When a fine-bowed vessel glides through smooth blue water, the bow seems to cleave the water effortlessly, slicing a passageway through which to gracefully pass. To an extent, the eye does not deceive, but to one degree or another all vessels — finely shaped or otherwise — push, or displace, water to make way for the hull. They push it ahead, push it aside, push it down. Sometimes they push it up in the form of spray. Some boats push more water than others, depending on size, hull shape and speed.

 On the open ocean, it doesn’t matter where this water goes, but in confined waterways it can interact with banks, bulkheads, bridge abutments, the bottom and the hulls of passing vessels in predictable and unpredictable ways.

Bank effect is one of several related phenomena involving pressure differences that form around a moving vessel in confined waters. The principle of fluid dynamics most closely associated with these phenomena is Bernoulli’s Law, which provides the mathematical explanation for the formation of these asymmetrical zones of pressure. In the case of bank effect, displaced water squeezing between the bank and the hull produces differences in pressure between the port and starboard sides, as well as between the bow and the stern. Under the right conditions, these pressure differences can dramatically affect the vessel’s handling.

Bank effect has two main elements: bow cushion and stern suction. We have all seen how a vessel pushes water forward, creating an area of high pressure that laps up the stem. In open water, that bow wave dissipates harmlessly. In a narrow waterway, the bow wave encounters the embankment on one side, forming a cushion that forces the bow away from the bank and toward the zone of relatively low pressure on the other side of the bow.

Next, the water from the bow wave is squeezed between the boat and the bank as it flows aft to fill the void left by the passing hull. To make that trip on time, the water must flow faster on the side closest to the bank than on the side away from the bank. This triggers a drop in pressure farther aft, causing a lateral attraction toward the bank. The propeller pulling water from beneath the hull can add to this effect. The combination of the bow pushing away and the stern sucking in creates the potential for a vessel to sheer abruptly toward midchannel. This may not be significant — unless there is another vessel, or two, right there.

Most of the time, bank effect doesn’t amount to much. You might find yourself applying a little rudder to keep the vessel tracking straight. A small craft — a skiff, for example — may never experience it. In general, the likelihood of a memorable encounter with bank effect depends on three things: hull shape, speed and waterway configuration. Shallow water (a relative term) and prop action also can contribute.

Hull shape: A boxy, full-bilged hull with a bluff bow will displace more and, therefore, push more water aside than a sleeker vessel of the same length, beam and draft. The fuller shape forms a chamber between the hull and the bank, constricting the flow of water and enhancing low pressure near the stern. A sleek hull with a lot of deadrise and a fine bow, such as a sailboat hull, will tend to do less of this.

Speed: A faster-moving vessel will develop a bigger bow wave than the same vessel traveling slower. Displaced water can’t get out of the way, so it piles up between the bow and the bank, amplifying the bow cushion. Meanwhile, more water must squeeze faster between the bank and the hull, amplifying the low pressure aft. Additionally, a fast-turning propeller will pull more water from beneath the hull, helping to sustain the low-pressure zone. 

Waterway: On a broad waterway lacking distinctive vertical banks, a boxy, fast-moving vessel may experience no bank effect. However, the same boat hugging the bank on a smaller waterway with well-defined banks may be susceptible. And, of course, the nature of a given waterway is rarely consistent. The conditions for bank effect may not exist in one stretch of water, but a mile or two later, they can.

The nightmare scenario for bank effect involves multiple vessels. Picture two vessels meeting port to port, just as a third decides to overtake. Each is displacing water according to its own size, shape and speed. Each is creating zones of high and low pressure. By necessity, any three-way meeting entails a certain amount of crowding, among one another and with the banks. With currents and eddies as secret ingredients, the recipe is complete for one or more vessels to respond unexpectedly. Anything can happen at any moment. And small tolerances equal short reaction time.

 What can you do to minimize this risk? First, don’t be the guy attempting to overtake while two vessels are meeting in close proximity. Second, if things are shaping up for trouble, pick up the VHF and say something that will make the situation safer, or use your horn. If none of these apply, slow down, get out of the picture, keep your hand on the throttle and be prepared to maneuver aggressively. Once bank effect has taken hold, slowing down may not help. You must be prepared to jack up the rpm momentarily with hard rudder so the vessel will respond. This means keeping some of your ponies in reserve.

Occasionally we have to take a big boat into a small waterway. This is a good time to be attentive to bank effect. Decreasing speed is a good place to start. One general rule to minimize risk is to maintain at least one vessel width from the bank, but no general rule can be absolute.

The hydrodynamics of bank effect are irrefutable, but the real world holds so many variables that nothing may happen when you think it will, and something might happen when you think it won’t. It is possible to operate for years on waterways, rivers and canals without ever observing bank effect. Like skeptics of the green flash, a person may cease to believe bank effect exists. And then one day it happens, and you are a believer.

This article originally appeared in the May 2017 issue of Soundings.