FIN flex

Hello I just built a fin for my IOM, foam wraped in carbon fibre. I did not have any uni-directional so two layers of standard weave. Now that the lead has been added if I hold it as if it were in the boat it flexs of center around 2cm is this two much. My cedar glass fin (much heavyer) does not move. Just curious, do I need to add more carbon fibre?


Yeah way too much flex. I don’t know what overall foil thickness you have, but you will have use so much standard weave to acheive stiffness that the foil will likely end up to thick. 2cm will flop around and you won’t get any windward performance. and it will be likely that downwind you will get rig shake. A suggestion might be to use the current foil as a mold and relay with unidirectional.

So would the whole fin be carbon or is there still foam or bulsa core? I will double check to see what the actual flex is off center when it warms up in the garage. I will try to get a picture as well.

Yes solid carbon. To get a fin that will give high performance you will need to have the fin thickness no more than 6- 7mm. I dont beleive you can acheive that using a core. I’d have some serious concerns “when it warms up in the garage”. If the carbon has cured correctly warm garages shouldn’t change anything.
But in sayng all of that it depends on what you intend using your boat for. If its just for fun and a bit of casual racing use it like it is and try it out.

I say when it warms up in the garage, just to say It was to cold to go out there in my pajama’s and take a picture. The fin did cure in side the warm house. I only sail an A rig on my home made triple crown IOM. When I say home made, everything has been scrounged up and has not cost much except for the radio and servos. I got my carbon fibre from someone I know who does large repair jobs on aircraft and custom parts for helocopters, racing bikes etc. for free as remnents destin for the trash bin. The sails are mylar film used to protect the comuter screens delivered to our office, mast is aluminum picked up at the scrap yard $1 per pound, so there is 6 masts in the garage for $2. Yes this is not going to win me races but there are no boats to race in my area, its fun to experiment and take the kids to the pond.

What orientation did you use for the carbon(90 or 45 degrees) and how long has the epoxy cures? Oh, and how thick is the fin? Good going on the scrounging material. Some of us appreciate that talent.(Damn smilie won’t work)

Don the carbon runs 90 degrees, i think (not much) it is around 6mm. It has cured in side the house for 1 month so far. I was thinking I get this much flex when the fin and lead are straight out from the work bench, but when the boat sails it is at 30-45 degrees so there would be less deformation (is that a word?) It also has a 5mm carbon rod in it, thought that would add to the stiffness.


Good on you. I got the impression you were looking for high performance and looking to win the World Champs. What you have will work grat for casual racing. Congrats, for using ‘what have you’ to get your boat up and running. Its good to see in this sport as most people seem to think the more expensive it is the faster it goes!

If you do want to upgrade your fin, might I suggest using a hardwood timber core. With couple of layers of standard weave you will get something with way less bend/twist than you currently have. Just remember if you do use hardwood it can often be fairly oily so you will need to strip as much oil out of the timber you can with acetone or similar. You may also find the epoxy takes a little longer to cure because of the oilyness.

TF, thanks for the complements. I did make a fin of 3/8 cedar strips glued with gorilla glue, then shaped, then two layers regular glass. Then I shaped a 5 pound window sash weight (cast iron). It worked okay but thought i would make somthing over the winter. The new lead is a 8pnd fishing weight cut down the shaped with a hammer, looks good.