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On the Wire - Feature

Rudder Stiffening
Flexy Rudders? Add Unidirectional Carbon Fibers

By Gary Friesen

Adding in some extra exoskeletal structural members (external stringers) for rudder stiffening.

I've been working on a rudder assembly modification for my cat.  The rudders that I have been experimenting with have been working pretty well for me but they have a little bit more flex in them than I would like them to have.

I decided to stiffen them using some carbon fiber "tow."  Tow is a strand of hundreds or thousands of carbon fibers which run all in same direction.  Many strands of tow can produce a pony tail like wad of fibers which can be formed into various shapes such as square or round.

I decided to introduce two sub surface stringers along the chord of the rudders.  I started by using a router with a high carbon steel bit to dig out a groove in each side of the rudder.  The groove is about 1 1/4" wide and a little less than 1/8" deep.  It runs most of the length of the blade and is in line with the pivot bolt hole and the tip of the blade.
The depth of the groove is enough to get beyond the gelcoat and into the glass, without go so far as to remove all the original glass.  This will allow my epoxy, tow composite to get a good grip on the existing external structure of the rudder.  It should allow me to later reform and cover the surface using gelcoat.


Next I gathered enough tow to fill most of the groove, when the tow is compressed.  I taped it down just outside of where the groove begins and cut it to proper length.





I mixed up some epoxy and lifted the pony tail and wetted the rudder surface.  Then I laid the tow in place and poured epoxy on top of it.



I used a squeegee and a bit of warm air from a blow drier to work the epoxy into the tow.  I had to tread lightly on the tow to avoid snagging and drawing out carbon fibers from the tow.



And once it was saturated, it was ready to set up.





In the close up view, you can see the waviness that the tow produces when the strands are not all in perfect alignment.  You can also see traces of fibers that have been dragged out of the tow and onto the surrounding areas.

After the epoxy hardens, I will wash the oils off with soap and alcohol.  Then I will sand the project to form.  Finally, I will use some layers of gelcoat to build up the area enough to sand it down into the proper curved shape that it once was.

Gary Friesen

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