People are so nice to answer my questions and followup questions, and I try to stop asking questions and pitching palm readers when knowledgeably people tell me it's just a stupid idea. Btw, in February I will have to show up in Spain and still like the idea of getting there by ship, flying is simply not an option.

 

Besides the questions I ask here, there are many more Google has answered. If you really think I'm asking questions for the sake of asking questions, I've attached proof that I bought a quite expensive book just to understand how waves actually work.

Yes, perhaps bored was the wrong word. You've asked some interesting things.:)

Princess . . .

 

I spoke with a senior Captain who was involved in the Star Princess fire enquiry - although a significant number of cabins were destroyed, the sprinkler systems in the cabins and the steel structure of the superstructure meant that there was pretty much no fire or heat damage to the corridors. Good to know!

VP

Yes, perhaps bored was the wrong word. You've asked some interesting things.:)

Thank you :) This thread proofs once again a fool can ask more questions than seven wise men can answer (not sure if that's a known proverb in the US)

You have me curious - why is flying not an option? That leaves you very few options when traveling to overseas locations.

It's not very convenient, and definitely not very rational, but I'd rather be on a balloon flight than being in airplane. A chat with a captain who insisted that the drive to the airport was more dangerous than the flight itself, and then went on to describe the really scary things she (yes) had experienced in her career didn't help much either :) A safety drill on a ship might save my live so I take that seriously. On a ship, in the worst case I can jump off with something that floats and hope for the best. When the Costa Concordia was already sinking