Vibrations motion sickness

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mike disch

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I read that the higher you go on the ship, the less the vibration. Is this valid? If so, what's the explanation, as it seems counter-intuitive. (One would think that the higher you get from the center of gravity, the greater the motion, like a wobbly spinning top, where the greatest movement off center will be at the top, less and less the lower you go.)
 
Mike, I think you'll find the vibration being spoken of comes from the engines and propellors. Mostly the latter by way of a phenomenon known as cavitation. Depending on how fast the ship is going, propellor cavitation can generate a lot of vibration and a helluva lot of noise.

Even with computer designing and testing techniques, nobody has ever licked this problem, only managed to keep iut down to levels where it doesn't threaten to tear the stern apart. Back in 1912, propellor design was as much guesswork/art as it was science and some ships had some serious problems with this. The Lusitania is but one example. The vibration was so bad that after trials, the ship had to have the 2cnd class accomadation back aft just about gutted out and reinforced so it would be habitable.
 
I'm sure it was. Unfortunately, my source didn't mention cost figures for the alterations, but they had to be as substantial as they were unwelcome. Still, it was either make the changes or put a ship in service that nobody would book passage on.
 
Which also could have been costly.

I suppose, then, fixing it up would be the better of the two. If they wanted business, that is.

Do you know of any that were scrapped because they couldn't get them habitable?
 
Mmmmmmmmm...offhand, no. Which doesn't mean it never happened...only that I'm not aware of any case histories.

By this point in time, the science of shipbuilding was advanced enough that such things were pretty rare. As expensive an investment as a ship is, if a given hull was rejected by the buyer, the builder would have made every effort to sell her to somebody else rather then scrap the ship. That way, at least the losses are minimized.
 
I can definitely understand that.

Thanx for the info!
happy.gif
 
Ships can "vibrate" under different conditions of helm. One lake freighter had a reputation for vibrating under neutral or left rudder, but was supposedly "smooth as silk" under right rudder. In studying the testimonies over the years I have found "ghosts" of information that Titanic did experience some "vibration" when the rudder was put over. The references are somewhat indirect and inconclusive...just ghosts...but this would not have been unusual nor would it have been the cause for official comment among the crew or builders.

--David G. Brown
 
The Hamburg Amerika liner "Deutschland" of 1900 is a good example of a ship that was a financial liability to her owners due to serious vibration problems. These were bad enough that prior to her refit as a cruise liner, she was often referred to as the "Cocktail Shaker" by both passengers and crew. The experience left such a bad taste in the mouth of Albert Ballin that he instituted a wholesale change in policy at HAPAG similar to that adopted by the White Star Line, favoring comfort and size over high speed.

TGOL has an article on the Deutschland here:

http://www.greatoceanliners.net/deutschland.html

Regards,

Scott Andrews
 
On day 1 of the Senate Inquiry, Charles Lightoller gave testimony that part of this problem was addressed with a trial-and-error balancing of weights - "The builders knowing the exact weights on board, additional weights are placed on each side of the ship. A pendulum is suspended in the most convenient place on the ship with a plumb on the end of it, and a method of registering the difference with the plumb line; a number of men then transfer the weights from one side of the ship to the other, bringing all the weight on one side and transferring the whole of it back again; and with this, I believe the builders are able to draw up a stability scale." It looks like even the builders were used to being caught with the problem unsolved completely until the first time the ship sailed.
 
Lee:

You (as per Lightoller) seem to be describing the Inclining Experiment. This was a standard static stability test applied to all newbuildings before handover. It would normally be conducted in the fitting out basin under the inspectorate of the classification society and insurers. As far as I know it is still applied today.

It was a confirmation of the builders' computation of the weights built into the ship and their moments of inertia bearing chiefly upon the vessel's lateral stability. It had no discernible relevance to vibration.

A textbook on theoretical naval architecture should provide a succinct or fuller description of the process.

Noel
 
Lee -- Lightoller was describing an inclination test which has to do with transverse stability and not vibration.

During the design phase the naval archtects use mathematics to calculate the ship's metacenter and righting arm. By 1911 they were quite good at these calculations, but no ship ever quite matches the math. So, every vessel must be physically "inclined" to prove that its real characteristics match those calculated on paper.

The process is simple. You start with weight on the centerline of an upper deck and note the ship's waterline. Then, you move the weight outboard and the ship heels, or "inclines." The new waterline is noted and all the data cranked through some formulae. The result is a real-world metacentric height for the vessel.

A ship with a high metacenter is very "stiff." It has an unpleasant snap roll. If the metacenter is too low, the ship has a long roll with a "loll" before it returns upright. The trick is to come up with a metacentric height that provides good stability (so the ship won't roll over) and a comfortable motion at sea.

A "metacenter" is a theoretical point on a perpindicular erected from the ship's keel. You can't see it, but it has a profound effect on stability and comfort.

--David G. Brown
 
Brian & Scott ~

Great links!! What trials shipbuilders go through to make a ship faster and more comfortable for its passengers. All for not, in some cases, I suppose.
 

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