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I read further on the physics of rowing. There's quite a lot of resources out there.
Specifically, I was confused by the center of mass/gravity vs center of buoyancy and the metacenter as mentioned in http://www.atm.ox.ac.uk/rowing/physics/basics.html#fig61. I'll look more into other articles on this topic. I think this is an important concept to understand, because if we make the argument that the center of mass stays the same on an erg on slides, we'll need to show how our erg models the movement of the COM for rowing on water.