Ships and earthquakes

[blackshoe]

In deference to Helene, I'll start a new thread with this tale.

 

Back when I was a junior officer in my first shipboard assignment (USS Worden (CG 18) in the late 70s), I was the conning officer one day as we were running West from Yokosuka to Chin Hae Korea, so South of Honshu. The captain was in a hurry, so we were doing 25 knots. Suddenly the ship started to shake, as if we'd run aground or hit something. I rang up "all stop", and as the engines stopped, so did the shaking. We looked in the waters around for debris, or a dead whale or something, but saw nothing. The Engineering Officer of the Watch reported nothing damaged in the plant. So we cautiously, in 5 knot increments, got underway again. No further weirdness occurred, so we went on our way. About an hour later, the CIC Watch Officer came out and told us he'd found out what happened - we'd run over the epicenter of a 4.9 earthquake. Water depth, as I recall it, was about 168 fathoms. I don't know how far down the actual quake was. Point is, there was hardly any visible water displacement at all.

[shintaro]

Last October whilst Cruising from Taiwan to Hong Kong on the Legend of the Seas at around 24 knots (the Captain was getting to Hong Kong early due to the Typhoon heading our way )

So we were on the Edge of the Typhoon

 

The Seas where running at around 20-30 feet But the Ship was not even rolling.

 

Until around 2200 hrs when everything started crashing about throughout the Ship only reason was we were responding to a May-Day call behind us and we were turning round across the seas We and others spent around 10 hours plucking Survivors from a Cargo ship crew of 26 which we saw Sink as we got on station.

 

The reason we felt the Seas was only because turned in a Tsuanami as long as the Ship was heading directly into the Seas it wouuld hardly notice 30 foot Waves

[blackshoe]

I suppose I should have mentioned that the seas in the incident I described were no more than a foot or two at most.

 

As for noticing 30 foot waves, you might not notice in a modern cruise ship, but warships don't tend to worry about keeping the crew comfortable.

[phil_20686]

Nearly all of the energy in the water is carried in a longitudinal pressure wave similar to a sound wave. Since water is incredibly incompressible the water displacement is small. However, the outerhull of the ship will have flexed significantly to accommodate the pressure wave as it passed through. However, "significantly" is probably only, at most, tens of centimetres. I would expect most ships hulls are designed to flex a lot more than that to deal with high seas. Most ships are double hulled these days (certainly all war ships) so probably would be no effects on the internal hull. I would expect the water displacement to be of the order of millimetres to centimetres, but that is pure intuition, not an "estimate".

 

In shallow water the pressure wave cannot accommodate all the energy, so they start to excite transverse modes. This makes the waves grow to a large height in shallow water aka a tsunami.

[shintaro]

I suppose I should have mentioned that the seas in the incident I described were no more than a foot or two at most.

 

As for noticing 30 foot waves, you might not notice in a modern cruise ship, but warships don't tend to worry about keeping the crew comfortable.

 

 

rofl aww the poor sailors :D

[y66]

Do you think your ship was over the epicenter when it started shaking or moving toward the epicenter?

[blackshoe]

According to the data collected by the CIC Watch Officer, we were directly over the epicenter. Not sure what the margin of error would be in that calculation.