Why Your Boat Wanders - The Science Behind Low-Speed Steering Oscillations

Boaters often ask about low speed steering oscillations in their boat.

“When I am driving very slowly and holding the wheel in a straight line, the bow wants to swing back and forth?”

I’ve addressed this phenomena before, but lets provide more details on both the underlying cause and how you can remedy it.

To be sure we are all on the same page, here is when it happens and how it feels. As you are driving your boat through a slow zone, or under 1,000 RPM and in a straight line, your bow slowly swings to one side. As you try to correct it, the bow begins to swing in the opposite direction. You find yourself continuously using your steering to correct the ongoing swing as your bow continues its minor oscillations. This action is referred to as “hunting." What causes it?

Your boat encounters different forces as it travels through the water at different speeds, and different parts of the boat come in contact with the water. Sitting still in water, the “footprint” of your boat on the surface of the water, or looking up at your boat from underwater will look something like this:

To move forward, you advance the throttle slightly and the boat will move ahead slowly, but at minimal throttle it will not dramatically change this footprint. It remains what is termed a displacement hull. At this slow speed it is pushing itself through the water by splitting the water and moving through it. But, most runabout hulls are designed to operate as a planing hull by raising itself relative to the water and running on the water surface. Once your boat has raised itself onto plane to travel faster, it’s footprint on the water surface is dramatically reduced. Here is the smaller running footprint, superimposed over the footprint at slow speed.

You can see from the above sketches that when ‘hunting’ occurs, you are driving slowly and your contact with the water resembles the top drawing. With this footprint, there is a greater portion of the boat hull in contact with the water.

Higher contact with the water is your first clue as to the cause. We know that when water molecules travel past the sides of your boat in the water, they create what is called a “boundary layer." Within this boundary layer, the water molecules at the very surface of your hull are moving very slowly, if at all. This layer flows smoothly, creating little resistance and is called "laminar flow."

As you move to the layers further out from the boundary layer, you come to boundary layers of molecules that are moving progressively faster and are tumbling over each other in turbulence. This type of boundary layer is called "turbulent flow." Laminar flow has much less drag than turbulent flow. It is this difference in the magnetic molecular attraction between boundary layers that creates a difference between the attraction in the surface water molecules and the outer molecules within the boundary layer.

This difference in the boundary layer pulling power from one side to the other will differ as your boat points in a different direction, increasing or decreasing the distance the boundary layers have to follow. A slight difference in steering input will momentarily give one side an advantage in how much pulling force is created and the bow will swing in that direction. But once the forward direction of your boat begins to change relative to its centerline, there becomes a greater distance for the water to travel backwards along the opposite side of your boat. This provides a greater attraction area or pull to that opposite side and your bow pulls back the other way, due to the same molecular magnetic force in the boundary layer, but on the opposite side. Thus, "hunting" is caused by this "viscous pressure drag" alternating in its pulling force from one full boat side to the other. There sure are a lot of interesting hydrodynamics going on with your boat.

The next question becomes: why do not all boats have this annoying trait? As stated, there are two types of flow inside a boundary layer on a boat. One is laminar, when the water molecules slip smoothly over one another, and the other is turbulent flow when the water molecules move around randomly, with layers mixing together and particles crossing paths. If the architects of your boat designed a hull where the boundary layers on each side flowed along the hull in such a manner that cancelled out, or reduced differences in the flow patterns within the boundary layer, then you have a well designed boat. Hunting can be one way for you to test for a well-designed boat before you buy it.

The good news is, even if your boat hunts, as a masterful captain there is a driving procedure that can eliminate it. If while driving along at low RPM one day, you find your bow wants to swing right then left, the instant it begins to turn in one direction (lets say to the right), give the wheel a tweak to the opposite (left.) Not a directional turn, just a short tweak away from the direction that the bow is starting to move. In this case the bow swings right and you tweak left. Then, almost immediately, and just as the boat is completes the right movement it started, turn slightly to the right some more.

This breaks the built-up the molecular force that is about to begin pulling your boat back to the left. Your steering tweak will cancel the swing, but not steer in the other direction. Once you catch on to performing this corrective input it will become second nature and the next time your boat wants to take you for a ride, you will no longer look like it is your first time at the wheel. #tips #quicktips