Returning to the initial purpose of a rowing model - to compare the relative influence that modifying a given variable has on boat speed - a problem is encountered. Assuming that we are studying the effect of a longer outboard length, while all other variables are held constant. A rowing model may predict a faster relative boat velocity (as compared to the standard outboard length). On the basis of this result, can you conclude that a longer outboard is in fact better than a shorter outboard? Of course not. The longer outboard oar will require more effort by the rower, and so it cannot be said with certainty that the increase in boat speed will not be offset by the added energy expended by the rower.
In order to properly account for the interrelated human and mechanical components of the rowing stroke when comparing the effects of changing a variable (outboard length, for example), an efficiency term is defined. In theoretical terms, the oar blade propulsive efficiency is the ratio of the power delivered to the water, in the form of shell propulsion, to the effort exerted by the rower at the oar handle. Broken down into components, this efficiency term becomes,
where,
Ppropulsion is the amount of power delivered to propelling the boat forward,
Pexerted is the effort exerted by the rower at the handle,
Fpropulsive is the component of the blade force acting in line with the boat motion,
vshell is the boat speed,
Finput,handle is the force applied by the rower acting at 90o to the handle,
linboard is the inboard length of the oar,
ωoar is the angular velocity of the oar
Simply put, a faster boat speed for the same effort exerted represents an increase in efficiency (as does the same boat speed for less effort.)
Knowing the interrelated behaviour of these different, measurable rowing quantities, the calculation of the propulsive efficiency of an oar allows the comparison of different blade shapes, outboard lengths, etc.
Before we look into the possibilities that comes from knowledge of the oar propulsive efficiency, the next section deals with some distinctly different phases of the rowing stroke.