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More on the free surface effect

Sep 10, 2013

To the editor: I always enjoy reading articles by Ralph Naranjo, including his recent piece on bilge pumps (“Pumps and priorities,” Issue No. 209, Ocean Voyager 2013). I would like to offer some further explanation, however, on his remarks about free surface effect (FSE).

First of all, the science of stability of ships is full of mathematics and formulas. Not all readers care for this, so let’s keep it simple.

As Mr. Naranjo stated, it is true that when you move a weight — like free-flowing water — away from the centerline of a boat, the center of gravity (G) shifts a bit. However, stability of a vessel is primarily determined by what is called the distance between G and the metacentric height (M). If the distance MG is 0, the vessel will be very unstable. Peculiar as it sounds, a little bit of water can easily capsize a large ship. This is what happened to two European ferries. The ferry Herald of Free Enterprise sank off Zeebrugge, Belgium in 1987, killing 193 people. A similar incident happened with Estonia in 1994 in the Baltic Sea at a cost of 852 lives.

Both of these ships were of the so-called roll-on/roll-off type, where trucks and cars drive through a large door/ramp in the bow onto the flat cargo deck. In fact, pretty much the whole bow is the door. The bow is open while in port and closed and secured while at sea. Both ships capsized because seawater entered the cargo deck through the bow door while underway. It was estimated that only a few inches of water had spread out over the full width of the main cargo deck, but enough for the FSE to cause the ships to capsize. It was not the weight of the water, but a sudden decrease in the metacentric height, caused by a couple of inches of free-flowing water. This seems counter-intuitive, but imagine trying to carry a large tray or frying pan with a two-inch high edge that is filled with one inch of water. Even if you keep the tray or pan balanced, you will feel a wobbly effect and will have trouble keeping the tray/pan upright.

FSE usually has much more effect on stability than the weight of water in the bilge. Since bilges are in the lower part of the vessel, below the center of gravity, the weight of bilge water may actually increase the stability (but not the list).

FSE plays an important role on the stability of oil tankers and cargo ships with double-bottom water ballast tanks. The formula for FSE contains a factor of width of the tank to the third power. In other words if the tank is divided in half by a center bulkhead, FSE effect will only be one-eighth as strong.

I am sure by now most people’s eyes have glazed over, but I wanted to show that there is a lot more to FSE than briefly stated in Naranjo’s excellent article.

Years ago, I was a chief officer on a cargo ship. After loading several thousand tons of Scotch whiskey, beer, rum and cigarettes, I calculated that our stability was getting pretty minimal. Then a last-minute order came to load several large earthmovers on deck. I informed the captain that this was very dangerous. He replied we had no choice. I immediately made sure that all ballast tanks where either completely empty or completely full to minimize the FSE. We made it from Liverpool to Amsterdam during stormy weather, with the ship often heeled dangerously when hit by a wave. After that voyage I started looking for a safer ship on which to ply my trade.
 
—Don Dykstra is managing director of Back-Haul Shipping in Houston, Texas.

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