Chine Walk Tech

(This article is a work in progress. A discussion thread can be found here: http://www.speedboatsim.com/main-forum/chine-walk-tech-article-discussion/)

What is chine walk?

Chine walk is a dynamic roll instability exhibited primarily by vee bottom watercraft at high speeds. This is illustrated in the following video from the Windows PC simulator, “Design it, Drive it : Speedboats”:

Shown here is severe chine walk.  Chine walk generally starts as a small oscillation which increases in magnitude as the speed of the boat increases beyond some threshold speed (referred to in this article as “chine walk threshold speed”) unique to the particular hull design and boat setup. At speeds below this threshold, the roll motion is inherently stable at all practical roll angles and the roll oscillations associated with chine walk do not occur. At speeds slightly above this threshold speed, the chine walk may be a mild, harmless rolling motion. As speed increases further beyond the chine walk threshold speed, chine walk progressively increases in severity and can quickly become dangerous. The rocking can become violent enough to eject the driver and/or passengers in a serious accident, cause the boat to spin or roll over, and in a worse case scenario at especially high speeds could even lead to a blowover similar to this:

Chine walk most commonly occurs with boats having a padded design, probably because the waterline at any given speed tends to be narrower, but it can also occur in boats without a pad. From the simplified model used later in the article, it appears likely that all vee bottom hulls will chine walk if they’re going fast enough.

What causes chine walk?

Chine walk appears to be primarily caused by the relative motion of the boat’s center of gravity and the hydrodynamic lift centroid in the lateral direction. At low speeds, the waterline across the beam (width) of the craft is relatively wide and progressively becomes narrower as the boat’s speed increases. The boat lifts out of the water as it speeds up. The lifting and corresponding narrowing of the waterline is partly due to aerodynamic lift and partly due to hydrodynamic lift. To illustrate the narrowing of the waterline due to hydrodynamic and aerodynamic forces, the next video shows an acceleration run up to the range of speeds where the boat begins to chine walk. Once the boat has planed, attention should be paid to the blue outline visible through the boat which shows the intersection of the boat hull with the water. The waterline gets narrower (and smaller overall) as the boat’s speed increases. Once the waterline has become sufficiently narrow, chine walk inevitably develops.

A simple computer model separate from Design it, Drive it : Speedboats was developed by the author with the purpose of investigating and illustrating chine walk in a simplified case. The same narrowing of the waterline with increasing boat speed discussed previously can be seen in the simplified model used for the rest of this article.  This represents looking at the vee of the hull which is partly under the horizontal waterline.   The green dot near the center is the boat’s center of gravity.  The two arrows extending diagonally are the hydrodynamic forces which lift the boat.  The longer arrow pointing upwards near the center is the resultant force (the sum of the other two).   This is the total hydrodynamic lift force referred to in the rest of this article.

A change in the boat’s roll angle will cause the centers of hydrodynamic lift and gravity to move laterally relative to each other. At low speeds where a wide waterline is present, the hydrodynamic lift centroid moves laterally at a higher speed than the center of gravity when roll velocity is non-zero. This keeps the position of the center of hydrodynamic lift outside of the center of gravity in a manner resulting in a stabilizing moment on the craft. This motion is demonstrated in the next video where the boat is rolled manually with a mouse:

As the boat’s speed is increased, the waterline width narrows which reduces the lateral velocity of the hydrodynamic lift centroid at any given roll velocity and angle. Given enough speed increase and corresponding narrowing of the waterline, the center of gravity at small roll angles will move laterally at a higher speed than the center of hydrodynamic lift, resulting in the center of gravity being positioned further away from center than the hydrodynamic lift centroid at any given roll angle. In this situation, across a potentially narrow range of roll angles, the moment about the boat’s center of gravity is reversed and the moment becomes destabilizing.

In other words, as the boat begins to roll away from a 0 degree roll angle, if the center of hydrodynamic lift moves further laterally than the center of gravity moves, the boat is stable in roll. If it doesn’t move far enough, it is unstable.

In the video below, the boat’s speed is increased for the first seconds which raises the boat and narrows the waterline.  The center of hydrodynamic lift moves toward the center of gravity and eventually switches sides, crossing from the right to the left side of the center of gravity which is unstable.   The boat is then rolled manually through a range of angles to show the area inside and slightly outside of the unstable region.  The key point to take away from this is that at high speeds where the waterline is sufficiently narrow, there is an area around 0 roll angle where the boat is unstable, meaning it has a tendency to roll away from center.

This can be further illustrated by setting the boat’s roll angle to some non-zero value and examining an increase in speed by raising the hull further out of the water:

In the above video, the narrowing of the waterline due to increasing boat speed causes the hydrodynamic lift center to first move in the direction of decreasing the length of the moment arm about the center of gravity.  In this case toward the center of the boat.  Given sufficient narrowing of the waterline, with a right hand (clockwise) roll angle as shown in the video, the hydrodynamic lift center will move from the right side of the center of gravity to the left side. At this point, the roll torque about the center of gravity has reversed and become destabilizing instead of stabilizing. Chine walk, a forced oscillatory roll motion, can then be expected to occur due to the presence of the unstable region near zero roll angle that has developed. The boat has reached its chine walk speed threshold as soon as an unstable region of roll angles has developed.

Why does chine walk get worse as the boat’s speed increases?

It may be useful to examine this in terms of a “critical angle.” The author defines this for the purpose of this article as follows: The critical angle is the boat’s roll angle at which the torque resulting from the hydrodynamic lift and center of gravity couple changes from destabilizing to stabilizing. It is the crossover point, the roll angle at which roll torque is 0 and a further increase in roll angle will result in the torque changing signs (going from unstablizing to stabilizing). At roll angles closer to zero than the critical angle, the moment is destabilizing. At roll angles further from zero than the critical angle, the moment is stabilizing.

This is more easily illustrated in the following video in which the critical angle is 5 degrees:

In this example, the roll torque is destabilizing between roll angles of -5 and +5 degrees. The boat doesn’t “want” to return to a roll angle of 0 degrees naturally the way it would at lower speeds with a wider waterline. The boat rolls away from 0 degree roll angle within this unstable zone of roll angles. At roll angles exceeding +/-5 degrees, the roll torque reverses direction and becomes stabilizing. This interplay as the boat rolls though the unstable and stable regions results in chine walk, the forced roll oscillation.

The initiation of chine walk in the first place is another matter for consideration: If the boat is running with a roll angle of 0 degrees and 0 roll velocity, it will remain in that state so long as the torque sum about the center of gravity is 0. The presence of any disturbance that results in a non-zero torque on the boat will then result in a non 0 roll angle and roll velocity developing resulting in development of chine walk. This torque could be anything. It could be due to a change in propeller torque (maybe an area of cooler or warmer water was just driven through), center of gravity position due to movement of passengers, fuel, etc., a change in the position of the center of hydrodynamic lift caused by a small wave striking the hull, a change in the aerodynamic forces acting on the boat (a gust of wind, etc.,), a change in engine rpm with boat speed (and therefore engine and prop torque), a change in throttle or engine trim or steering, or any combination of these.

The key is “any disturbance.” Once there’s a disturbance away from the perfectly balanced case, the roll velocity will become non-zero and the chine walk roll oscillation will progress on its own even if the torque disturbance that initiated it all is removed. The basics are illustrated in the next video. Note that the roll angle of the boat in the video was being controlled manually by manipulating the angle directly by hand while the video was recorded, so the roll velocity and angles obtained are not a result of physical computations. The roll angle is just being eyeballed and controlled with a mouse by hand. This simple model is merely intended to illustrate the conditions where the roll acceleration reverses as it traverses through zero roll angle, the critical angle and beyond, and to explore the various effects and tendencies in a general sense.

The roll oscillation in this model should therefore be expected to be a sort of wobble rather than a smooth trace resembling a sine wave. It would be interesting to expand the model to include plots of the motion and generate roll acceleration and velocity traces. This has not been done by the author at the time of this writing.

Returning to the question: Why does chine walk get worse as the boat’s speed increases?

As the boat’s speed increases in this model, the critical angle increases due mainly to the narrowing of the waterline and coinciding reduction in the lateral speed of the hydrodynamic lift center. This is illustrated in the following pictures. The first picture has a wider waterline to approximate the situation at some “low” speed in which chine walk development might first become noticeable to a pilot. In this example the critical angle is about 5 degrees. (A real pilot will detect much smaller roll angles, the angle is artificially large here just for illustration):

Then the speed is increased resulting in a narrower waterline and slower lateral movement of the hydrodynamic lift center. The critical angle increases to 10 degrees:

As the boat’s speed increases further, the critical angle will continue to increase:

So as speed increases, the roll angle that the boat will accelerate toward before slowing down for a rebound oscillation increases. In addition, the roll torque at any given roll angle within the unstable zone (roll angles between +/- the critical angle) can be expected to increase as well. Here are the previous three “boat speeds” with roll angle set to only 3 degrees:

Slow:

Faster:

Fastest:

Here we see the magnitude of the destabilizing moment increasing with boat speed in the model. So at 3 degrees roll angle, the roll torque and roll acceleration increases with boat speed. More precisely: The roll torque increases with a narrowing of the waterline in this example.  This is assuming no aerodynamic lift.

Aerodynamic lift effects:

This article so far has made the simplifying assumption that changes in boat speed change the waterline width while keeping the magnitude of the hydrodynamic lift center constant. With this simplification, the weight of the boat is supported entirely by hydrodynamic lift, none by aerodynamic lift. Similar trends in the variation of critical angle can be observed by varying the weight of the model boat in a manner roughly similar to what would be expected in the presence of aerodynamic lift. This is still a significant simplification in that it effectively works as though the aerodynamic lift center is located at the boat’s center of gravity, there is no aerodynamic roll moment present. However, an interesting development comes out of this even with the simplification which will be explored briefly now.

First, a video illustrating the difference in the model between varying the weight of the boat and varying the hydrodynamic force per unit area of wetted surface is shown. I.e., first without aerodynamic lift in which the hydrodynamic force remains constant, then changing it in a way in which hydrodynamic force decreases with speed as would be expected in the presence of aerodynamic lift. In the first case the hydrodynamic lift forces change magnitude. In the second case they do not.

The general effects on the critical angle are similar in both approaches, as are the general effects on roll torque in the regimes examined so far on the particular boat model parameters used so far in the article. However, there is a noteworthy difference when assuming the extra lift with increasing speed is generated aerodynamically versus not:

In the “hydrodynamic lift only” case, as the boat’s speed is increased while the roll angle is held within the unstable zone, the destabilizing torque increases as shown in earlier videos. However, in the “aerodynamic lift included” case, the roll torque increases much as before for some time, but it begins decreasing again once the waterline becomes sufficiently narrow:

At the point at which the boat was completely flying and not in contact with the water, the hydrodynamic force would be zero and therefore the torque due to that force is also 0. This limit can no longer be considered hydrodynamically unstable (although it will likely be aerodynamically unstable). Chine walk due to hydrodynamic lift no longer exists at this limit, and as can be seen in the video above, it should be expected that the destabilizing roll torque will increase until the boat has lifted sufficiently out of the water for the roll torque to begin decreasing again, eventually reaching the limit when the boat is no longer in contact with the water.

A further analysis of this would be interesting to explore, but has not been done at the time of this writing. However, from the general trend in the simple model it appears reasonable to expect that if the ratio of aerodynamic lift to hydrodynamic lift was sufficiently great, there may be a second threshold speed at which the chine walk effect is reduced again because the hydrodynamic torque could begin decreasing at some boat speed, at least so far as the hydrodynamic lift center is concerned. The author is not aware of this ever occurring in practice with real boats however, so the practice of attempting to eliminate or reduce chine walk by increasing speed beyond some even higher threshold speed than the chine walk threshold speed is not recommended!

Effect of deadrise angle on critical angle:

The next video shows some effects of changes in deadrise angle on the critical angle. The roll angle is held constant at a large roll angle (10 degrees) for easy visibility while the deadrise angle is varied.  Starting deadrise angle is 20 degrees. The roll torque transitions from stable to unstable as the deadrise angle is increased. It may be reasonable to assume that the chine walk threshold speed would increase as deadrise angle is decreased. This appears to be the case in the more sophisticated model used in “Design it, Drive it : Speedboats” too, and a primary reason for that appears rather obvious by examining the much simpler vector model.

Effect of center of gravity height on critical angle:

The next video shows the effect of changes in the center of gravity height. As the center of gravity height is raised above the green cross, it moves from one side of the hydrodynamic force center to the other. The stabilizing torque transitions through zero and becomes destabilizing as the center of gravity is raised sufficiently. This suggests that lowering the center of gravity will tend to increase the chine walk threshold speed.

Other effects not included in the simple model:

The simple vector model used here likely would not explain chine walk oscillations becoming progressively worse with time. The author has not analyzed this closely at the time of this writing, but it at least appears that the forced oscillation is explained reasonably well by the model simply due to the existence of an unstable zone near zero roll angle. As the speed of the boat is increased, the unstable zone grows (the critical angle increases) while the magnitude of the roll torque within that zone also generally increases. It should, however, be noted that this component of the torque may very well be self damping in the simple model. If this is the case, then the chine walk oscillation, once begun, might remain stable in magnitude between two roll angle values in this type of model. This has not been examined by the author. The simple model does not include any motion simulation, so this is uncertain.

Design it, Drive it : Speedboats is a much more comprehensive model than the simple vector model shown in most of the videos here. It includes drag forces, aerodynamics, usually an offset center of gravity not cleanly lined up with the bottom of the vee, p-factor effects on propeller torque, etc.. In the more complex model, chine walk oscillations generally start mildly and become more severe over time even if the speed of the boat is held constant. In this case, however, there is generally a speed range very near the chine walk threshold speed at which chine walk is mild enough to result in regular oscillation that doesn’t increase in magnitude over time. A gentle chine walk oscillation with little drama that doesn’t require any driver correction. At higher speeds, however, the oscillation becomes self exciting: The peak roll angle generally increases with each oscillation. This can quickly lead to disastrous consequences if not corrected.

One likely reason for this difference in the models may be that the yaw angle (rotation around the vertical axis in world coordinates) changes while the boat is rolling. This can be expected to cause additional lateral hydrodynamic forces to be exerted on the hull that are not included in the simple vector model. Were these extra forces included in the simpler model, they would be expected to have the effect of changing the position and magnitude (and probably the direction) of the hydrodynamic force center which would change the roll torque differently than predicted otherwise. These effects are included in Design it, Drive it : Speedboats and tend to exacerbate chine walk.

Can chine walk be cured?

This analysis suggests that chine walk would occur in every vee bottom boat hull (pad or padless) so long as a great enough speed can be attained. Once the waterline is sufficiently narrow, an unstable zone near 0 roll angle should develop. The critical angle should increase as the waterline continues to narrow with further increases in speed, tending to make chine walk worse. For the purposes of boat design and setup, it may be useful to think of the onset of chine walk in terms of a “chine walk threshold speed” used in this article which is the speed at which the unstable zone first develops. Changes in hull design or boat setup, engine trim, etc., will influence this chine walk threshold speed. Eliminating or reducing chine walk could then be thought of in terms of increasing the threshold speed to something just beyond the top speed of the craft. I.e., increasing the width of the waterline in order to allow greater movement of the hydrodynamic force center laterally would seem to be a good place to focus. With Design it, Drive it : Speedboats, keeping a close eye on the width of the waterline as the design is altered seems to work well.

The chine walk threshold speed and the top speed would appear are be mutually exclusive design variables unless the blowover speed is greater than the chine walk threshold speed. It must be noted that this widening of the waterline at the rear of the boat (perhaps through a decrease in transom deadrise angle) increases hydrodynamic drag. (A wide, narrow water contact area generally produces more drag than a longer, narrower area). So if the goal is to maximize top speed, some amount of chinewalk would appear inevitable if sufficient power is available.

The author would therefore not characterize the existence of chine walk as indicating a design flaw. Rather it is likely an inevitable consequence of a never ending narrowing of the waterline as speed is increased in any vee bottom hull. If the hull is designed to maximize top speed with a given amount of engine power available, in many cases the chine walk threshold will be less than the top speed and therefore the boat will chine walk.

It would then be up to the driver to continuously counteract the roll oscillations via steering corrections while operating the craft within the unstable zone. A sufficiently skilled pilot can operate such a craft without any visible roll angle oscillations ever developing. It is enough to use roll acceleration cues without waiting for any significant roll angles to develop. A steering rhythm can be developed that is unique to each boat. Practice makes perfect!