A worthwhile addition to the issue of peak power measurement and to the issue of accuracy of the C2 power measurement :
To summarize these issues: a paper from Zagreb University found for a group of young experienced rowers (men and women) an average peak power of 23.5 W/kg for 6 "all-out" strokes and stated that the power displayed by the C2 performance monitor underestimates the true peak power by a factor of ~3.
I found another paper, published in 2007, that measured more or less the same peak power with different tools. It is from the Kinesiology department at the University of Texas
in Austin. The tool was a modified C2 erg. The main modification was that the flywheel with fan vanes was replaced by a more heavy, solid brass disc with 15 slots. These cleverly spaced slots enabled to measure the rotation speed of the flywheel by an optical sensor. Because of the much higher sampling rate per rotation (C2 uses 3 magnets and a Hall sensor) and because the rotation speed data were not processed in real time, the accuracy of the Austin power measurement is likely much better and more detailed than from the C2 system. The high sampling rate enabled them to measure instantaneous power accurately.
The participants were young male varsity rowers (more details in the paper). After a 3 min warm up on a conventional C2 rower, each participant moved to the modified rower and performed five trials consisting of 6 maximum-intensity strokes. Between the trials there was 3 min of low intensity rowing. To help the rowers, the seat had a high friction surface and a seat belt. The ergometer was bolted to the floor.
The figure below is taken from the paper and shows the instantaneous power during the 6 strokes from a representative trial.
Because the first stroke starts with a standstill flywheel, it is much slower than the next and power is also lower. The authors distinghuished between maximum stroke power, maximum pull power and maximum instantaneous power. Maximum stroke power is work during the drive phase divided by the time of drive + recovery phase, which is the same as the way C2 calculates power. The maximum pull power is work divided by the duration of the drive phase only. Maximum instantaneous power is the highest peak value in the 6-stroke sequence.
The average maximum stroke power for these 11 well-trained athletes was 812 W or 9.8 W/kg. The maximum pull power was 1995 W or 23.9 W/kg. The last number is very close to the Zagreb study (23.5 W/kg), which makes it very likely that the Zagreb study also calculated the power for a single stroke and discounted the time of the recovery. The average maximum instantaneous power was 3489 W or 41.9 W/kg. This is already close to instant peak power measured in elite countermovement jumps (CMJ), 50-65 W/kg.
Knowing this, it is not difficult to calculate the underestimation of pull power by the C2 performance monitor using data from RowErg. The RowErg shows stroke rate, drive length and drive speed. From stroke rate, the stroke time can be calculated: Ts=60/sr. From drive length and drive speed, drive time can be calculated: Td=Ld/Vd. Pull power relates to stroke power by a factor Ts/Td.
The main objective of the Austin study was to measure force-velocity relationships on a C2 erg. Because a solid brass disc is much less decelarating from air friction during the recovery, participants have to pull over a wider range of velocities. The figure below shows the relationship for the pull phase (solid squares). This graph helps to better understand what a change in the drag factor does. With a higher drag factor, the rotation speed of the flywheel drops faster during the recovery. Hence, after the catch the handle speed matches the speed of the flywheel earlier in the drive, but due to the lower speed we can pull with a higher force. Conversely at a lower drag factor, for the same acceleration of our body it takes longer to catch up with the flywheel but our higher body speed (=handle speed) does compensate the lower pull force. It is not only likely that the optimal drag factor depends on body weight but also on agility. My guess is that seniors benefit from using a higher drag factor than adviced in the tables.