15 Important Quantities and How to Measure Them
Direct Measurements
Basic measurements have long been available with moderate priced instruments. A lot of additional information can be derived from the basics by calculation.
- Bearing: compass direction the bow of the boat is facing, degrees magnetic.
- Boat Speed: forward motion through the water, usually in knots. Not the same as speed over ground unless there is no current in the water.
- Apparent Wind Speed: speed of the wind felt on the boat as a combination of air motion over the water and the motion of the boat through the air, usually in knots.
- Apparent Wind Angle: direction of the apparent wind relative to the boat, usually degrees from the bow of the boat, negative if from the port side, positive if from the starboard side.
- Heel: difference in the roll angle from horizontal, usually degrees, positive for starboard side down.
- Position: location of the boat in world coordinates, usually expressed as latitude and longitude in degrees, ignoring elevation.
- Depth: either as local depth of water or additional depth of water under the boat, metres or feet.
There are lots of additional measurements that can tell us more about what is happening on the boat, requiring instrumentation that is not as common.
- Inertial Quantities: pitch, roll, and acceleration are key to performance.
- Forces: standing and running rigging, as well as control lines all carry forces that directly or indirectly relate to performance.
- Shape and Deformation: sail shape determines lift and rig shape determines sail shape.
- Ranging: relative position to other boats is essential to tactical decisions.
Many class rules restrict access to certain measurements or to results automatically calculated from the measurements. This is usually justified as requiring competitor skill, or maintaining a reasonable cost for participation. The cost argument is much weaker now that almost everybody has high performance GPS and IMU instruments built in to their phones.