Northwest Fisheries Science Center

The Main Deck

Acoustic and trawl adventures in the Northeast Pacific

This portal tracks the research and sea-going activities of the Fisheries Engineering and Acoustic Technologies (FEAT) Team from NOAA¿s Northwest Fisheries Science Center.  Follow us as we use acoustics, trawling, and oceanographic sampling to learn about the Northeast Pacific Ocean.

Back deck of Bell M. Shimada
Acoustic echogram of hake
trawl catch

Dampening the roll

By Ensign Phil Manougian
February 7, 2016

Since Bell M. Shimada has so many interesting and unique features built into its design, it’s only fair I share another one with everyone.  Shimada has a special stabilization system that dampens heavy and medium rolls when in rough seas, which is something we encounter at least once a trip.  Operating in the North Pacific Ocean gives us a high likelihood of having to deal with swells causing the ship to roll - sometimes up to 30 degrees – and dislodge items to fall off of things, into other things, and under more things.  Goodness gracious it’s important to be secure for sea!

The anti-roll tank on Shimada is located just forward of the bridge, and one deck below.  The tank, which is filled with fresh water, runs the full breadth (or width) of the ship and is fitted with two internal partitions called baffles.  The baffles have large holes cut out that allow water to pass between the different compartments, but the holes limit how quickly the water can flow from side to side. 

Liquids flowing around in a tank unimpeded create something called the Free-Surface Effect, which can be dangerous if not accounted for in large tanks of fuel or water as it can cause the ship to list more severely than expected.  In this case, when the water flowing from side to side in the anti-roll tank hits the baffles, it is forced to slow down and pile up as it passes through the holes.  The motion of the water inside the tank is thus delayed compared to the roll of the ship.  By controlling how much water is put into the anti-roll tank, the CO can manipulate the roll period so that as the water is piled up on the starboard side, the ship is already rolling to port.  This opposite loading of the tank helps to dampen the overall motion of the ship.  Below is a diagram of the water in the tank and the outside of the ship, for reference.

Source:  Flume Stabilization System and Liquid Level Indicating System Model LLIS-9227-1T

One fun little fact about this tank - it’s just forward of three staterooms on the 02 deck.  The occupants can hear the water in the tank rushing from side to side and hitting the tank walls with a thunderous SPLASH! during rough seas.  Some of the crew find it relaxing, while others… not so much.

Tagged: Winter hake survey

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