Length and Effective Length Are Not The Same Thing
Sander Roosendaal, Czech masters rower and data guru, created a rowing analytics platform that incorporates data from many on-the-water and off-the-water devices, including the NK SpeedCoach and EmPower Oarlock. The free to use platform and Sander's insights into his own rowing data are available on the Rowsandall website.
For one of his recent training sessions, Sander rowed three 1000-meter pieces (the length of a masters race) and recorded the data from his EmPower Oarlock onto his SpeedCoach GPS 2.
There was a slight head wind for the first piece, a slight tailwind for the second piece, and during the third piece, Sander was rowing through a wake. Here is a graph showing each stroke of the three pieces, with the drive length of the stroke plotted according to the total distance Sander had rowed at the time he took the stroke. Total drive length (the distance from the catch to the finish, in degrees) is plotted in blue and the effective drive length (total length minus slip and wash) is in red. (With respect to the EmPower Oarlock system, slip and wash are the length of drive at the catch and finish, in degrees, where there is less than a threshold force on the blade - 100N for sculling and 200N for sweep).
You can see from the plot that the first two pieces were pretty consistent in terms of length and effective length. The third piece is interesting; not because length and effective length decreased (we would expect the stroke to shorten while rowing through a wake), but because effective length was impacted more severely than total drive length. Digging deeper into other data recorded by the EmPower Oarlock, it became clear that Sander was both cutting off the end of his stroke (reduced finish angle) and also washing out. These data didn’t just reveal that he was shortening his stroke when the water got rough, it revealed how he was shortening his stroke. Now, instead of simply trying to "stay long" through wakes in the future, unsure of whether to focus on the catch end or the finish end of the stroke, he can dial in on using his core to maintain good connection with the blade through the finish.