IOM home built Fin weight

Hello, all,
As I work on winter project, 3’ of ice still on lakes. I was wondering
what people are getting there IOM fin’s prior to adding the lead? I think
there might be some home made fin’s out there so I thought I would
ask. I have seen on some other page that 108g is estimated fin weight.
Is this realistic for home built?

I haven’t achieved 108g myself but I tend to overbuild and my next fin will be more lightly built that my current fin. If you can do better than 160gr and still have the stiffness in two dimensions that you need, you have a usable fin in my opinion. Remember that even if your fin is a little heavy, the weight is still low - not as low and ideal as lead in the bulb, but low nonetheless. My priorities are achieving my desired thinness and foil shape with appropriate stiffness. Hitting my weight target comes after that.

Hi Andrew,
My first IOM fin came in at 120gms. That had a ply core and balsa cheeks and 2 layers of 5oz cloth and resin. The first layer of glass was longitudinal and the second layer was 45 degrees. It was quite stiff - not quite equal to a CF fin, but not bad.

I don’t have an IOM, but has anyone given though to using RC helicopter rotors…the key being that they need to be symetrical IE for stunt choppers…
some of the larger blades

* Length: 660mm
* Width: 60mm
* Weight: 175-185g
* Grip: 12-14mm
* Hole: 4mm
* Airfoil: Symmetrical
* Tip: Curved
* Common Application: 60-90 size RC Helicopters.

660 is double the legal length(outside the hull) so if yout ake the 660 a divide in half you got 330 with a weight of 92… just whether or not the foil shape is good or not…i guess you’d need to test…

60 mm wide is not enought for a IOm fin, were the root at hull bottom sometimes is around 90 to 100mm.

The blades often are are too thin walled and you will need to put a lot of epoxy at the bottom to fit securely the bulb of 2.5 kg.

may be a tail rotor as a rudder could work.


I see. Just me thinking out loud…:slight_smile:

Hoe stiff should this fin be? How much deflection would be on the 160g fin?

Almost none - which is why I know I can go lighter next time. My fin is stiffer than any of the professional fins I’ve compared it to, when held at 90 degrees with the bulb on. This means it is unnecessarily stiff.

I will save weight on the next one by trying a lighter core and by vacuum bagging. I’m happy with the choice of carbon - two opposing layers of unidirectional at about 20 degrees to vertical, covered by a single layer of twill at 45 degrees. You don’t need to use twill as there is no structural advantage, but it don’t 'alf look cool with a natural finish. Trouble is I had a few blemishes so ended up painting it anyway.