## Wave Dynamics 101: For the lay person

Here is a lay persons version of ocean wave mechanics.   I’ll be as simple as I can but please get back to me with anything that is not crystal clear.

Waves are caused by wind rushing across the waters surface.   First little ripples called Capillary waves start to wrinkle the surface.   This gives the wind something more to grab onto and it builds up higher and higher waves from there.    The higher the wind speed the bigger the waves.   But it takes a while for waves to build up to full height.    There is a height for a given wind speed where the wind blowing at that speed for longer will not make the waves grow any bigger, which is called a fully developed sea.    That does not happen very often.   Most often the wind is stronger than is needed and the waves continue to grow, or the wind is dying down and the waves are starting to spread out and get longer.

Now here is where it gets complicated.       In Ocean wave science we talk about the wave period which is the time between crests passing a spot and the wave length which is the distance in feet or meters between crests.

Ocean waves we care about are between about 2 and 30 seconds in period.   If you convert that to the frequency that other sciences use for radio or sound waves, everything is a tiny fraction of one cycle per second so it’s kind of hard to wrap your brain around what it means.   Three seconds or 7 seconds is something you can count off so it’s easier to understand.

Waves with a longer wavelength or period travel a lot faster than shorter waves.    On any typical day there are hundreds or thousands of different waves coming from different directions at different speeds that are all passing through one another and adding to or subtracting from the other waves at any given instant and locations.   Crests passing the same spot at the same time make the apparent wave higher, the valley between the crests called a trough subtracts it’s height from the apparent wave as it passes through

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.    Run a couple of hundred different waves through a spot at the same time, coming from different directions and you get the sort of jagged messy surface of the real sea.   If all the crests add together at the same time you get a monster wave or a monster through called a rogue wave.   Thankfully these don’t happen very often.

We talk of Seastate 3 of seastate 5 or seastate 4.   Seastate 3 is any wave from half a meter high up to 1.25 meters high.    Seastate 4 starts at 1.2500001 meters high and goes up to 2.5m high, 5 is from 2.5 to 4m etc.   When the wave height doubles the energy goes up by a factor of 4 or as the square of the wave height for those more mathematically inclined.

There are really two main types of waves that will affect a ships motions, seas and swells.   Seas are generated from the local wind field and can be predicted by the local weather conditions and how far over open water the wind can blow before it reaches your spot.   In harbors and bays, the short distance called “Fetch” keeps the locally generated wind waves from getting very big.    Swells are a different animal entirely.    They propagate in from up to thousands of miles away from distant storms.   Because they are not locally generated, they cannot be predicted by local wind conditions.   For areas of the world where there have been wave buoys in the water for 30 years or so, you can predict swells probabilities of occurrence from what they have done in the past.

Wave of all kinds are also altered (slowed down) when they start to feel the bottom coming up.   In the extreme case the bottom of a wave is slowed down so much near the shore that the top and pack of the wave catch up and the wave breaks onto the beach.   This is also why tsunamis pass unnoticed by ships out at sea but build into monsters as they start to slow down by feeling the bottom and all that energy traveling behind them catches up and pushes them upwards into giant breakers.

For the area off Nantucket Shoals, where the Doria went down, there are significant shoals to the north of your position starting with Georges Bank that starts about 120 miles or so east of Cape Cod and then comes in as close as the Great South Channel which is like a valley along the forearm of Cape Cod if you think of it as big arm sticking out into the ocean.   Parts of Georges Banks are only 3 to 6 feet deep, such as at the top of Cultivator Shoal or Georges Shoal.   Then south of Nantucket you have the Notorious Nantucket Shoals which have eaten the bottom out of many boats through out history.   The Shipping lanes pass to the south of this and probably it has a significance in that the Stockholm did not try to turn North sooner and miss the Doria because he wanted to make sure he was clear of the Shoals before he headed in that direction.

Ship motions are very sensitive to the wave period.   A big ship like the Andrea Doria would typically have a roll period of longer than 15 seconds by deliberate acts of design.   People get seasick due to the vertical accelerations of their bodies going up and down.   This effect is greatly magnified if the acceleration is shorter than 1/10th of a second.   A passenger ship wants to roll and pitch slowly so that people don’t get sick.   One of the reasons that the Titanic had a Dummy smokestack was so that they could get some more light weight inertia up high.   This is like a tightrope walker holding a long pole.   It helps slow down the motions so that few passenger complained of “Mal du Mer”

SEAN KERY MS PE

Senior Hydrodynamicist

CSC  Fellow