Best lay up for IOM fin

1st option : build two halves consisting of 2 layers carbon then 1/16 balsa, then on more layer carbon at 45 degrees. then glue both halves together once cured
2nd option: build two halves cosisting of 3 layer of carbon one or two at 45 degress, then glue them together after curing with 1/8 balsa core
3rd option: build same as option 2 but use micro baloons and epoxy instead of balsa.

or any other suggestions?

The two halves are made in two moulds then the molds are clamped together to make fin?

Gidday AB, I need to make ten fins (keels & rudders) and thinking whether to go to the trouble of moulds also, not experience with making moulds so following suggestions with interest.

Current experience is sandwiching 2 layers of carbonfibre between 3 x 2 mm thick blasa strips then gluing 2 mm dia carbon rod on leading edge of the fins, also narrow 1 x5 mm strip of alloy (non-corrosive) on tail edge of the keel fin only, then shape fin profile by hand sanding, using tail edge metal and leading edge CF rod strip to maintain straight edges.

Once desired fin profile is reached, wrap wetted layer(s) of fibre around entire fin and cover with plastic film (plastic rubbish bag for smooth finish) and place 1 piece foam block covering the entire fin surface ( one each side) and then sandwich set-up betwween 2 solid wood pieces (same size as foam) and compress with woods clamps so you have even pressure in this simple press type construction. Once cured only need to trim excess off tail edge of fin and head & foot,

It’s real basic but you have clean result with excess expoxy squeezing out under the pressure, not air traps, waterproof and nice smooth finish. If you have vacum set-up even better (I don’t) I really don’t fancy all that sanding for 10 sets of fins, hence if some-one could explain/show a easy to make mould … I would be really greateful.

Another option I have been exploring is CNC option however CAD design reference is required and availability of equipment.

Cheers Alan.

Alan, here is my reply from another forum on fin molds.
Photos of my last mold:

I would not use wood as a core. Water seems to find pin holes and will swell balsa and distort the skins when it gets in.

I like the two layers on bias and one unidirectional on each skin, but maybe lightweight filler and epoxy as a core, or even some divinyl cell foam as a core. I have even built a couple that were hollow except for a spar near the middle, but the skins were not stiff enough to hold their shape over time, and they are no longer nice and smooth.

My next one will be skins like above, with a layer of epoxy, and microlight fairing compound spread inside each skin like a thin “core” with a layer of glass over it. When cured, the two halves joined with epoxy but leaving an air space in the center. The reason is that when clamping the two sides together, a small amount of extra filler will prevent the two surfaces from mating properly and would result in a foil that is thicker than the mold design. A slight hollow would allow any air trapped or excess epoxy, a place to go and still have the two skins meet properly at the leading and trailing edges. Even better would be to join the two skins, still in the mold halves, and then inject a lightweight filler mixture inside the mold, filling it from the inside out. I just have not figured out how to do that yet.

I also am eliminating PVA from the process. It adds thickness to the inside of the molds and flanges that could distort the designed shape. I’m going with Frekote FMS and 700NC (or 770NC) semipermanent mold releases on any new molds I make.

I wish I could afford a set of machined aluminum or steel molds! But the layup and the details around mating the two sides together will determine the finished product. If you have too much “un-compressable” filler inside the mold, you will not be able to bolt the two sides together. The splooge around the edges may squeeze into the area around the leading or training edge and keep the two halves from mating properly. Using PVA release film will add thickness to each mold half and possibly change the designed thickness of the fin.

There needs to be a way to join the two mold halves without the possibility of excess epoxy oozing into the flange area as they are bolted together. Still thinking about the exact process here!

When I made my molds, I made a flange at the trailing edges that is 90* to the parting edge. I’m hoping that any excess resin will squeeze out of this thin gap, yet with the molded flange at 90* it will give a lot of stiffness to keep the trailing edge nice and straight.

I would also recommend putting one layer of regular FG against the mold before the carbon, or perhaps a thin layer of epoxy, let set then do layup. this will give you a nice hard layer to sand if you need to, without breaking into the carbon fibers.

[QUOTE=hew565;57466 Even better would be to join the two skins, still in the mold halves, and then inject a lightweight filler mixture inside the mold, filling it from the inside out. I just have not figured out how to do that yet.[/QUOTE]

Very interesting Hew thank you, I like the idea you described in quote above, have you thought about injecting filler inside the skins (while still in mould) through a hole at the head of the fin which you could then plug (while still wet) with the fin bolt ?

I once tried gluing two halfs of balsa together with spar in centre to hold the fin profile (like an enevople) and then poured in epoxy, unfortunetly the epoxy needed 1 hour to go off and it eventually soaked through the the balsa and lost it’s shape, hence a mould would eliminate this problem.

Cheers Alan

Hi A.B.

Firstly, nice description and photo’s Hew!

In answer to the best layup then all the carbon should run up and down the fin, but there is a need to use one layer to prevent the trailing edge from breaking by using a woven cloth diagonally to bind the unis together.

In my experience I have found that Microballoons epoxy mix is too heavy and a ‘structural’ foam is preferred. Either Divinycell or SP corecell is fine and the density you need is either 60 kgm3 or 80 kgm3 ( Kilograms per Meter Cubed) or P400 - P500.

My preferred approach that I use when building any foil is use a split female mould and layup the skins first, then cure. Then fill with the core material, cure. Then sand the excess core and laminate flat to the foil mould mating surfaces before then joining together with a thin resin silica mix. This ensures that there are no voids and the fin is true to the design and the moulds.

Another approach again that I will try on my next fin is to use just 2 unis @ 0, fill with foam and sand flat. then use a single layer of RC200 @ +/- 45 to bond the two halves together. this will remove 1 layer from the overall layup and help push the lightness further without affecting the stiffness in any way.

Good luck, Jim.

I just pulled the fin from my mould, turned out great. What i ended up doing was , three layers of carbon on each side or 1/8 inch balsa, with one of the layers at 45 each side. My mould was built running a piece of plywood through a table saw sideways with the blade only up 1/8 then took a hand plane and faired out the trailing edge. the layup went like this:
2 layers 4" carbon overlapping on the ends, then 2" carbon, then microbaloons and epoxy over balso core, then close both sides the clamp over night. Turned out so good I am going to clear coat as the finish, also lightest fin yet.


Hi Andrew, well done ! how heavy ? and what was the final thickness of the fin ?

Hey if you could post a pic of you mould, that would be great.

Cheers Alan

Kiwi, the finn weighs in at 325 kg but i neet to trim it down to length so sould be good. Everyday the epoxy cures it seems to be getting stiffer. I will try to attach pictures. Fin thickness is aroung 6mm.

Hi Andrew, thanks for the specs & final result looks good.

Using my method explained earlier I had approx 500 mm long x 100 mm wide fin with 8 mm profile weighing in at 189 grams and Jim holds the record using his method explained above weighing in @ 110 grams !!!

The search continues on how to “add lightness” as H.B would say …

Cheers Alan

Kiwi, I was still asleep and no coffee when I posted the pictures and weight. I correct myself it cam in at 125g.

Looks good Andrew, my best weight has been 124gr. I’ve used epoxy/microballoons and styrofoam pre-expasion balls and still end up at about 125. The weight must be in the layup. I noticed on Sails Etc site that they don’t use the diagonal lays. Their idea being that if the bulb is balanced on the fin that there won’t be large torsional loads. I’m wondering if instead of filling the fin if I couldn’t just run a bead of thick filler hither and yon around the interior and leave it at that. Has anyone had a problem with the trapped air expanding? Also, kind of an idea/question. Why clear coat it, why not just polish it? Why add extra weight? Does CF absorb water if exposed?

Hi Don,

Yes,I have had my hollow fin expand and “puff” up. It was noticeable because I had a spar running down the center. It held the two skins together while the hollow sections expanded while laying in the sun. Also had one suck water inside through a pinhole when it was placed in the water. It was a pain getting it dried out and sealed!


A twist to your obsevation from Sail etc. what if we had a real twisty fin, would that not change the angle of attack when sailing upwind allowing the boat to point higher. I know the rules say the sin should not pivit, but noth said about excesive twist?


Hi Andrew
Like everthing else in this hobby it’s been tried already.

Down towards the bottom of the page, Larry Robinson’s remarks.