Why in and out of the pools every 10 minutes?

[chengkp75]

I was wondering about that. That's a US standard isn't it, so it doesn;t necessarily apply (although it certainly would make sense).

 

The EU has a program of cruise ship sanitation very similar to the USPH regulations, and so does the WHO, which is the standard for ships everywhere else in the world. Both require "anti-entrapment" drains among their requirements for pool construction and operation, which again are very similar to the USPH regulations.

[chengkp75]

Unrelated, hope you don't mind me hijacking your expertise for a question. On another thread someone is asking about bringing a Brita pitcher on board (due to the latest NCL feature clawback--no more beverages of any kind brought on board). It made me curious if Brita type filters accomplish anything with ship drinking water that's already been processed.

 

What a Brita type filter will do is remove the chlorine from the water. Unlike shore water supplies, which are only required to be chlorinated to a specified level at the processing facility, and which dissipates before the water reaches the end user (homes and businesses), cruise ships' water is required to have a residual chlorine level at all times, and this must be at a measurable level (0.5ppm) at the furthest point from the chlorine injection equipment. So, naturally, if your cabin is closer to the engine room than the bridge, your water may have more than 0.5ppm chlorine. Many people complain about the taste of the chlorine, so a Brita filter will help with taste only. This is similar to the filters used by the ship at the water dispensers in the buffet, the water stations the wait staff use in the dining venues, the ice makers, and the bar guns around the ship, but these filters only provide a taste benefit as a side effect, because removing the chlorine before the water goes into these pieces of equipment reduces the chlorine scale that forms, and reduces maintenance.

[Ex techie]

The real problem with larger pools is not weight. It is a phenomenon known as "free surface effect", which is caused by the water in the pool, being a liquid, being able to change shape to conform to the container it is in, regardless of the orientation of that container. Think of a glass of water. When you tip it to one side, the water level on that side goes up, and goes down on the other side. This moves the center of gravity of the water ever so slightly (it is after all, only a glass of water), but when you multiply this by a hundred tons, it becomes a very noticeable effect, and the higher in the ship it is experienced, the worse the effect becomes.

 

Ship stability depends on knowing where the center of buoyancy of the ship is, and where the center of gravity of the ship is. The relationship between these two points in space determines whether a ship returns to upright when rolled to one side, or whether it continues to roll over. If the center of gravity is allowed to move to a significant degree, the stability of the ship is negatively impacted.

 

The heeling and eventual capsizing of the Costa Concordia was caused by free surface effect. This was caused by free flowing water in the engine room, low down in the ship. Had the weight of water in the engine rooms been solid, the ship would have sunk on the rock off Giglio as it did, but it would have done so upright. Now imagine a much smaller weight of water, but placed much higher in the ship, and you have an idea why cruise ship pools are kept small, and they have multiple pools.

 

Chief,

 

As always, you have the on the money answers!

The best I could come up with was dynamic weight shifting and you had the precise terminology of "free surface effect"!

 

So if DCL were to use the Dream class ships as an example for design, and wanted to increase the size of the pools by 25-30% again for example, what would they need to do to accomplish this?

 

How could they stabilize the ship more to compensate for the extra mass and increased free surface effect of a wider and longer pool to the same level of comfort with the increased pitching and rolling, and obviously making sure she would return to upright!?

 

Is it as simple as moving the pools lower and more centered?

 

ex techie

Edited July 4, 2016 by Ex techie

[Loonbeam]

Adding on to this, its my understanding that the ship water generation systems remove near 100 percent of sodium (trace levels may be less).. Assuming this is true, unless salt is added back in (is it?) ship water should be pretty close to sodium free...

 

What a Brita type filter will do is remove the chlorine from the water. Unlike shore water supplies, which are only required to be chlorinated to a specified level at the processing facility, and which dissipates before the water reaches the end user (homes and businesses), cruise ships' water is required to have a residual chlorine level at all times, and this must be at a measurable level (0.5ppm) at the furthest point from the chlorine injection equipment. So, naturally, if your cabin is closer to the engine room than the bridge, your water may have more than 0.5ppm chlorine. Many people complain about the taste of the chlorine, so a Brita filter will help with taste only. This is similar to the filters used by the ship at the water dispensers in the buffet, the water stations the wait staff use in the dining venues, the ice makers, and the bar guns around the ship, but these filters only provide a taste benefit as a side effect, because removing the chlorine before the water goes into these pieces of equipment reduces the chlorine scale that forms, and reduces maintenance.

[AdoraBelle]

Adding on to this, its my understanding that the ship water generation systems remove near 100 percent of sodium (trace levels may be less).. Assuming this is true, unless salt is added back in (is it?) ship water should be pretty close to sodium free...

 

 

Thanks, guys.

 

(FWIW, I'm a fan of the ship's water on NCL Getaway, whatever they do it. My overall cruising experience is limited, but my swelling on cruises appears to be due to food. :)

 

I'm just barely beginning to crack the surface of tap water (etc) properties for unrelated reasons (hydroponics), and all I know is that water from my Brita filter has fewer "TDS" than my tap water, and wondered where ship's water fell in that sense. Either way I'm still fine with drinking it and won't be hauling a pitcher/filter around with me.)

[Host Mick]

Removing the equivalent weight or "a lot of weight" of larger pools from elsewhere off the ship would not counter the dynamic weight shifting of increased volumes of mass (water) moving around, port to starboard, fore to aft and heaving up and down.

 

ex techie

 

Actually it can be. Liquid free surface effect can be converted to moments and compensated for elsewhere. Naval architects take it into account all of the time. Ships are full of sloshy liquids.

[chengkp75]

Chief,

 

As always, you have the on the money answers!

The best I could come up with was dynamic weight shifting and you had the precise terminology of "free surface effect"!

 

So if DCL were to use the Dream class ships as an example for design, and wanted to increase the size of the pools by 25-30% again for example, what would they need to do to accomplish this?

 

How could they stabilize the ship more to compensate for the extra mass and increased free surface effect of a wider and longer pool to the same level of comfort with the increased pitching and rolling, and obviously making sure she would return to upright!?

 

Is it as simple as moving the pools lower and more centered?

 

ex techie

 

Actually it can be. Liquid free surface effect can be converted to moments and compensated for elsewhere. Naval architects take it into account all of the time. Ships are full of sloshy liquids.

 

While, indeed, ships are full of "sloshy liquids", they are more contained than a swimming pool's water, and there are some design characteristics involved. Tanks that contain liquid that are nearly full or nearly empty present less free surface effect than tanks that are half full (the nearly empty tank has little weight so its movement is minimal, while the full tank has the "shape" of the fluid constrained by the top of the tank (pools don't have tops), which limits how far the liquid can move). So, operationally, you try to keep the number of "slack" (between full and empty) tanks to a minimum to reduce the free surface. Also, some tanks are wide and some are narrow (in the athwartship direction). Narrow tanks are more forgiving of free surface than wide ones, so the wide ones are even more restricted in the amount of time and number of tanks that are allowed to be slack.

 

Making a pool longer is not so critical, since longitudinal stability is extremely rarely a problem. Width of the pool, for the reasons noted above, is the critical factor.

 

Placement fore and aft would not matter much, but lower definitely is better than higher. The problem with placing pools lower is that you then have to carve out the center of the ship (a la Oasis) to provide an open air pool, or enclose the pool and get the chlorine heavy atmosphere of a community pool (even worse as chlorine levels are higher).

 

Yes, you can add weight lower down, or reduce weight higher up to compensate for the added weight and free surface of a larger pool, but this will then change the initial stability of the ship, and lowering the center of gravity causes the ship to be "stiffer", meaning that while it is harder to get the ship rolling (requires more force), once it starts to roll, the righting force is greater, so the motion is faster, and you get an uncomfortable "snap roll" that naval vessels experience. Pool water up high actually makes the cruise ship ride "feel" better (slower rolling), but will also allow the ship to start rolling in less severe weather, and "hang" at the ends of the roll. And that is why they need stabilizers.

[CanadianDee]

The real problem with larger pools is not weight. It is a phenomenon known as "free surface effect ...you have an idea why cruise ship pools are kept small, and they have multiple pools.

 

Thanks so much for the brilliantly simple and relatable explanation. Makes total sense.

[Steelers0854]

The EU has a program of cruise ship sanitation very similar to the USPH regulations, and so does the WHO, which is the standard for ships everywhere else in the world. Both require "anti-entrapment" drains among their requirements for pool construction and operation, which again are very similar to the USPH regulations.

 

 

 

Thanks for being active on this board and sharing your knowledge. I always enjoy seeing a post from you as I learn something new and know the answers are going to be accurate.

[moki'smommy]

Hey Chief,

 

This question really relates to your response on another thread.

We were told that DCL purchases water to fill the storage tanks in US ports only, but uses exclusively ship generated water when visiting other ports. Of course, what is told to guests is not always accurate, even at the now defunct "behind the scenes" type tours.

 

In another thread you stated that it costs less to make water on the ship than to purchase it in ports and that if the heat is not used to make water, it is discharged into the ocean (thus, a form of thermal pollution.)

 

Would you think that we were misinformed about purchasing US water, or is there a reason that DCL would purchase water if it is cheaper to process sea water? Or perhaps do they refill the tanks while in the US because the Caribbean/Bahamian cruises don't allow sufficient time for them to process enough water to meet the ship's needs?

 

Obviously, not a big issue....just wondering.

[AmazedByCruising]

Now imagine a much smaller weight of water, but placed much higher in the ship, and you have an idea why cruise ship pools are kept small, and they have multiple pools.

 

Almost.. except for multiple pools? If one big pool was divided into 10000 tiny pools in a 100x100 grid, I can see the water can't move much and can't spoil stability much either (except of course that it's still heavy and usually high up).

 

Yet from what I've seen, there are multiple pools but they are usually not side by side but like one aft and one midships (and some small hot tubs of course). That wouldn't help against sideway movements of the ship?

 

(I know I have asked whether pools would actually add stability before..)

[Ex techie]

While, indeed, ships are full of "sloshy liquids", they are more contained than a swimming pool's water, and there are some design characteristics involved. Tanks that contain liquid that are nearly full or nearly empty present less free surface effect than tanks that are half full (the nearly empty tank has little weight so its movement is minimal, while the full tank has the "shape" of the fluid constrained by the top of the tank (pools don't have tops), which limits how far the liquid can move). So, operationally, you try to keep the number of "slack" (between full and empty) tanks to a minimum to reduce the free surface. Also, some tanks are wide and some are narrow (in the athwartship direction). Narrow tanks are more forgiving of free surface than wide ones, so the wide ones are even more restricted in the amount of time and number of tanks that are allowed to be slack.

 

Making a pool longer is not so critical, since longitudinal stability is extremely rarely a problem. Width of the pool, for the reasons noted above, is the critical factor.

 

Placement fore and aft would not matter much, but lower definitely is better than higher. The problem with placing pools lower is that you then have to carve out the center of the ship (a la Oasis) to provide an open air pool, or enclose the pool and get the chlorine heavy atmosphere of a community pool (even worse as chlorine levels are higher).

 

Yes, you can add weight lower down, or reduce weight higher up to compensate for the added weight and free surface of a larger pool, but this will then change the initial stability of the ship, and lowering the center of gravity causes the ship to be "stiffer", meaning that while it is harder to get the ship rolling (requires more force), once it starts to roll, the righting force is greater, so the motion is faster, and you get an uncomfortable "snap roll" that naval vessels experience. Pool water up high actually makes the cruise ship ride "feel" better (slower rolling), but will also allow the ship to start rolling in less severe weather, and "hang" at the ends of the roll. And that is why they need stabilizers.

 

Great info as always Chief!

 

It is my understanding that the tanks low down with the large amounts of "sloshy liquids" also have baffles in them to reduce the amount of free surface effect and are compartmentalized to limit the spaces volume to more manageable sizes rather than say a few big tanks, there are several connected smaller tanks? Is this true?

 

I've probably asked this before and you have replied, but why do ship pools need to be more heavily chlorinated compared to land based pools?

 

Thanks for your insights and sharing your knowledge!

 

ex techie

Edited July 5, 2016 by Ex techie

[karl_nj]

Or perhaps do they refill the tanks while in the US because the Caribbean/Bahamian cruises don't allow sufficient time for them to process enough water to meet the ship's needs?

 

That's what I was told at one of those "Lectures with the Chief Engineer" on the QM2 -- when that ship is doing crossing they are able to make enough water because of the itinerary (no ports). But when the ship was doing Caribbean cruises they had to buy supplemental water because they weren't at sea for enough time.

 

Of course, QM2 was mostly designed for transatlantic crossings compared to the Disney Dream/Fantasy which I assume were designed for Bahamas/Caribbean cruises. This could effect the specifications for the equipment installed onboard.

[moki'smommy]

 

Of course, QM2 was mostly designed for transatlantic crossings compared to the Disney Dream/Fantasy which I assume were designed for Bahamas/Caribbean cruises. This could effect the specifications for the equipment installed onboard.

 

The reason that the Dream and Fantasy are doing just Bahamas and Caribbean cruises is that in order to get the terminal expansion needed for the larger ships, they had to agree to a contract with the port authority stating that the ships would remain based there for at least 5 years and guaranteeing a number of dockings and passengers during those years. And the fact that they are continuing to command large cruise fares for most dates and nearly filling the ships on most dates doesn't make it a problem in DCL's mind. Yes, there are some sailings where they have to offer discounts to fill the ships, but these are not the norm.

 

I don't think that the ships were specifically designed for these short sailings, but at this point DCL is still fulfilling the contract ...although the Fantasy is now 5 years out. Hmmm....not sure what the current contract is. The original one should be completed.

 

It will be interesting to see what happens 5 years from now when the first of the new ships is due.

[chengkp75]

Great info as always Chief!

 

It is my understanding that the tanks low down with the large amounts of "sloshy liquids" also have baffles in them to reduce the amount of free surface effect and are compartmentalized to limit the spaces volume to more manageable sizes rather than say a few big tanks, there are several connected smaller tanks? Is this true?

 

I've probably asked this before and you have replied, but why do ship pools need to be more heavily chlorinated compared to land based pools?

 

Thanks for your insights and sharing your knowledge!

 

ex techie

 

No, fuel, potable water, or ballast tanks (or any other miscellaneous tanks onboard) don't have baffles, other than the required frames for structural strength of the hull. So, wing tanks (running up the side of the ship) will only have the 18-24" deep frames on each side, while double bottom tanks (those under the engine spaces) will have frames that take up nearly the full height of the tank, and have only small "manholes" through them. But, this is for strength, not slosh control. Nor are they compartmentalized. However, yes, the naval architects will balance tank size and tank number to control possible free surface, so there tend to be more smaller tanks than few large ones.

 

As to pool chlorination, its because the USPH says so. They have studied pool contamination and pool borne disease, but they only have jurisdiction over ship's pools, while shoreside pools fall under state and local jurisdiction. So, while their studies have shown that the chlorination levels they recommend are better for sanitation, states and towns don't want the complaints about chlorine eyes, or bleached out swimwear, or swimwear that rots out from the chlorine, so they go with a lower level that I believe comes from the pool industry.

[chengkp75]

The reason that the Dream and Fantasy are doing just Bahamas and Caribbean cruises is that in order to get the terminal expansion needed for the larger ships' date=' they had to agree to a contract with the port authority stating that the ships would remain based there for at least 5 years and guaranteeing a number of dockings and passengers during those years. And the fact that they are continuing to command large cruise fares for most dates and nearly filling the ships on most dates doesn't make it a problem in DCL's mind. Yes, there are some sailings where they have to offer discounts to fill the ships, but these are not the norm.

 

I don't think that the ships were specifically designed for these short sailings, but at this point DCL is still fulfilling the contract ...although the Fantasy is now 5 years out. Hmmm....not sure what the current contract is. The original one should be completed.

 

It will be interesting to see what happens 5 years from now when the first of the new ships is due.[/quote']

 

I think what karl means is that there is a decided difference in design between a "liner" and a "cruise ship", since the liner is designed for heavier sea conditions during routine crossings rather than one or two repositionings a year.

 

As far as water making equipment goes, though, there's not much difference between the QM2 and most cruise ships. The Norwegian Sky, for instance, uses about 700mt of fresh water per day, but has a water making capacity of 1600mt per day. Yet, given the itinerary in Hawaii when she was the Pride of Aloha, we had to take hundreds of tons of water in every port. She was at sea only 60 hours per week, so that is a maximum of 35% of the cruise, and 35% of 1600mt/day is only 570mt/day, so there was a minimum of 130mt/day shortfall, required from shore.

[moki'smommy]

Makes sense--thanks!

[Ex techie]

No, fuel, potable water, or ballast tanks (or any other miscellaneous tanks onboard) don't have baffles, other than the required frames for structural strength of the hull. So, wing tanks (running up the side of the ship) will only have the 18-24" deep frames on each side, while double bottom tanks (those under the engine spaces) will have frames that take up nearly the full height of the tank, and have only small "manholes" through them. But, this is for strength, not slosh control. Nor are they compartmentalized. However, yes, the naval architects will balance tank size and tank number to control possible free surface, so there tend to be more smaller tanks than few large ones.

 

As to pool chlorination, its because the USPH says so. They have studied pool contamination and pool borne disease, but they only have jurisdiction over ship's pools, while shoreside pools fall under state and local jurisdiction. So, while their studies have shown that the chlorination levels they recommend are better for sanitation, states and towns don't want the complaints about chlorine eyes, or bleached out swimwear, or swimwear that rots out from the chlorine, so they go with a lower level that I believe comes from the pool industry.

 

Great information!

 

ex techie

[alaska_planner]

As crowded as the pools are, it also allows the lifeguards to have the occasional clear view to the bottom of the pool

 

yes...in my years way back when as a lifeguard that's why we cleared the pool...to have a complete view of the bottom..

not that we'd probably have saved any lives that way, but it does get difficult in a very large pool with zillions of wild kids to have a clear view.

 

so we would clear the pool, claim it was for a few minutes of adult swim, then let the wild animals back in....