Wire your boat for impeller use with the SpeedCoach GPS 2. Impeller input allows for speed and distance to be measured based on the boat's travel through the water, rather than over land as with the GPS. This means that upstream and downstream splits will be equivalent as they will be unaffected by current. If you row on a river or tidal water and wish to compare your performance to target standards or past rows, use of an impeller provides the most precise and relevant information. Your SpeedCoach GPS 2 offers an easy menu selection for impeller or GPS speed input, allowing you to switch back to GPS mode when you train in an unwired boat.