Robotic Fin

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Status Suspended
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Niche Mechanics
Purpose Fun

By Semi


Make a robotic fin for powering a small boat. The finished assembly should be battery driven and clamp on like a normal trolling engine. Desired power output is ½ to 4 hp.


Fish, whales and sharks beat their tails to move. Fish also use their smaller fins, but that's outside the scope of the project.

There's a company that makes a flapping pair of fins for pedal-powered kayaks called Hobie Mirage - this variation will probably not be tried.

The idea has been researched - the academic name for the device is 'oscillating foil'.

In papers it's been calculated, first by aerodynamic formulae (Bernoulli's theorem etc.), and later with finite element analysis. The conclusion is that hydrodynamically, this device can be slightly more efficient than a propeller (in theory).

I have skimmed a couple of papers on the subject, but sadly the newest ones aren't freely available.

It seems that the academic view of this device is a flat, hydrodynamic shape (like a wing, only symmetric) attached in the front and in the middle. It is then moved from side to side in the front, and then the middle attachment is used to create the 'bashing' action that pushes the water.

This seems mechanically complex, so instead I will make a flexible tail and only attach an actuator one or two places along its length.

The tail should have different flexibility different places. The rearmost part should be very flexible for best efficiency.


The mathematics is complex, but the principle simple, so trial and error will be the way to go.

First create one or more prototypes. The full-scale model can't be tested before spring 2011 anyway :)

A computer is needed for control anyway, so make it run and collect data for various settings. Maybe use fuzzy logic, neural networks or genetic algorithm for optimizing.



An arduino will do nicely in the beginning. Later, more processing power may be needed.


I should use a modular design so I can swap parts for different sizes, shape and flexibility.

There are two options. Either make a huge slow-moving horizontal tail (whale type) or one or more vertical tails (shark or tuna).

First attempt will be with a tuna tail prototype and later probably two tails side by side. That way they can cancel out the lateral movement hopefully.

An example of something like this is MIT's Proteus Penguin Boat


Obviously, servos can be used, but I am thinking about using solenoids. Basically the idea is to attach an iron rod to the fin, and then make the iron rod move in and out of a coil on each side. Don't know if it'll work.

Electroactive wire seems interesting, but doesn't seem to have enough movement. Also, it might not be as efficient as a solenoid. Time will tell.


Number 1

Under construction.

This is a single shark's tail made from polyethylene sandwiched together in places to create varying stiffness.


2010 11 06

Tried using a long (8.5cm) solenoid with ~300 turns of 0.5mm copper wire for pulling a 5mm steel rod.


  1. steel has far worse permeability than iron, so less power.
  2. A solenoid actuator only pulls the steel rod until the end is flush with the solenoid end. Multiple solenoids in series might be a good idea.


[ NEW! Article about calculating a fish fin. Very detailed. (WARNING: PDF)

Thread that discusses fish tail propulsion for RC and small boats

Article about MIT 'Penguin' twin fish tail propelled boat community (last post seems to be from 2009 or so :-/)

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