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With grateful thanks to the Rec.Sport.Rowing community and Carl Douglas in particular.

Lift is generated when the flow of a fluid aver a surface results in a fall in that fluid's internal pressure. Lift is always considered to act perpendicular to the surface over which operates. Where there is a rise in internal pressure in the fluid, the lift is locally negative. Lift can act in any global direction, that direction being determined by the flow direction & the alignment of the surface over which the flow occurs.

A wing or foil (or a sheet of plywood) experiences a net lifting force (perpendicular to its surface) when the consequence of flow of a fluid (air, water, whatever) over its surface is to create an area (or areas) of reduced pressure. Because the fluid is flowing over (= along) its surface there will also be fluid drag - in the flow direction and parallel to the surface concerned.

The internal pressure in a fluid is induced to fall when the fluid's flow accelerates, because part of its pressure energy is converted directly into the necessary increase in its velocity (kinetic) energy. So lift is a consequence of the local or general acceleration of fluid flowing over a surface.

Planes fly because the air flows faster over the curved upper surfaces of their wings than over the (usually less curved) lower wing surfaces. This creates a useful pressure difference between the upper & lower surfaces which keeps the plane in the air - if it keeps moving fast enough. I've already mentioned drag. Whereas lift, on a decent wing or foil surface, incurs very little drag, turn the wing flat onto the flow & it generates no lift & lots of drag. Much the same with a parachute, or a falling stone. Drag is, of course, a force which can be used, e.g., to propel a boat. Unfortunately, the drag force on a non-lifting body is always bought at a very high cost in wasted energy. And that wasted energy represents work which you did & which is simply wasted.

If drag were an efficient means of moving things through a fluid, then planes would be propelled by reciprocating paddles which moved forward feathered & backwards squared. But drag is an energetically lousy way to generate a force, so planes are propelled by engines which use fast-rotating discs, studded with aerofoil blades, each of which generates lift by slicing at an angle through the air inside (gas turbine), or outside (propeller), the engine. This ends up giving a huge volume of air some backwards momentum to balance the forward force from the net lift forces on all those aerofoils.

The point about this explanation is that the blades which provide the propulsion are not moving backwards (as an oar does in mid-stroke) but are moving forwards & sideways through the air. And the lift forces that each blade generates do not act in the direction the plane is moving but at an angle to that direction. The side-forces on blades moving from left to right at any moment are cancelled out by the side forces acting on blades moving from right to left, etc., leaving only the forward component of force. Yet this is a highly efficient process, or we'd use other ways to propel planes. In just the same way, the oar blade at the catch is moving both sideways (as you swing it away from the boat) & is driven forwards (please note) through the water by the forward movement of the boat. At this stage of the stroke it is acting in the water like a shallow slice of an aircraft propeller. The water is flowing along the blade from tip to root, & being accelerated by having to flow the longer way, over the convex back of the blade - thus generating lift. Although there is a strong side-force component, this is always balanced by the blade on the other side of the boat, so moves nothing & thus does no work & absorbs no work either. In this phase of the stroke the oar works very efficiently to convert your work into propulsion (low drag, so low energy wastage, but high lift, so minimal slippage).
When the blade has swung near to square off with the boat it no longer has much of a flow component along its length, so no lift is generated. All you get is masses of drag - provided the blade slips substantially, face first through the water. But that drag generates great quantities of turbulence, all of which is wasted energy. And the blade now slips, which is the direct reflection of that wasted energy since slip is work that you do which does not move the boat, only the blade & surrounding water. This is the parachute scenario. Sadly, a high proportion of the rowing world thinks & teaches that this is the bit which actually moves the boat when, in reality, it is the bit which is the least efficient & wastes the greatest proportion of the effort you put in.

The situation approaching the finish is somewhat like that around the catch, but with the flow directions reversed.

Note to readers: Dreher oars and sculls are designed with the principle of hydrodynamic lift in mind. That is one reason why the curve on the back of a blade should be completely free of obstructions and as smooth as possible. Look closely at the back of a Dreher oar and you can see that the join between shaft and spoon is virtually seamless and there are virtually no ridges and protrusions from the profile of the back and front of the spoon around the neck of the blade.

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