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Close Pass

Part 1: Passing boats, the suction effect and the science of hydrodynamics.

The success of the Autobahn and high-speed motorways in Europe has proved that going fast in a car is not inherently dangerous by itself. On the Autobahn, the majority of vehicles are traveling at the same or similar speeds. According to the Federal Highway Research Institute, among other factors, “disparity of speed is a contributing cause of accidents on U.S. interstates and highways.”

In other words, fast vehicles and slow vehicles occupying the same roadway at the same time significantly increases the risk of accidents. This condition also exists with boats on the water, but can be even worse because of the way water moves around boats and ships.

One of the places where fast and slow boats are frequently in close proximity is along the Intracoastal Waterway during seasonal migration. Winter brings on the exodus of boats in the Northeast and mid-Atlantic, moving south to warmer waters. This migration, which reverses itself in the spring, consists of a wide variety of boat types and sizes. At numerous points along the ICW, many of these boats are occupying narrow, confined stretches of water, with each boat traveling at different speeds. Add to this a dose of commercial traffic, including slow-moving tugs pushing large barges, and you can begin to see the potential for trouble.

One of the keys to safely handling a boat in crowded waterways is having an understanding of how water moves around a boat. Our lesson here is not about the obvious results of someone’s wake affecting your boat, although that’s serious enough. This is about the more subtle and potentially more dangerous effects of how water, and specifically the shallow water of the ICW, affects the handling of your boat.

Most boaters are aware that when boats move through the
water, they push water downward and outward, away from the boat. The larger and heavier the boat, and the shallower the water, the more water is pushed away.

Boaters may be surprised that the same water being pushed away can also draw boats closer to each other.

Without getting too deep into the science of hydrodynamics, let’s take a minute to understand why this is happening. As
water travels along the side of a moving boat, the resulting turbulence creates a vacuum along the hullside. This turbulence has a suction effect, which basically draws anything nearby toward the boat. There have been numerous occasions when I’ve seen a larger, faster boat attempt to pass a smaller, slower boat going in the same direction, only to watch them be drawn much closer to each other than either planned.

When passing in an opposing direction, the wakes from the boats’ bows will cause the boats to repel each other, but when traveling in the same direction, the boats will be drawn toward each other, with the smaller boat feeling the greater effect. Be mindful to give yourself extra room when passing or being passed in the same direction.

An especially challenging situation is when one boat is passing alongside and is just about to overtake the other boat. Because of the wake coming off the bow of the boat being passed, the passing boat has to apply more throttle. At that point, hydrodynamic forces also tend to deflect the passing boat’s bow away from the boat being passed, and draw the stern toward it. As soon as the passing boat’s bow passes the slower boat, the reverse occurs and the stern is repelled, with the increased drag suddenly disappearing. At this point, the helmsman has to respond quickly to keep from unintentionally steering in front of the other boat.

On a recent passage along the ICW, I watched a small, single-screw, displacement-hull trawler passing a tug, and feared I was going to watch the tug run over the little trawler. It didn’t appear that the trawler owner had any idea what was happening; he only knew he was suddenly being pulled in front of the tug. His problem was that he passed too close in a narrow section of water. The water flowing around the tug was acting on the underside of the trawler, causing a decrease in stability, and increasing the potential for the two to come in contact with each other.

Another way of stating this is: When two boats are next to each other and their bows are abreast, the water passing between the two hulls is traveling faster than the water on the outer side of both hulls. This situation can cause the bows to swing away and the sterns of both boats to be drawn toward each other. The suction effect draws the boats closer together.

If you have not experienced this, or are not aware of this potential, you may not know what is causing it and how to react. Always make sure there is enough room to pass without being influenced by the hydrostatic forces of the other boat. Be certain that you have enough power to overcome those forces should you encounter them, and always call the vessel you’re passing to be sure its skipper agrees that the pass is safe for both boats.

In the next issue, we will discuss how to handle your boat in the wakes of passing boats. 

This article was originally published in the November/December 2022 issue.