The usual solution is to brace the feet solidly. A handy bulkhead or hull frame might do. Custom cleats for the feet to push against attached to the hull are also common. If the rower now turns downwind he may find his same power will push the boat at 5 mph. As shown in Figure 3 he will now have 3.6 pounds of force at the blade of the oar.
Less force is required there now since the wind is helping to push the boat along.
Now let's imagine the rower encounters a headwind that slows the boat to 3 mph. As shown in Figure 4 the blade will balance now with 5.5 pounds of force at the blade. That's an increase over the first condition and it's that extra blade force that will help push through the headwind. And one can see that the total length of the oar might not really enter into the above discussion, only the location of the pivot point effects the gearing of the oar. But the length of the oar does have effects.