How to… Skim a stone
1. The perfect pebble
The size and shape of your stone plays a vital role in the number of bounces achieved by a skim. The ideal stone should be as circular, smooth and flat as possible. This will help to reduce drag while in the air, as well as improving spin rate, angular stability and the lift reaction force as it hits the water.
2. Position matters
The next thing to consider is the angle of incidence between the stone and the water’s surface. This is pivotal as it will largely determine how much of the pebble breaks through the surface during the collision process – a factor that dictates the friction, lift and angular torque generated. Ideally, a low angle of incidence is ideal – ie as close to flat as possible without throwing on a parallel plane – while high angles generally lead to a poor result. The latter inevitably leads to angular destabilisation and limited perpendicular reaction force on contact so it sinks quickly.3. Velocity
Once you’ve nailed your position, the next factor to consider is velocity. This is important to achieving a high number of bounces, but not as critical as most think. The stone has potential energy and, by converting more of this into kinetic energy, obviously it has more to release during the skimming process. However increasing throw power at the expense of other factors can be a false economy. As such, you should only throw the stone at a power you feel comfortable with.
It could be argued that spin (angular motion) is of far greater importance than velocity for achieving a high number of bounces. When a stone comes into contact with the water, the reaction force grants it a degree of torque, creating a potentially destabilising momentum – this can affect its angle of incidence and surface friction for subsequent bounces. With a high spin rate at the point of release, the pebble will have a much more stable touchdown on the water.
5. First contact
Unfortunately from here on in the stone’s progress is out of the skimmer’s hands. When the stone collides with the surface, a section of it will become immersed in the water, as determined by the angle of incidence and tilt. These factors, along with its velocity and angular momentum, determine how much is submerged, and consequently how much friction and lift are generated. As a general rule of thumb, a small amount of submersion leads to reduced friction with the water and enhanced lift, while greater submersion leads to high friction and little, if any, lift.
6. Energy dissipation
The previously mentioned properties of the water reaction force will lead to energy dissipation, slowing the stone’s velocity and – in some cases – spin rate. Both of these, in partnership with any alteration in the stone’s angle of incidence or tilt will lead to a different second contact and water reaction force. As you’d expect, the first contact is critical in determining the general success of additional bounces, with the pebble travelling shorter distances through the air and minimising the potential destabilising effects of wind.
7. Natural obstacles
Despite these steps outlining the core physical processes involved in skimming a stone, as well as some tips for improving your success rate, environmental factors – such as waves or high wind – will make achieving a lot of bounces much more of a challenge. That said, with a bit of luck and at least a basic understanding of the physics at play, anyone can improve their skimming skills.