If you’ve spent any time out on the ocean, you’ve most likely experienced all six of the different motions of a ship, but you may not have been aware of their differences or how they’re defined. After all, the ocean is fluid and in constant motion, and what might feel like a steady rhythm of movement is really the boat rotating around one of its three axes: lateral, longitudinal and vertical.
From gentle sways to more intense motions, the forces at play can significantly affect the vessel’s stability as well as the comfort and seasickness levels of everyone aboard. These motions are most commonly influenced by wave size & direction, wind direction, vessel design, and the water’s depth. Knowing what causes these motions and how to mitigate them can make for a safer and more comfortable journey.
What Are the 6 Types of Ship Motion?
Ship motion refers to the movement of a vessel across the three-dimensional axes of the water. Each motion is linked to different directions and forces acting upon the boat. These six types of motion are:
- Pitch – up and down motion, rotating along the vessel's lateral axis.
- Roll – side-to-side tilting along the vessel's longitudinal axis.
- Yaw – rotation left and right around the vertical axis, affecting the vessel’s direction.
- Sway – lateral movement, where the entire vessel moves side-to-side.
- Surge – forward and backward motion along the vessel’s longitudinal axis.
- Heave – vertical movement, where the entire vessel moves up and down.
Pitch Motion
Pitch refers to the up-and-down motion of the bow and stern, as the boat rotates around its lateral (side-to-side) axis. Pitching is typically caused by heading straight into waves that are travelling the opposite direction. In rough seas, this movement can become quite severe, leading to discomfort and even safety risks if not controlled. It can also be affected by hull design and the speed the boat is travelling, particularly on planing powerboats, which experience bow-rise at slower speeds and flatten out as it gets up on plane.
How to Address Pitch
To mitigate pitch, ensure your vessel is balanced, distributing weight evenly from bow to stern. Adjusting your course & speed relative to the wave patterns can also reduce pitch. Utilizing an automated trim system, like Zipwake, will also adjust the boat's angle relative to the water and automatically react in real time to the motion of the boat in the waves, mitigating pitching while keeping the boat stable and running at the ideal angle.
Roll Motion
Roll is the side-to-side tilting or rocking motion of the boat, occurring around the longitudinal axis (running from bow to stern). This is one of the most uncomfortable ship motions and the one most likely to cause seasickness and affect the ship's stability. Roll is almost always caused by waves striking the side of the boat, but can also happen on smaller narrow boats when weight, like a person, moves from one side to the other.
How to Address Roll
Stabilizing roll is crucial, and there are several systems that can be installed that are designed to minimize it. Of course, altering your course to reduce the impact of waves hitting the side of the ship can also help, but changing course isn’t always an option, and won’t help much in big swells or truly rough conditions.
The best way to stabilize roll on larger yachts and ships are with fin stabilizers or gyros. While they are extremely different technologies, both stabilizer fins and gyro stabilizers aim to accomplish the same goal of producing forces on the hull of a boat in the opposite direction of the roll that is caused by a wave passing underneath. You can learn more about how they work in our Fins vs. Gyros article.
On smaller, faster powerboats, an automated trim & roll control system like Zipwake can provide roll stability while underway. Because the system reacts in real time to the motions of the boat, deploying and retracting the interceptors to counteract the rolling or pitching of the hull, it can help keep the boat flat in wavy conditions.
Yaw Motion
Yaw refers to the twisting or rotation of the boat around its vertical axis. Essentially, yaw causes the boat to turn from side to side, affecting its ability to maintain a straight course. Strong currents, winds, or improper steering can lead to significant yaw. Yaw by itself is unlikely to cause seasickness but can exacerbate it when combined with roll & pitch.
How to Address Yaw
To control yaw, ensuring proper steering and using navigation systems like autopilot can help maintain a steady course. If steering manually, slight adjustments rather than drastic turns can also help control the yawing motion. If you’re navigating a sea with following seas off one stern quarter that is causing the boat to turn as the wave picks the boat up, slight wheel movements in the opposite direction of the waves can help as well.
Sway Motion
Sway is the lateral movement of the vessel, side to side, without any tilting. Sway is usually caused by strong winds or currents pushing the vessel off course. Like yaw, it can interfere with steering and maintaining a straight path.
How to Address Sway
To counteract sway, effective course adjustments and the use of thrusters or stabilizers can help keep the boat steady. Staying aware of wind and current conditions and adjusting accordingly is key to minimizing sway.
Surge Motion
Surge refers to the forward and backward motion along the longitudinal axis of the vessel. Surge is typically caused by sudden changes in speed, such as when the boat accelerates or decelerates in response to waves or the throttle.
How to Address Surge
To reduce surge, it’s important to maintain a consistent speed and avoid sudden changes in acceleration. Proper throttle management, bringing it back slightly as a following sea pick the boat up, then accelerating slightly as the boat falls off the back side, can help keep the vessel moving at a more consistent speed through the water.
Heave Motion
Heave is the vertical movement of the entire vessel, where it moves straight up and down. This motion is typically caused by wave patterns directly beneath the boat. While heave can be uncomfortable, it doesn't usually cause seasickness like pitch or roll.
How to Address Heave
Heave is the only motion that cannot, and really should not, be mitigated. The only way to avoid heave is to avoid conditions with swells all together, otherwise anything that floats will move up and down with the rising and falling of wave activity.
Final Thoughts
Understanding the six types of ship motion—pitch, roll, yaw, sway, surge and heave—is essential for any seafarer. While all motions play a role in how a boat moves through the water, pitch and roll tend to have the greatest impact on comfort and safety. By using stabilizers or pitch & roll control systems like Zipwake, adjusting your speed, and modifying your course, you can significantly reduce these movements and ensure a smoother journey.
Whether you're navigating open oceans or cruising along coastal waters, mastering these motions will help you enjoy your time on the water with more confidence and control.