How to have a race when your boat doesn't move!
Concept 2's performance monitor (PM) uses signals from a sensor to detect the motion of the flywheel, much the same as a cycle computer uses a sensor mounted on the front forks of your bike to detect the motion of your wheel. Both have a built-in "clock" to measure time intervals required for the calculations.

However, unlike the cycle computer, the PM can't use distance or speed to monitor performance simply because a rowing machine doesn't actually travel anywhere ... there is no speed or distance to measure! Instead, the PM uses the power applied to the handle to gauge how hard you're rowing.

How is it able to determine this?

Power supplied to the handle: a measure of rowing intensity
Physics tells us that work is done when a force applied to an object causes it to move. For example: a person pulling on the handle of a rowing machine does work. Now, without getting too technical, it turns out that the work done in pulling the handle during the rowing cycle can be calculated with knowledge of:

• The motion of the flywheel during the drive
• Some physical quantities associated with the rotating flywheel-fan assembly (predominantly something called the 'moment of inertia' - which depends on the mass and how it's distributed over the assembly)
• The motion of the flywheel during the recovery.

By dividing the amount of work done at the handle by the time taken to complete a stroke the PM calculates the average power over the stroke. The power, in Watts, is one of three different measures of rowing intensity that the PM can be set to display.

'Row, row, row your notional boat'
In order to introduce a more familiar concept to the measurement of an indoor rower's performance, a formula is built into the PM which links the stroke power to the speed that an on-the-water boat may typically travel if the oarsman were to supply the same power to propel his or her boat. For the mathematically minded, the relationship between power and 'boat speed' used by Concept 2 is:

    Power = 2.8 x Speed ³    (Power in Watts, speed in metres/second.)

For example, if the power at the handle is 350W, the 'speed' will be 5m/s, because 2.8 x 5³ = 350.

Although this 'speed' is an abstract quantity, its mathematical relationship to the actual power at the handle makes it a perfectly valid quantity for comparing rowing intensities.

Now that we have a means of deriving a 'speed' for each stroke, it becomes a simple matter to calculate the 'distance' travelled by our notional boat over a given period of time. The use of the above formula linking power to speed and the accuracy with which Concept 2 rowing machines measure power are what make competitive indoor rowing over fixed 'distances' a practical reality.

Two more expressions of rowing intensity: pace and energy expenditure
During a race, knowledge of your pace (i.e. the time taken for your notional boat to travel a fixed distance), rather than your speed, makes it easier to predict your finish time. So in preference to displaying speed, the PM can display the pace in minutes and seconds per 500m - for each stroke, averaged over a split or averaged over the entire row.

Using our earlier example: at a 'speed' of 5m/s you would take 100s to 'travel' 500m. So a rowing intensity of 350 Watts is equivalent to a pace of 1:40/500m.

Finally, an attempt has been made by the PM designers to relate the power exerted at the handle to the rate of total energy expended by the user while rowing. The units are Calories per hour. This is based on two arbitrary assumptions:

1. The rower expends energy at the rate of 300 Cal/hr performing all functions other than applying power to the handle. (This includes basal metabolism plus the energy required to perform the rowing movement up and down the monorail.)
2. Of the additional energy required to supply power to the handle, only 25% manifests itself as mechanical power at the handle.