Looks like the plain wooden oar off the rack at the tackle shop. Paddle it all day. Then twist the throttle — and a fin buried inside the blade starts to wag, and drives the boat like a fish.
The design rule that drove every decision: the outer perimeter of the oar must be identical to a normal store-bought oar — same blade, same shaft, same loom, same feel in the oarlock. All the machinery lives inside the blade, behind a flush hatch. No exposed prop near your hands, nothing to foul a net, nothing to catch a curious eye at the dock.
This started as “why not put a propeller in the paddle?” Every propeller idea died on the same rocks. The fin is what survived.
Lab oscillating-foil drives reach roughly 85–90% propulsive efficiency at low speed — often beating a screw propeller in exactly the calm, shallow, weedy water you fish in. It’s how every fish and whale on Earth already moves.
With the motor off, a one-way louver in the blade face does the same trick your wrist does when you feather a paddle — automatically. Push to drive, pull to glide.
Now the motor drives the internal fin back and forth. On each sweep the fin passively feathers, so it slices thin on the way out and pushes hard on the way in — a wave of thrust, just like a tail.
A fin has to reverse direction every half-stroke. Reversing a motor 100+ times a minute is hard on cheap hardware. So we don’t — the scotch yoke does the reversing mechanically while the motor spins one steady way, exactly like a piston engine turns spin into back-and-forth.
A servo slamming end-to-end wastes energy at every reversal and hammers the gears. The yoke gives free, smooth, sinusoidal motion — the exact profile an efficient fin wants — from a plain brushless motor and one speed controller.
Crank = the wag. Feather pivot = the thrust. The fin hangs on a free-floating hinge with a soft stop, so it angles itself on each sweep independent of the crank. Decoupling them is what keeps the whole thing simple.
The blade is a sealed clamshell: two molded halves over a rigid spine. Everything tucks into a ~10×4″ envelope, with the fin exiting through a slot at the tip.
| Concern | Blade-screw prop | Concealed fishtail fin |
|---|---|---|
| Back-force fight | Severe (flat blade vs thrust) | None (feathers on return) |
| Paddle vs power shape | Conflict (flat vs twisted) | Same shape works for both |
| Low-torque motor | Stalls / needs gearbox | Ideal (slow big sweeps) |
| Low-speed efficiency | Moderate | 85–90%, beats screw |
| Weeds & shallows | Fouls, cavitates | Sheds weeds, works shallow |
| Hidden inside oar | No — must be exposed | Yes — lives in the blade |
| Hands-near safety | Spinning blades | Enclosed, gentle sweep |
| Reversing wear | — | Scotch yoke (steady spin) |
No hand-waving. Here’s what actually makes this tricky to build.
The fin slot and the shaft into the blade are the leak paths. Wiper seals + a slightly pressurized/flooded-and-drained blade cavity, or a magnetically-coupled drive so no shaft crosses the seal at all.
If the fin doesn’t pivot freely (with the right soft end-stops) it just slaps water flat and goes nowhere. Getting the pivot friction + stop angle right is the whole ballgame — too stiff kills thrust, too loose flutters.
Mass in the blade end ruins paddling feel. Battery + motor must sit as far up toward the loom as possible, or feel becomes tip-heavy versus a wood oar.
The passive louver face and the fin slot share the same blade real estate. They have to nest so paddle mode stays flush and power mode has clear sweep room.
The patent-worthy nugget is the combination: a stock-oar envelope hiding a scotch-yoke-driven, passively-feathering fin, with a flow-actuated louder for manual mode. Each piece exists somewhere; nobody’s packed them into a plain oar.