At the junior level (in the United States), regattas require each rower to weigh in at least two hours before their race; they are sometimes given two chances to make weight at smaller regattas, with the exception of older more prestigious regattas, which allow only one opportunity to make weight. For juniors in the United States, the lightweight cutoff for men is 150.0 lb.; for women, it is 130.0 lb. In the fall the weight limits are increased for women, with the cutoff being 135 lb.
Although the oar can be conveniently thought of as a lever with a "fixed" pivot point in the water, the blade moves sideways and sternwards through the water, so that the magnitude of the propulsion force developed is the result of a complex interaction between unsteady fluid mechanics (the water flow around the blade) and solid mechanics and dynamics (the handle force applied to the oar, the oar's inertia and bending characteristic, the acceleration of the boat and so on).
The rowing machine itself is unlike any other on the market with its patented water filled flywheel. It is hard to exactly copy the action of a scull on the water, but the mechanics of the flywheel spinning in water comes in a close second on dry land. The fact that the water is 800 times denser than air means that there is no need for any extra resistance or dampening that you will find in normal air rowers. The faster you pull, the more resistance is generated giving it infinite variability. However, if you want to be able to practice rowing with a faster stroke, you will have to reduce the amount of water in the tank unlike an air rower where you just have to adjust the baffle.