CF Fin problems

I decided to try and make a thin fin for my US1M. I carved out a 1/4"(6mm) soft balsa blank and put on one lay of 4 oz uni carbon vertically and one lay of 3 oz cloth on the diagonal. I let it set for about a week. It was a little bendier than I’d hoped but I finished it anyway. Now that I have a shiny finish on it I can see the carbon crumpling on the compression side when I bend it. Do you think my problem is lack of adhesion, not enough carbon or maybe I should use a more compression resistant blank? I tried to use minimum resin and vac bagged the uni lay to 20 in. I’ve read that carbon is not good in compression so maybe more lays or heavier cloth would help. Any ideas would be appreciated.

Don , I think you need more plys of heavier carbon/glass.

The first one I built was per the one meter building guide.

I used 1/32 ply covered with 1/16 hard balsa for the core and then coverd it with 2 plys of 4.7 oz uni carbon, then 1 ply 5.9oz Bid carbon on the bias ,and then 1oz glass. It came out stiff, I dont remember the exact deflection , but it was stiffer than specs for the one in the one meter guide.

The core really just ads shape, most of the strength comes from the skin, but the core has to be strong enough not to compress in normal use, like pinching it when you pick it up.

when dried, the structure would be stiffer than the original core alone.
Sounds as if the resin has not been properly cured. Still rubbery.

Also, with minimum lay-up composites (1 layer, to 4 layers), it is very easy to suck out too much resin.


I think you will get better results making a plug and then a mold.

Laying up a solid fin, besides being stronger, stiffer, will give you a better finish especially at leading and trailing edges and will be much more damage resistant.

What I have found ,is its easier to keep the the original shape of the foil using a mold over cutting a core and then covering it with cloth/ epoxy/ vacuum bag ect. Because as you build up the layers you are going to change the shape some. It works great for wings , but the skin thickness is alot thinner.

I am a beginer, Nigel does this for a living, so listen to him.

It might still be a little rubbery but it’s had a week and it sanded OK.

Do you think the two single lays of CF should be stiff enough?

I hate to experiment with this stuff-it’s about $15 worth of CF in each fin and if I have to double it my pension won’t take the strain.

Have you made a fin from a mold? That is my plan once I get a good one. How would you join the two halves? The trailing edge would glue up fine but the leading edge is straining my brain.

I’m in the middle of waiting for my first CF fin to cure in a mold I made! what timing, eh?

I made a trial mold and laid up a fin from glass and epoxy. I was pleased with the results, and made a new plug and mold for my IOM. On the trial, with just glass, two layers 6oz bias, and laid a bunch of uni glass tows up the middle of each side, result was plenty stiff enough!

New layup is 6oz bias, 6-8oz uni CF (not sure as it was a freebee), and another layer glass on bias, on each mold half. I laid a strip of glass uni tow down the middle of each side for good measure, and a strip of 1/8 spruce embedded in epoxy to act as a spar. Then epoxy gell around edges and bolt halves together.I’ll let you know the results when I crack the mold tomorrow.

I didn’t mean to hijack the thread, so my thought is that the core may be too soft, or not enough resin in the cloth. CF is hard to wet out, and it is difficult to know when it is wet through and through. a balsa core may be soft enough to let the skin deform a little. That’s why I am trying a layer of 45* bias glass each side, to help hold the cf in it’s plane. I have a solid core in the center of the airfoil, but it is actually hollow in front and behind the “spar”.

Yes , I just finished a high temp mold for use with prepreg carbon( got a source for alot of free prepreg of different weights).

I am practicing different layups with cheap glass first.

When I get good enough I will start using carbon( I want to get at least a few fins before I burn up the mold in the autoclave) and then try the prepreg in an autoclave .

I have been getting the parts together for a small autoclave.

We will see, it might be a disaster.

I have learned alot so far, we will see how much:rolleyes:.

As far as doing the layup for the fin ,I have found there are a bunch of different methods you can use.
I think the two basic ones are either making two hard half shells in the mold and then joining them back in the mold either with various structures inside (like spars )or just filling them with epoxy /microballons/ or epoxy/ carbon( tows or chopped carbon) or a core(like foam) and epoxy /carbon.

Or doing the whole layup at once.

Look here:

I wish nigelpheron would post a build thread for his fins( hint hint)

Thanks guys, I think I’ve got it now. One final question. When I’m laying the cloth in the mold does it have to fit exactly or can it hang over a little to be trimmed off later. My thought here is that the hang over part is going to increase the thickness of the fin which is not what we are here for. Trying to fit the CF perfectly inside the mold after it’s been resined ain’t going to happen either. There must be a happy medium.

I didn’t try to get the cloth up to the very nose or tail of the foil. I left it short and used epoxy with thickener to fill the space when I put the two halves together. it did ooze a little bit into the gap, but since the halves were bolted together, not much and it was very thin. On my trailing edge, I ended the mold at the TE. This leaves a thin slot where the cloth/resin can ooze out without deforming the TE. Trial and error method continues!

Don if you are carful you can keep most or all of the fiber inside.

You will have a tiny bit of epoxy or epoxy/ carbon spludge ( depends on your filler) squish out, and you will have to work out the amount of filler that goes inside( first by measuring the volume of the fin then by trial and error). You will still have epoxy flash to sand off.

To little filler and you will have voids, to much and you will have a hard time squishing out the excess without moving the fiber from where you want it. Keep track of your layups fiber and filler( volume) so you can repete it when you get good results.

I made vents top and bottom ( on the parting line of the molds) for the air and extra epoxy to come out.

Don I am just learning too and I have only made a few glass fins so far in the mold, but each one has gotten better.

The other way is to do what nigelpheron does and is also described in Chris Jacksons book. Make two shell half first , trim them and then add some kind of structure ( like a spar ,leading and trailing edge reinforcments, fillers, whatever) and put them back in the mold to set.

Nice, I like your idea of the sloted trailing edge.
hew , your fin is hollow in parts?
How do you keep the fiber tight up against the mold surface?
Or did you make two shell halfs?


I laid up each half and joined them when the resin had gotten thick enough that the skins would not separate from the molds. They are not completely filled and do have some areas of air between the two skins. weight for the first one to come out is 165g. This includes a 6/32 SS bolt that was embedded in the top. I will use a wing nut or thumb nut of some sort to hold it into the hull. It seems pretty stiff so far.

Here is an interesting photo of one of graham bantock’s fins :

Looks like the two halfs are cured and then the reinforcment is added, then it looks like carbon powder? and epoxy are “pastry bagged” around the edges and on the spars , then the other half added, and from what I read, put in a metal mold and then in a hydraulic press.

Look close at the leading and trailing edges, it looks like they join them with a flat surface backed up by the carbon sauce. After they are cured ,I think they then trim the joined flat surfaces off so they just touch and the joint is reinforced by the sauce behind it.

If you did it that way you would have to make a resess on the edge of the mold.

I looked at the sails ect site and found cross sections of some rudders with instructons on trimming them and it looks like the do them the same way.

What do you guys think?

Are you looking at the same picture as us? I wouldn’t have quessed that was even a fin. What am I missing?

Don ,
l know it is not a conventional shape, but I think it is a fin.
Look towards the top of the “fin” and it looks like a small foil peice is being use to temporarily center a threaded rod.

I also found this in an article on fin building talking about fins made by sails ect.( graham Bantock)" this is made up of three moldings, including a corrugated core which are then placed in a compression mold to produce a hollow ,but extremely strong and light fin"

Its not refering to the photo , but I can see the similarity to the photo.

Don I am kinda retarded somtimes. Look at the web address, it says “laminating Ikon fin”

Yup, you’re right. I found all the things you’re talking about. Also if you look here
Item 370B and click on the (I) you will see a drawing of that fin, although it’s for a 10r.

Whilst looking at the drawing of Sails Etc. fins all of a sudden the dimensions sunk in. This fin is 135mm(5 1/4") at the top and 75mm(3") at the bottom. I’ve been making mine about 3 1/2" top and 2 1/2" bottom. This is very wide at the top. I got my measurements from the US1M construction files. Granted mine is a US1M and Sails Etc is an IOM. Can anyone try to explain the difference? I’m thinking that they are trying to move the max lift towards the top of the keel. Assuming that the center of gravity of the boat is going to be somewhere along the length of the keel this would try to put most of the lift above the COG. This may help pointing and righting??? Any thoughts

Hi DOn.

I think fin shape is like religion or automotive brand loyalty. Some people have their favourites.

There are IOM fins that are tapered, and some that hae parallel sides. Some are thicker at the top and thinner at the bottom. Some have a sharp entry and some are rounded.

Some argue that the fin can be narrower at the bottom (both cord and cross section) as the water pressure increases with depth.

I don’t know that it makes much difference. It is more important to have a stiff fin. Yours sounds a bit narrow, especially at the bottom and will allow the fin/lead to ‘wiggle’.

My advice is pick a foil section and make a parallel fin (for easy of shaping). Go for about 3 to 3 1/2 inch wide to provide enough ‘bulk’ to be stiff, yet not so wide as to increase wetted surface. Thickness seems to settle at about 6 to 7%. Finished weight for an IOM fin should be between 100 to 200 grams, the lighter the better.

Will we see you this weekend at Salt Spring for the Western Canadian IOM regatta?

Probably not but we’ll see