Bank Suction

Topics: Affect, Ship, Hull Pages: 2 (431 words) Published: March 3, 2013
Weird things can happen to a boat when she runs through a narrow channel, dredged cut, or canal. The larger the vessel, the weirder it gets. And when increased speed and shallow depth are plugged into the formula, the situation can become downright dangerous. It all has to do with the strange effects of "bank suction."

Bank suction is the tendency of a vessel traveling near a steep underwater bank to move sideways toward that bank while an opposing force pushes the bow away.

Although it usually affects large ships, it can affect smaller yachts and boats as well.

The combination of close proximity to a steep bank, shallow depth and increased speed are the critical factors that determine the severity of the bank-suction effect.

Bank suction starts when a vessel strays too close to a bank, restricting the water flow on its bank side. The water-flow velocity increases, causing the later between the vessel and the bank to squeeze out of the area faster than it can flow back in. This causes the water level to drop between the vessel and the bank, and consequently the vessel is pulled sideways into the low-water area. At the same time, there occurs a second phenomenon known as "bank effect." Here the bow wave reflects off the bank and causes twisting effect on the entire vessel as the bow sheers away. If you do nothing to counter these effects, your vessel could actually cross the narrow channel and run into the opposite bank. Several shallow-water effects can also come into play. First is the tendency of a vessel to sink below her normal draft as her speed increases in shallow water. Just a few knots above idle speed is enough to cause the hull not only to sink down, but to “squat” significantly at the stern. As a vessel sinks and squats down, steering becomes less responsive and the turning radius increases. Water flow under the vessel is restricted; it can't flow as is normally does at greater depths and tends to flow more around the hull sides, thus...
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