Oar blade motion and forces

Path of the blade through the water

As we begin to understand the flow of water around the blade, it helps to look at its path traced through the water during a stroke. Imagine being above the water and looking down as a boat rows past, what we would see is that the blade moves in a figure-9 pattern.

While the blade is buried in the water during the drive, the blade moves simultaneously parallel (in the x-direction) and lateral (in the y-direction) to the direction of boat travel. The movement of the blade parallel to the direction of boat travel is known as slip. Positive slip is blade motion in the same direction as the shell velocity, while negative slip is opposite the shell velocity. The lateral motion of the blade is caused by the rotation of the oar.

Forces on the blade

If we now look at the instantaneous motion of the blade relative to the water at any point during the stroke (so, at any location along thefigure-9 trajectory), we can calculate what the blade 'experiences'. By vector addition of the velocity of the boat (v_shell) to the rotation of the oar blade (v_blade), the result is the velocity that the blade sees (v_relative). The angle that this velocity makes with an imaginary line connecting the blade heel to the tip is known as the angle of attack (α_nominal).

The overall force that the water creates on the blade (F_net,blade) can be broken down into several components. Acting in the direction of the relative flow on the blade is the drag force (F_drag), and acting perpendicular is the lift force (F_lift).

The combination of drag and lift forces on the blade creates the propulsive force (F_propulsive), acting along the line of motion of the shell.

When we combine the instantaneous motions of the oar and the shell with the forces on the blade, an oar blade propulsive efficiency can be derived.