Marla Crouch: Pitch and Roll, June 24, 2013

NOAA Teacher at Sea
Marla Crouch
Aboard NOAA Ship Oscar Dyson
June 8-26, 2013 

Mission:  Pollock Survey
Geographical area of cruise:  Gulf of Alaska
Date: June 23, 2013

Weather Data from the Bridge: as of 2100
Wind Speed 6.30 kts
Air Temperature 11.7°C
Relative Humidity 73.00%
Barometric Pressure 1,004.20 mb

Latitude:  56.42N   Longitude: 158.20W

Science and Technology Log

Who can tell me the direction longitudinal and transverse waves move?  Think about the electromagnetic spectrum; what is the relationship between wavelength and frequency?  The physics of these wave actions are experienced in fields other than earthquakes (seismic), and light (optics) and sound (audio).

Picture provided by NOAA NWS Prediction Center
Picture provided by NOAA NWS Prediction Center

There are two different types of water waves that mariners regularly encounter, wind waves and swell waves.  An analogy for wind waves and swell is a wind wave is to weather as swell is to climate.  In other words, wind waves are local and swell occurs over a great distance.

Waves are formed when repeated disturbances move through a medium, such as, air, earth and water.  As the wind moves or blows across the open waters, energy is transferred from the friction of the moving air particles to the waters’ surface creating wind waves.  The speed, and fetch (unchanged direction) of the wind, and the distance the wind has traveled unimpeded; influence the amplitude and frequency of the waves.  As wind speed picks up so does the amplitude of the waves.  Wind waves can be identified by their white caps.  Wind waves have short wave lengths.

Graphic courtesy of Tammy Pelletier, WA State Dept. of Ecology
Graphic courtesy of Tammy Pelletier, WA State Dept. of Ecology

Swells are a formation of long wavelength surface waves, which travel farther and faster than short wavelength wind waves.  Swells can be formed by storms that occurred somewhere else in the ocean.  For example, Tropical Storm Leepi formed off the China coast south of Japan, and was active June 17 – 19, 2013.  The energy from Leepi’s 40 mph winds and rain radiates outward from the storm, like the ripples that form when you drop a rock in a puddle of water, creating swells.  Swells can travel in a multitude of directions as they bounce off landmasses back into the open waters.

Wind waves and swells transfer energy to ships, such as the Oscar Dyson.  The energy causes ships to pitch, roll and yaw.

Pitch, roll, and yaw are three dynamic ways crafts, such as airplanes and ships move in a fluid.  In my “Surf your Berth” blog I used a teeter totter as an example of pitch.  If you think about the way energy moves in waves, pitch is a longitudinal wave where the energy is moving front to back, so that the bow of the ship goes up and down.  Roll is a transverse wave, the energy is moving side to side, rolling the ship from port to starboard (left to right).  To describe yaw, think about sitting in a chair that swivels.  Yaw is the swiveling action of you in the chair moving in the chair or a ship rotating around a vertical axis.  Watch the horizon in the video to get an idea of what pitch looks like from the vantage point of the bridge of the Oscar Dyson.

If you turn your field of view 90°, so you are looking either port or starboard and see the same motion that is roll.

The officers of the Oscar Dyson work to navigate through both the wind and swell waves to give us the smoothest ride possible.

Personal Log 

Recently we experienced sustained wind speeds between 30 and 40 kts.  Needless to say, we were a pitchin and a rollin.  Chiachi Island afforded us calmer seas, as we reached the lee (wind shadow) side of the island.  I noticed something different in this last encounter with rough seas, instead hearing the water race past the hull, this time the water slammed into the side of the Oscar Dyson.  The crashing transformed some of the wave’s kinetic energy into thunderous claps of sound…BAM!…BAM!  What caused the difference, I’m not sure, maybe we were in a convergent zone, before reaching the lee, where wind and seas raced around the island creating a collision akin to clapping your hands together that buffeted the Dyson from both sides.

What do we do aboard ship to cope with the pitch and roll?  We anchor ourselves.  Chairs at our work stations, do not have casters and are tethered to the desk with a cord.  The dressers in our berths have straps to keep the draws shut and the closet doors lock into their closed position.

On a ship the dining hall is called the Mess.  Here the chairs are tethered to the floor.

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