Hydrostatics


#21

I’ve been (one time only) in 12-14 meter seas and 70 kts of wind but I had some difficulties (discovered the ship rolls heavily in short steep head seas)

By what you describe it looks like that this was a case of parametric resonance.


#22

Yes, it was. That’s how I learned about it.


#23

Learning the hard way… It took quite a while before this phenomenon, as well as the dangers of following and quartering seas, were taught in the nautical schools. Any idea whether this was a 1:1 or 1:2 resonance? The last one is much more severe. Do you remember what your GM was at that time?


#24

Don’t recall exactly, Min required GM is 0.9 meters or so, we were fully loaded so likely around 1.2 or 1.4 meters.

Seas at the time were “phenomenal” about 14 meters . We were hove to into the swell, the ship was not noticeably rolling at all. Not really pitching so much as climbing up one swell and then down the back side. The ship was climbing up the face of one of the bigger seas and the ship just suddenly took a sharp roll to stbd, it seemed like it was just falling over. Then over to port, only worse, then back to stbd, deeper than ever.

Main engine tripped out on low lube oil. The ship rode better with the seas on the beam.


#25

That is a frightening experience especially the suddenness and violence of this phenomenon. Also in case of the ‘APL China’ the vessel was rolling and pitching moderately with a consistent motion. Then with no warning the vessel began to roll and pitch heavily, with maximum rolls developing in only a few roll cycles.

The reason that I asked about the GM was that it seems that at high GM-values, a 1:2 resonance can occur in head seas at slow or moderate speeds. In the case of the P&O’s ‘Nedlloyd Genoa’ the master, by ballasting, increased the GM and so decreased the roll amplitude, but the natural roll period was getting closer to twice the encountered wave period, and therefore the risk of parametric rolling was increased. Slowing down increased the encountered wave period bringing it even closer to half the vessel’s natural roll period.

One of the problems with modern ship design is the development of container ships with large bow flares and hanging sterns designed to maximize container stowage surface on deck and keep low resistance and so reducing fuel consumptions also at high speeds. I sailed mainly on box type tankers which are not very sensitive to parametric rolling.


#26

Parametric rolling is a second-order problem usually easily solved. . Parametric rolling in 5-6 meter seas is likely an easy problem to avoid and to solve.

Heavy weather is a first-order problem. I think of things like a bad crew as being a first-order problem. A bad crew makes problems both more likely to occur and more difficult to solve.

Like a bad crew heavy weather makes problems both more likely and more difficult to solve.

While I was in the wheelhouse trying to control the ship the mate had the crew in the holds tightening lashings, a task both made necessary and more difficult by the ship’s motion.

While all that was going on one piece of used equipment caught fire. Battery came loose and shorted out. Fortunately quickly extinguished, at that point the crew would not have been able to give a fire full attention.


#27

Oops, that was quite something. Trouble never comes alone or/and Murphy’s law in action. Nice theory classifying crew as first order problem!

Matthieu is the man. His famous second order differential equation explains it all. It is a broad concept, there also is the phenomenon of parametric resonance in forced (electronic) oscillators. It seems that also freak waves are a simple second order problem.

Lashings in general can be a problem in itself. On a lot of ships the quality due to wear and tear leaves to be desired.


#28

5 posts were split to a new topic: Air France Flight 447