Building Fin

Today I built a two sided plywood press to make a fin for my new boat. I uses 3/4" plywood and ran it through the table saw sideways, and raising the blade little by little after each pass. When I put the two pieces I have 1/4" in the middle. then too a block plain to taper out on edge more flat. Looks real nice.

So I was wondering if anyone has used the 1/2 checkerboard looking carbon fiber, i was going to get a few layers and squish it between the two boards?

Your thoughts would be great.

A picture or a diagram would be useful to clarify what you’ve described - I can interpret your description several ways?



I will take some pictures tonight. To try to explain this a little better, I took the rip fence off of my table saw, then put a new fence at right angles to the blade, and slid the plywood sideways across the blade, every pass I increased the depth of the blade. This created a nice curve cut out, then I took my block plain and flattened out the trailing edge.

The carbon fibre I was talking about is a 5 oz 1/2 strip by 1/2 strip checker board looking mat woven together will try to find pictures of this as well.


Good idea Andrew, but the leading edge may be a bit pointed. The more pointed the leading edge the more tendancy the fin has to stall. Is it possible to run the plywood through at an angle to put more curve in the leading edge? It would still be a bit pointy but you may be able to file some off after it’s moulded. Nice way to get symmetry though so don’t give up on the idea.

so you are using the curve of the blade to make the foil section of the your fin… essentially using your table saw like a “router” more or less… nice way to make a female mold…

you are right the leading edge will be a little sharp, but a light sanding once it is out of the mould should be fine.
Yes this I did use the curve of the blade almost like a router, just make sure you pt the fence on the correct side of the blade you would not want the piece to take of away from fence.

A nice idea - and one I often have used to create simple profile crown molding.

At the pointed end of your shaped curve - leading edge of finsihed fin, you “could” insert a strip of ply (or solid wood) about 1/4 inch thick. The edge that is closest to your leading edge of the fin, can be routed with a cove bit that will provide a nice radius. If the added piece is beveled on both sides that touch your plywood “mold” it should create an angle to the overall mold that goes from the leading edge of 1/4 inch (or 1/8 —your choice) down to the trailing edge which would be zero thickness (nominal).

So another question is . Do i put arbon on each piece, vacumn bag it the join the two pieces together after. Or do i fill both side up with carbon, glass and epoxy, then squish it all together, clamp and have a solid fin in one shot. This is for a IOM if anyone is wondering?

Also has anyone used the 1/2" x 1/2" checker board looking carbon fibre. My thought was to use one lay on out side the strip it down cut to 80cm by length of fin, then separate the longs from the shorts, then put all the longs in then take shorts and put at 45 deg. this would be like uni carbon? then fill in the middle with micro balloons of balsa?


Putting carbon down the middle will only improve stiffness and strength fore and aft. You want your material on the outside and inside of the bend, so that would be on the sides of the fin.

I’m thinking it might work to lay up a layer of the carbon cloth on each side of the mold. Perhaps let it set up and then put in the goo. Might be good to put in a layer of glass cloth at a diagonal if your bulb center of gravity is much fore or aft of the fat part of the fin. Will help with the twist. And if you do this, you might as well use unidirectional carbon for the part that goes up and down. Or maybe uni glass would be good enough. Carbon is lighter and stiffer though, so you could use less and put the weight saved into the bulb. You could mix microballoons into the epoxy that goes between the cloth to save some more weight to put into the bulb instead. Or even, if your setup and your epoxy can handle heat, roughly shape some styrene foam, oversized, put it and a bit of epoxy between the carbon sheets, and heat just enough to soften the foam. I understand some peole do this to get nice wings. Maybe an extra layer of cloth would be good in this case.

I haven’t done the above tricks for making the fin, just ideas!

If you can figure out how to do it, a very good airfoil shape at the Reynolds numbers seen in, say, US1M fins, is make an ellipse with high point around 22 percent back, rest of airfoil is just straight line tangent to trailing edge. Probably 5 to 7 percent thickness is ok. The drag is pretty low, so you can afford to make a somewhat wider fin. These results according to playing with Xfoil.

To clarify, if the fin is 2 inches wide, you’d want the high point of the ellipse to be 0.44 inches back, thickness of fin 0.10 to 0.14 inches thick.

Most of the checkerboard stuff is cosmetic - unless it includes Kevlar, and if it does, Kevlar opens up a whole new host of problems and issues with cutting, sanding or finishing. Unless you are vacuum bagging Kevlar to get a nice, smooth resin-rich exterior, it will “fuzz up” on you when you hit it with sandpaper.

Stay with straight carbon, and you will have far fewer headaches. I used the checkboard stuff to wrap an extension tiller for my catamaran (8 foot long, and I spent countless hours trying to get it smooth and shiny. Vowed I would never do it again if it required an external finish to whatever I was doing. - Just my experience - others may have found the product OK to work.


I thought the checker board material was a stronger weave, as it is such a thick wide weave cloth? I was also thinking I could use it like uni directioanal if I seporated the weaved pieces.

There are basically two kinds of weave - “Twill”, “Harness/Satin” and “Plain”, and within each you might see 4HS or similar designation that further defines the weave.

You might be further ahead to purchase uni-directional tapes and only buy what you need for the fin, rather than paying the weave price, and trying to pull it apart to use as a single direction reinforcement.

Depending on the manufacturer, I would guess your photo is a “satin” weave, but have also seen reference to crowsfoot or harness weave and other types of proprietary names. - a good link to explain weaves in greater detail.

If you want to generate Oh’s and Ah’s pondside, you “could” try some of this stuff - but clear finish only - of course.

Hi Andrew
Starting from the mold out I use two lays of carbon cloth at 45 degrees(to stop twisting) and two lays of uni directional carbon. Lay that in both halves. Cut it accurately so nothing is hanging out. While that is starting to harden mix up some epoxy and micro balloons to about peanut butter texture. You can put in some graphite for color if you want. Smear that into both halves and clamp together. Leave the ends of the mold open so the excess filler can squeeze out.


Do you put a bolt in the same time as the epoxy in peanut butter consistancy or after fin is done?

Here are the pictures of my fin mould, it needs to be cleaned up and painted.

Using micro balloons to fill your keel is fine but better to do this after the outside skins have cured… If you can, vacuum the skins too as this will compress the fibers and give a stiffer lighter keel fin… when using unidirectional fibers in a keel I have found that the trailing edge can be a bit too fragile so I now use a woven cloth just @+/- 45 in that area to avoid it breaking off.
To get the stiffest keel the unis should be the first layers then the “tie-in” layer of a woven cloth @ +/- 45 degrees after… Thats where they function at their best.
Here is a picture of my canting keel showing the method. Its pure uni carbon high modulus prepreg @ only 40mm wide and 4mm thick.

So if the Uni is the first layer, should i use a regular S glass on the outside then uni, then carbon mat at 45? could I put balsa in the core as well? is that lighter or is epoxy and micro baloons light enought?

the glass isnt necessary but you can put it there if you feel happier. Balsa core is a good idea but the weight question is a bit subjective as that depends on the grain density. My best guess is that it would be pretty close. As a bonus the grain structure running up and down will aid stiffness slightly more than the microballoons would.


I have heard of people putting straws in with the micro ballon mixture to take up the void? Any thoughts? I was also thinking my wife has some bamboo stakes to hold up the plants? Bamboo strong and light? Or how about the corrigated plastic that lookes like cardboard?

Or Balas vs. Foam?
So many question lots of great answers!!!

I like the idea of the straw… just one would be fine and hows this for an idea… On the full size yachts some have a hole bored down through the keel fin. When the boat is measured, if it comes up light, you pour lead shot down that hole to bring the boat up to weight… you remove it by using a vacuum cleaner if the boat is too heavy.
You could use the same idea and instead of carrying correctors inside the hull, pour in a few grams of shot down the tube in the keel. The best place for it…
Corrugated plastic would be fine but you would need to change the adhesives and use a product like Plexus to bond the pre-cured shells together… It sounds like more trouble than its worth to me…