building a IOM fin


I decided to give a go at building a IOM fin for my boats.

here is the process I opted for:

  1. build the cheecks of the fin in 1 layer of c/f 200g + 1 layer of f/g 50g - done that on a flat plank covered of magic cooking sheet (brilliant for cooking :slight_smile: and even better for doing flat laminate :rolleyes:) epoxied together using West Epoxy 105+205

  2. for a consistent thickness and adding longitudinal strength I tack together 2 pultruded c/f tubes of 5mmx3mm

  3. to make the leading edge thicker I epoxied together 4 layers of 200g c/f out cuts which I epoxy on the leading edge of the fin

  4. epoxy the whole lot together using thickened epoxy with 406 up to a peanut butter consistency

And because this morning the weather is brilliant over here - 18-20C I decided to to glue it together.

and now I cross my finger that my time in building this prototype fin will work !!!

any feedback, views and suggestions welcome


Hi Gilbert,
I will follows with interest your construction.
Already now I have a question about the weight of basic material before assembly !

Hi Claudio,
I’ll tell you tomorrow when I remove the clamps but I already see 2 issues

  1. the leading edge is not consistently glued in despite the number of clamps I used - solution use a plank of wood probably 4 or 5mm thick and 15mm wide to spread the pressure of the clamps along the edge for a better adhesion
  2. the back of the fin looks glued in too far in the fin profile - probably because the clamps were too far down on the profile - though I did put them on the edge but think I should probably just close the back edge simply with tape instead of clamps?

tomorrow all will be revealed !

in any case I look forward to your comments as I value your feedback Claudio

Merci et a demain

Ok here is the verdict - not quite what I was expecting ! :frowning: but not a bad first try

+/- weight is 110g not sanded the excess epoxy that squirted out or the excess tubes (not sure if this is good or bad - nothing else to compare)

  • stiffness is actually quite good - I wonder how much more weight it add if I lay the cheeks of the fin with 2 200g c/f instead of 1 + 1 f/g 50g
  • shape it seems as I expected that because I didn’t use a plank to spread the load the clamps did put pressure too far in the fin from the back edge and has pinched the shape a little bit instead of getting a constant shape

do you think that once sanded down and with a bit of filler towards the back to rebuild the shape and a coat of paint to make it look good it will work still ?

Hi Gilbert,

I never used this method and understand your difficulties, but the first remark is that assuming a 7% thickness out of 100mm chord the central tube needed will be of 6mm + Skin, while the ID will be less then 4mm. Of course for chorter chord a thinner profile the inner tube diametr will become an issue.
I would anyhow fix a carbon rod at the leading edge and eventually 2 , one being cut away at the end while at the trailing edge I will insert an aluminum ribbon of 0.5mm thick - 10mm wide in order to strenght the area when sanding and avoid chipping of thin carbon. This is what I do on all my fins with a balsa core.
At the end I’m not very sure about strenght and gain in weight.


Hi Claudio,

Thank you for your feedback - I am more a sailor than a builder so all this building is still quite new to me - I did tack the 2 c/f tubes together before epoxy them on the cheeks of the fin. The forward tube is at 20mm from the leading edge, the fin length is 45cm so I can cut it to suit my need and to comply with the 42cm IOM class rule in draft. The width is 70mm at the bottom to 95mm max. Its thickness is 6.5mm constant where the 2 c/f tubes are.

Part of my reasoning for 5mm OD and 3mm ID tubes was that I could get a 3mm tread in which I have to attach the bulb on the fin while keeping a fin thickness reasonably thin yet creating enough shape.

Do you think it is worth finishing it properly by filling the areas pinched by the clamps with filler and then a coat of paint ?

I like the idea of aluminium ribbon on the trailing edge - need to find where to get such an item (out of curiosity where do you get yours ?)


I’m having difficulty to understand your data when you says that the front tube is at 20mm from the Fin trailing edge.

I would have expected a bit more like some 30/35% of the chord as for the NACA 006 or similar, that is perfect for this job.

Further a Fin with 7% out of 95mm will give a thickess of 6.3mm therefore a tube of 6mm would be OK and inner Dia will be 4mm.

The point is that at the bottom the profile will very different if you employ the constant tubes thickness like with rectangular Fins and the profile percentage will raise at 8.5% and thus producing a different lift coefficient from bottom to top with consequents flux disturbances.

The Al ribbon is simply cut away from an Al sheet available in the model’s shops.

You can try to repair the pinched areas with some epoxy filler and painted over.


Hi Claudio, thank you for these infos

so for my second go:

  • will change the shape to be more rectangular to keep the profile constant
  • use 6mm OD tubes at c34% of the chord
  • I’m gonna try to find some aluminium - not easy here as hobby shops are very thin on the ground over here and don’t have much stocks so will have to turn to the internet like most other stuff for our hobby
  • use a plank to spread the load when epoxying the lot together

I’m with my v1 not being perfect cos I don’t have much experience in building so it’s great to learn


rectangular Fin may be OK with same overall surface. This will imply a shorter chord and as consequence lower thickness and smaller tubes. You should consider these aspects and probably the treaded rod may not fit in the tube. One possibile solution is to put it in between the two tubes.
I will draw something about…

I like that and this will be the base for my second attempt then - thanks Claudio :slight_smile:

back in France until 15th May but will post more once I’m back cheers

Eric Rosenbaum has written an article(s) on fin construction for the RG65 (you can find it here: and for the US1M that uses a very similar method. Both were published in the AMYA magazine, the RG65 can be found on line (see above), the US1M (MY #172), that would be more appropriate for your needs unfortunately no, but I think you can contact him directly or the US1M class secretary for a copy of the article (maybee). I can also try to find that issue and send you the material used, the technique is exactly the same between RG and US1M.

For what I can understand, it is not wise to use constant thickness all along the Fin with variable chord !
I’m non expert in fluid dynamics, but I think that the same profile proportions should be kept indipendently of the chord lenght and therefore suggesting tappering.


I too make my own keel fins for my US1m’s using a process described in an article by Eric Rosenbaum in the American Model Yachting Association quarterly publication, Model Yachting (Issue #172, Summer 2013) - back issues available here: It looks to be what you are doing. I have successfully produced several fins and have found the following will make things easier and give better results.

  1. Set out the skins with the internal faces down and the leading edges almost together, like spreading open a book with the pages facing down and the spline facing up, leaving just enough room for the edge of a carbon strip (see #2 below) and then use blue masking tape to hold the skins in alignment. Then turn them over so the internal faces are facing up and glue in the internal rod/tubes/whatever. I also add some carbon tow lengthwise along side of the tube(s) and if there is room, some on a bias to reduce twisting. (BTW, the Bantock fin I have has some sort of a corrugated carbon prepeg internally and it is REALLY stiff)

  2. Use a thin ribbon of solid carbon tape at both the leading and trailing edges. These sand to a much better edge and do not splinter like the skins might. I glue the TE down with CA but leave the LE to “float” so it stays on center. Before the skins are closed, the LE carbon strip is stuck down to the blue tape so the skins are flat and the strip is vertical, held only by the tape along one long side.

  3. Fill the foward 2/3 of the fin with a mixture of epoxy and your favorite thickening agent. I use colloidal (fumed) silica (Cab-o-sil). This is makes a thixotropic mixture that will stay where you put and not leak out. Microspheres are lighter, but I have found they are not as stiff.

  4. Close the fin and then use two long pieces of aluminum or steel (about 20 - 30 mm wide x ~3mm thick - the exact size does not matter) and just a few clamps to hold and close the LE. The metal pieces will keep the edge straight and there will be less chance that the clamps will cause the fin edge to have the indentations the clamps might cause. This is the key difference with this method and I have found it produces excellent straight and fair leading and trailing edges.

  5. The trailing edge can be held closed with blue tape. After the LE cures, fill the aft 1/3 of the fin with more epoxy mix, seal the edge with blue tape and use the metal stock and clamps to hold the edge closed and straight. It helps to cover the metal pieces with packing tape so that any epoxy mixture that does escape will not stick and can be easily removed so you can use the metal pieces again.

I hope this helps.


very interesting indeed thx

the tapering would be an issue with this concept as the c/f tubes are constant - yes unless the fin is exactly rectangular the profile will change alright.

Hi Steve,

Thank you for your contribution - this is my first attempt and though not what I had expected - it was a great learning and a lot of my issues would have been lessen by using a piece of aluminium as you describe in your process - I also epoxied the whole fin close together ie the LE and TE.

I am more enthusiastic after the feedback from Claudio on the shape and the post re the actual building technic that my v2 will be much better.

one live and learn hey - part of the pleasure of RC boats !

I would strongly advise against using balsa as fin.
i made a 6mm thick balsa core and it failed badly during horizontal shop test.
the balsa simply crushed and the carbon de lamed and buckled.

all my successful fins are Baltic ply core, pre shaped before laying up carbon.

i also insert long stainless steel screws(heads in) into the wood before lay up.
when I was in Fuji my boat constantly hit rocks at full speed and I did not suffer any damage

All my Fins are with balsa core + 4 layers of carbon 160g/m² crossed at 45°.
All still very sturdy and OK.
The assembly uses a press composed of wood plates of 25mm and soft foam of 30mm each side . 24h cure, lamination isolated with polyane film (garbage film)
The balsa core is a stack of balsa foils of 1.5/2mm glued togheter and manually shaped.
In between sometimes I used carbon tissue as marker during sanding, (see images), later I simply used colored paper since cheaper.
I will adds some pics

Here they are for a typical class M :


Have you tried vacuum bagging rather than using the press?

No, I don’t have one.