Carbon Fin 101

How do you make a big carbon fiber fin?

I can build just about anything on a large(ish) RC yacht like a One Meter or a Marblehead except for the keel fin. I want to be able to make an 18inch long (4"width@top, 2"@bottom) fin from carbon fiber.

Questions I’ve always had when looking at the ones I have from other builders:[ul]
[li]What’s inside; foam or wood (looks like foam)? How does it resist bending (what material) under such loads?[/li][li] Is it cast (autoclave?) or sandwiched in a vacuum bag?[/li][li] What’s the mold made of? Is there a type that allows easy adjustment to different sizes?[/li][li] Temperature cure a must?[/li][li] How to get the fiber so perfectly aligned and get a mirror finish right out of the mold?[/li][li] Do you build rudders similarly?[/ul]Would you guys direct me towards a resource or describe the process here?[/li]
Thanks, yar

Hi Yar.

I can tell ya how I do mine.

  1. My foil-shells are hollow. Rudder shaft is directly bonded to the skins.
    Keels are hollow on the wee foils, using their form as a contributer to
    their strength. Future bigger keels will have a spine to
    take the brunt of all loads, with the foil skin to give shape.

  2. The carbon fiber skins are made with a male and female compression
    mold. the lay-up is put in the molds, then the mold goes into a
    vacuum baggy and “squish”, all non-needed sauce comes out.

  3. My foil molds are made of carbon fiber, to retain their shape more
    To make it adjustable, I guess I’d make a long, tapering foil shape,
    then I could lay-up where ever on the shape, that gives me the
    size I need.

  4. I beleive so. Post-curing epoxies always yields best results, so long as it
    is done propperly of course.

  5. The alignment of fibers has only to do with your desired strength plan.
    From a cosmetic view point, it’s good not to have the outer “weave”
    layer, NOT have any ugly distortions. The parts come out as smooth as you
    mold surface is.
    To have a smooth finish built into the part, means extra
    resin, which means extra weight. To have a smooth finish that is as light
    as possible, the construction methods used on the part, are engineered
    only for maximum structural strength. The gloss comes with a light-
    weight sprayed on finish. (production family boats have the gel-coat as
    part of the lay-up, light weight race boats are painted.
    All my smooth finishes are done afterwards. My mold surfaces on those
    parts is at 400-Grit, so that the satin finish is cast onto the part. Makes prep work
    a little easier.

  6. Yes.

Never having done this I would start out by looking at a DVD of vacuum bagging wings.

The general method is to hot wire cut a foam core, then lay glass or carbon fiber onto a sheet of mylar with a little epoxy. Squegee out as much epoxy as possible. Twist up a cord of paper towel and place near wing (or fin) to act as a channel for air to exit. The mylar edges are sealed with silicone putty, using a wallpaper seam roller. Vacuum is applied via the paper towel port, and, with luck and a bit of skill you wind up with a highly finished end product. I forgot to mention wrapping the foam wing or fin with the prepaired mylar, but I’m sure you figured that out.

I think PVA is used as a release, but it is a long time since I’ve read a how to on this subject.


nigelpheron, I was hoping you’d reply since I saw your parts listed. PeteSchug, thanks for your input as well. Let me reread your posts a bunch of times and hope for more help.


How about foam - the blue isolation foam- foils (like RC planes wigs) with one or two CF layers and vacuum bagged? … After reading PeterShung post it seem that he was already suggesting that. But checking around there are a cheap do-it-yourself foam cutting plans (CNC and not) could something like this bee used to produce keel blanks? Obviously high strength aluminum CNC cut female molds is the best way for consistency but they are quite expensive and for a single keel only, with a foam cutter and a simple foil software you could generate, relatively cheaply, a library of keels to test for your particular boat.

Could this be an alternative?


Off topic, YAR where in Idaho are you? I’m in Logan (UT) 10 mils from the Idaho border…

Look here:
Part 2 ,page 19.
This is easy to build , is stiff and fairly thin.

From what I have read, I think alot of pro made keels are made in metal molds. I have asked ,and it seems most are tight lipped about how they build them( at least in the small details).

I did find one site,( lost it) that gave a very detailed description of using metal molds, carbon fibre and epoxy/ micro sphere slurry. Does any one know where it is? HaHa.

Also ask here, and look in the older posts too. There used to be a couple of guys building them for sale.

Again, thanks for the continued help. robert, read the link you provided - the laminate of woods covered by carbon is logical but that can’t be the “modern” way, then it talks about an aluminum “spine” yet the newer ones seem to be just foam with carbon skin.

In fin construction, if I use foam, is there some sort of stiffener inside the fin (like an aluminum or carbon tube) that runs the length of it? The exterior carbon “shell” can’t provide enough stiffness by itself, can it?


Gio, I live in Hailey (South Central ID). If closer to you, I’d see if we could sail together… if this dang snow ever melts.

There is the two mold method, if you want a perfect “cored” part.

  1. you make a plug.
  2. cast the molds.
  3. make the part from those molds.
  4. increase the size of the part, with the same lay-up thickness, you’ve chosen for your finished part.
  5. cast another mold set from the increased size part, so now you have a small and large mold set.
  6. using the small mold, cast the part with a two part foam, making the core.
  7. cut and wet you laminate, and place it in the large mold.
  8. wet the smaller core part, and put it inside the two other wet laminates.
  9. mash the two halves of the bigger mold together, and the core acts as the compression member, giving a perfect, monocoque, cored composite part.

Thanks nigelpheron,

Do you cast the molds in fiberglas, like making a hull mold?

robert says the pros make metal molds. It would be nice if someone would confirm this or give some info but I’m starting to believe his other comment…

… I have asked, and it seems most are tight lipped about how they build them (at least in the small details).
Except the 4 members here.


My molds are made from many different materials (wood, glass, carbon aluminum, sometimes cardboard, with teflon “Flash” tape.

My top-end “investment” molds are made from Carbide tooling surface coat, then re-inforced with carbon fiber. It is my medium, so I can absorb costs.

Those whos medium is cutting metal can do the same.

Either way, the goal is to have a mold that holds the shape, weather glass, carbon, aluminum, titanium, cold butter, etc.

The materials and methods don’t make a good product. It is the skilled use of the mediums, that counts.

On another note of methods, some claim that a CNC carved plug is more acurate than hand carved methods. The CNC part still has to get faired, and I have personally seen a perfect cut CNC’d plug messed up, because of the skill required to fair it, was not up to snuff.

This goes for hand carved pieces as well. I’ve seen good, and shite.

To sum, it is not the methods or the materials used,
it is the end product that counts. :zbeer:

[quote=yar;44062]Again, thanks for the continued help. robert, read the link you provided - the laminate of woods covered by carbon is logical but that can’t be the “modern” way, then it talks about an aluminum “spine” yet the newer ones seem to be just foam with carbon skin.

In fin construction, if I use foam, is there some sort of stiffener inside the fin (like an aluminum or carbon tube) that runs the length of it? The exterior carbon “shell” can’t provide enough stiffness by itself, can it?


Yar, I have built the fin described ( ply,balsa, carbon and carbon) on page 19 of the us one meter guide. One vacuum bagged and a couple not, and they were good stiff fins,( no internal structure except the wood) the vacuum bagged was a tad lighter and much smoother.

It is a good method for a home builder that doesnt want to make a mold.

I think that the wood core(balsa ply balsa) is easier, because you dont need a mold, the core on its own has more rigidity than foam or obviously liquid epoxy/ micro ballons and cabosil.

I guess the molds are more suited to production work, where you want parts to be identical and perfect finish ( possibly straighter too). I know vacuum bagging ads strength because of the lower resin to fibre ratio and fibre spacing, but do molds under pressure do the same thing( add strength)?
I am guessing they would( if you used a solid core) because you are just using a press, weights vs air pressure.

I know that fins (glass, not even carbon) provided on some kit boats dont have any internal reinforcments, just epoxy/ micro ballons and cabosil.

A spar is always going to make it stronger/ stiffer, but is it the most effcient place to add glass/ carbon?

If you look at plans for fins or fins for sale it will often say what the deflection is with a given weight, so you could use this as a comparison when building your own to see if you need to add more reinforcement.

What I have said is just as much question as comment, feel free to correct me.

Can we hear more from nigelpheron ?

How about a thread with pictures and instructions on fin building with a mold? I know its asking alot , but alot of people would like to know!

Yes the shape of the shells add stiffness to the part (form strength).
But this has it’s limits. It depends on what strength is needed.

The shells themself, can be layed-up using many different lay-upschedules, to suit the desired strength needed.


Need more strength longitudinally, add some uni’s in that direction.
Need more strength to control twist, add more diagonal (45-ish degrees)
layers of re-inforcement.
And the big stiffener, Cored-construction. works on the “I-Beam” priciple.

Keep in mind, the added advantages of using angled shapes and trusses as well.

Well I dug out my copy of " Radio Controlled Racing Sailboats" by Chris Jackson.
It has a chapter on fins and rudders and a pictorial on building a carbon fin. I cant scan it because its copyrighted, but here is a description.

Two fin mold halfs machined from tufnol ( with locating dowels and air holes top and bottom )are polished and given a release agent and then sprayed with acrylic to give them color.

Then resin is applied to the molds and a layer of thin glass on both( wet out),then uni directional carbon( vertical) on both then( wet out ), then roll out both with metal washer roller to spread resin evenly.Then the last layer of carbon is layed up on both ( looks like uni) on the bias( diagonal) about 45 deg.They used pieces to save material.

Then the central gap is filled with carbon fibre tows and thickend resin( it does not show but I assume a little in each side, enough to squish out ) and then the mold half are clamped together and allowed to set. it shows two 8" c clamps at each end of the mold.

The other method uses the two same mold halfs, but once the layups are done in each no filler is added, instead they are vacuumed bagged separately making two hard half shells. Then they are wetted with resin filled with micro ballon and epoxy and clamped till set.
Everything shows simple hand tools except the mold.

robert and nigelpheron,
Thank you for sticking with this thread.

EXCELLENT! The Jackson stuff is great. I want to vacuum bag yet the method described here could just work fine for my first fin (or two). The detailed text is just what I need. Much appreciated.


Found this article, bad photos, but some good fin building info.

I would consider using a carbon fiber helicopter blade.

Most heli pilots have blades saved from crashes. Blades come in matched sets, so get replaced two at a time. You may be able to talk a heli pilot out of an old blade or two. They are longer than we need, so you can saw off minor dings.

Modern model heli blades from non-beginner heli’s are all symmetrical in cross section and in the general range of size to make a good fin. Suitable heli’s come in sizes from small electric to probably to big for a footy. Someplace in there are CF blades that look like a million bucks and are perfect for the task.


I tried one, seemed pretty bendy. I’m now looking at trying some of Aerospace Composite Product’s fiberglass/nomex honeycomb with a carbon fiber tube at each edge.



The foil and the rudder on the “Nightmare” trimiran are made from a heli blade. 11 inches for the foil and 8 inches for the rudder. The full dimensions for the blade are: 19 inches of symetrical cross section, 22 1/2 inches total length, and 2 3/16 inches wide. Works pretty well for a tri, but looks too thick for a rudder on a smaller boat. Might work as a fin. This blade does not bend very much. Branded with: Thunder Tiger High Performance Carbon Graphite Blades. Hope this helps, Clyde

robert, this is really good. Very detailed on how to make the mold and the sequence of different carbon weaves. Even “without” the photos, it is the perfect textbook for CF 101.


Try here too:
They have always been very helpful when I have called.
Tell them what you want to do, they have tech support too.
Look and ask here too:

Here too:
and here: building Heli blades