Blackwood
Contributor
Preface:
I have no reservations that split fins are easier to kick than paddle fins. They "give" against the water. That's a no brainer. For some, that suits their leg strength, endurance, and style. For others, a blade works better. This thread (I hope) won't become "blades are better" or "splits are better" and I'll request that any such replies be removed.
Rather, I want to discuss the physics of split fins. In particular, I often read here and in magazines that split fins use hydrodynamic lift to thrust the diver forward just like an airplane or ship's propeller does. From Atomic Aquatics:
In the above diagram, the integral of the pressure distribution gives the net lifting force.
Unfortunately, even if the deflected shape of a split fin forms a hydrofoil, I can not for the life of me see how any lift it may generate would contribute to a net force in the forward motion of the diver (i.e. thrust).
Let's assume it does generate some lift. I'll consider the up stroke. Since split fins have a more-rigid outside edge, when kicking down, the split fin's blades deform up towards the middle like in this picture from Atomic's website. (The opposite is also true).
In their diagram, they show some stuff and then say "resulting lift" and put an arrow forward.
Compare the Atomic diagram with the airfoil above. See anything interesting? Like the direction of lift? The inwards deformation of the split fin forms a bit of a foil shape, but it looks to be in the wrong orientation.
If I can see any lift, it's the pink arrows perpendicular to whatever component of flow is in the direction of the orange arrows (foil cross section in red).
One could say that once the kick breaks the neutral plane, those pink arrows will be going on average "more forward," but even if that's true it's counteracted by the half of the kick which "lifts" backwards.
Atomic's annotations say "high pressure side" and "low pressure side," but that's not hydrodynamic lift. It's just moving a blunt object through the fluid which creates a wake (sometimes thought of as "suction"). The same thing happens with a blade fin.
And even if you can call it lift, how does it translate to DIVER motion? Seems to me that since the blade is so flimsy, it would just act locally (i.e. on the fin blades themselves) to straighten them out some.
My impression is that the physics between blades and splits are the same, not like "a paddlewheel versus a propeller". You push against the water, it pushes back against you (thanks Isaac). Since the blades turn off-perpendicular to the water against which the diver is kicking, that wake is reduced, easing the work of kicking. However, the more it turns the less it pushes against the water, which is why a long wide flutter is less effective than a short narrow one.
Can any split fin experts/designers/builders show me where my cursory analysis is incorrect? Any other thoughts?
I have no reservations that split fins are easier to kick than paddle fins. They "give" against the water. That's a no brainer. For some, that suits their leg strength, endurance, and style. For others, a blade works better. This thread (I hope) won't become "blades are better" or "splits are better" and I'll request that any such replies be removed.
Rather, I want to discuss the physics of split fins. In particular, I often read here and in magazines that split fins use hydrodynamic lift to thrust the diver forward just like an airplane or ship's propeller does. From Atomic Aquatics:
I have yet to read any explanation for how that happens other than "it's the Bernoulli principle" (which for the unindoctrinated is a theory of lift that in short states that the faster a fluid moves, the lower its dynamic pressure, and thus a shape which causes the fluid to move faster over one surface than the opposite will generate a net pressure differential across the shape resulting in a force).The Atomic SplitFin slices through the water with two wing-shaped surfaces, creating lift and forward propulsion more like a propeller.

In the above diagram, the integral of the pressure distribution gives the net lifting force.
Unfortunately, even if the deflected shape of a split fin forms a hydrofoil, I can not for the life of me see how any lift it may generate would contribute to a net force in the forward motion of the diver (i.e. thrust).
Let's assume it does generate some lift. I'll consider the up stroke. Since split fins have a more-rigid outside edge, when kicking down, the split fin's blades deform up towards the middle like in this picture from Atomic's website. (The opposite is also true).

In their diagram, they show some stuff and then say "resulting lift" and put an arrow forward.
Compare the Atomic diagram with the airfoil above. See anything interesting? Like the direction of lift? The inwards deformation of the split fin forms a bit of a foil shape, but it looks to be in the wrong orientation.
If I can see any lift, it's the pink arrows perpendicular to whatever component of flow is in the direction of the orange arrows (foil cross section in red).
One could say that once the kick breaks the neutral plane, those pink arrows will be going on average "more forward," but even if that's true it's counteracted by the half of the kick which "lifts" backwards.
Atomic's annotations say "high pressure side" and "low pressure side," but that's not hydrodynamic lift. It's just moving a blunt object through the fluid which creates a wake (sometimes thought of as "suction"). The same thing happens with a blade fin.
And even if you can call it lift, how does it translate to DIVER motion? Seems to me that since the blade is so flimsy, it would just act locally (i.e. on the fin blades themselves) to straighten them out some.
My impression is that the physics between blades and splits are the same, not like "a paddlewheel versus a propeller". You push against the water, it pushes back against you (thanks Isaac). Since the blades turn off-perpendicular to the water against which the diver is kicking, that wake is reduced, easing the work of kicking. However, the more it turns the less it pushes against the water, which is why a long wide flutter is less effective than a short narrow one.
Can any split fin experts/designers/builders show me where my cursory analysis is incorrect? Any other thoughts?