Ship Design Regarding Ballast.

Greetings all This is my 1st post to the site in the hopes someone may be able to answer an important question. Although, at this time I do not own a boat (which I hope to change in the near future), I have pondered on the effect pools on a cruise ship affect the ballast of that vessel? I assumed that when at sea, the water, be it fresh or sea, have an effect on the control of that ship and the ballast since it is on the top deck of those ships. Can anyone give me thoughts or links to go to to verify or debunk this theory? Thanks to all for the insight.

 

Welcome to the forum Timh1111. Yes, the mass of the water so high up on the cruise ship (assumed any, generic ship with one or more pools) does have an affect on the location of several key "centers", but in all honesty, the significance is quite slight, in relation to the mass of the rest of the vessel.

Lets take an example, one of the Royal Caribbean's newer ships with a gross tonnage of about 74,000 (165.7 million pounds). It's two pools contain about 15,000 cubic feet of water, which weighs about 415 tons (930,000 pounds). So, yes this hunk of water, way up high does affect the stability of the ship, but not nearly as much as the metal in the surrounding structures aboard, let alone the guests. This particular vessel carries over 2,700 passengers. Including their luggage, the resulting weight is more then the total of the water in both pools.

I've simplified the set of issues quite a bit, as it's actually a fairly complex set of variables, but rest assured, the mass of water in the pools, though substantial in and of themselves, isn't a significant concern in regards to stability of this ship, nor any other of the modern era.

Daniel, I've talked to you about posting my wife's picture on this forum before. Actually, that's an older picture when her hair was short.

 

Hi Par

A question on those ships. Their drafts are not nearly as large as you would think for a vessel that is so tall. I mean these guys are 12 stories high or more, and yet have only 3 stories under. What is their secret?

 

Sorry

 

Sorry what, that you didn't post more bikini pictures.

 

No, Daniel and I have an understanding about these types of pictures, that show only my better half and not me too. I'm demanding equal time.

The secret is their CG and GM. The sheer volume of machinery below and relatively light weight super structure (even though it doesn't look like it) make these things really "low slung", even with a few hundred tons of swimming pool on the upper most weather deck. Did I mention the gyros?

 

I wonder if the stability calcs assume that they are so big that no wave can flip them.

 

Thanks PAR...I posted this question because there was a blogger on another website that raised my eyebrows to this question. The poster stated that the size of the pools on the vessel should have no impact on the ballast of the vessel. I feel that the larger the pool (more water) and the higher up on the ship..the more instability with the water "swishing" back and forth (ESPECIALLY in open sea). Most cruise ships have up to 3 pools top side and as the ships get bigger, in most cases so do the pools with the added water when at sea. I just feel that adding a larger pool, holding more water topside would cause a major control and ballast problem to the vessel.

 

I thought they always drained the pools in heavy weather.


Good to see you back Paul !

 

Yes, they do drain pools in heavy seas and in all honesty they really don't affect stability with "sloshing", which we call free surface effect. The pools contain the water fairly well and the movement isn't sufficient to really get the contents very far from where it normal resides, before it's shed overboard and no longer in the stability equation. Free surface effect requires a good bit of movement, which you just don't see on ships this size. If sea states do get to this point, then the pools are emptied.

The effect the pool contents have on stability is their location, not free surface effect. Though the pools have a measurable impact on stability, not enough to be especially concerned about.

Oh, hell no, you bet they can get capsized, pitch poled and flipped. Every effort is made, usually by a team of designers and engineers, just to insure the hull form, the centers, etc. all contribute to nothing less then the most uneventful cruise ride, a guest could ever imagine. You have to realize, that one tragic event, where the boat flops on it's side (or whatever) and it's found the design was flawed. The resulting litigation would end the careers of many and crush a few businesses under the financial weight of responsibility. It's cheap insurance to be overly conservative in these matters, as has been repeatedly proven.

 

I was out on a storm in one of these monster and it was amazingly docile. Except for the chairs flying of the deck everything seem normal. It seem to shift sideways more than up and down. I could reference another cruise ship in front. On some occasions we where above it and others below it. Hard to explain but it most have been 80 feet from crest to trough.

 

Parametric roll, which depends mainly on hull form and GM, is more dangerous in high seas than the modest amount of water in the swimming pools.
This is an example: http://www.youtube.com/watch?v=kohr0KfVXZA
As you can see, though apparently too high and tippy, that beast recovers pretty well from large heel angles. So (generally) you can feel confident that it will take you to your destination, even when it rains.

 

Daiquiri, that ship appears to have something wrong with it, as it's listing to starboard and seems to favor a roll to the same side. Maybe some flooded compartments, bad giros, who knows, though she does manage to "get back up".

 

By crew or Poseidon.

I had a passage on QE2 in the 80ies. All three pools have been empty soon(except a few buckets), when she put her nose in for the 4th or 5th time. Dunno why the crew had not drained them.

 

PAR, you are right - it seems to favor the hell to the starboard. However, it might as well be the result of the combined effect of roll resonance and wind action.
Both wind and waves appear to come from the port side, about 30-40 degrees off the bow. It can be better seen in this video: http://www.youtube.com/watch?v=t_3-Re5KCj8&feature=related - see the direction of the airborne spray.
A strong wind coming from that direction will generate a force on the superstructure pulling it to the starboard side, with a consequent additional heeling moment. Higher the superstructure, more aerodynamic force it will generate. That could explain the asymmetrical rolling, imho.
Cheers!