Oars shafts contribute to boat speed purely as a lever.
Oar Shaft Drag is the extent in which an oar shaft encounters the water, counteracting the forward momentum of the boat. The rower’s effort moves the boat but with normal oars is required to move up to half of the oar shaft through the water.
You can see in the image below that half of the oar shaft is in contact with the water. The rower forces the shaft down into the water at the catch, pulls the shaft through the water during the stroke and then draws it to the surface at the finish.
Each of these movements effects boat speed, and when this is multiplied by eight oars and then by over 200 strokes in a race this drag effect is huge.
The role of placing a Randallfoil on a blade is to remove the oar shaft completely from contact with the water.