The loads are stornger than most people think. As a rough guide mount the fin horizontally, then stand in the middle of it.
My reasoning being that well powered up when you "rail up" the fin is lifting your weight.
And taking leverage vectors into account (distance from finbox to foot), can't generate more force than your weight (the board just turns over)

Thanks Decrepit,

That answers the question nicely, and provides an easy way to test the thing off the water.
I'd prefer not to try to slog back to shore with no fin if it breaks at the root.

I'll let you know how it goes

nebs, look in the tweed phonebook white pages for a guy last name featha. (no kidding) he is the eccentric obsessed totally over the top backyard foil analysis yippee go for it these wierd foils hydro go big smooth fast but grinding stylish fins always looking for new ideas guy. he's a legend, his foils are outstanding, they work and they are totally unmarketable, give him a call.

Hi Notwal,

The rocks thing is easy, just run a strip of fencing wire down the leading edge, file to the right profile and bob's yer mothers brother. You should be able to do this on a fibreglass fin as well.

You're almost right with the articulated idea. I want to run some stiff steel down inside the leading and trailing edges, and make the rest of the fin out of flexible nylon, those white chopping boards would be about the right stiffness I think. This means that the fin should get a camber on it as you push against it, in a similar manner to an old dacron sail -- the leading and trailing edges stay tight while the middle bellies out to give you some form of asymmetry.
The nice thing about this design is that it's totally passive, no moving parts.
The hard part is making it strong enough, although with the right diameter of steel rod this shouldn't be too much of an issue. It all comes down to engineering now

quote:

---

Originally posted by nebbian

Hi,

I've got an idea for a new type of fin. Basically it's got some very flexible parts, and some stiff parts. It's a radical departure from your typical fibreglass fin, so the normal techniques don't apply.

I've got an old fin I can use the Powerbox base from, so that's no problem, but I'm worried about the stresses involved.

Does anyone know how much sideways force a typical fin has to withstand? If you cut off the fin from the powerbox part, and drilled a couple of holes and inserted some spring steel, would it withstand the bending moment?
Are there any special techniques for building fins, especially the area between the fin and the powerbox?

How strong does it have to be? I'm looking at about a 32cm fin.

---

I'd probably try to make the head as one with the fin, i dont like seperate headed fins they break eventually (plastic with fin inserted). Cutting fin off at base and attaching new fin with dowell points will be a bit hit or miss also. Powerbox shape is relatively easy to shape.

Regarding you're fin design- I have heard/read of this being attempted before but I dont recall any results and whether it was good or bad. I'm thinking the nylon might be a little too flexible especially if you taper the fin. Shape-wise vando's speed fin/new freestyle fin design would be a good start for this kind of fin- wide and short.

So you are going to get a sheet of nylon and insert stainless rod into it?

Since you mentioned hollow how about using a sheet of carbon, put this down the leading/trailing edge and also in the centre of the head and maybe 1/4 way down into body of fin. You could maybe taper it slightly as it goes down the body so you get some sort of progressive flex happening or even cut it at an angle with carbon at front extending further down than trailing edge.

Also since the head is wider you could fill it out either side of the nylon with glass, i think this will make it strong enough. I do see bonding all these materials together being a problem though, the beauty of nylon is that ideally it could be molded around the other materials but you're not going to have that option.

Just some food for thought.

You b*gger decrepit,

Now I'm thinking of a fully stressed carbon leading edge, a couple of battens, and some monofilm for the body of the fin. Pre-shape, luff curve, fin twist... ahhh my head hurts!!

Think I'll just suck it and see, try some stainless or carbon rods. Surely with our incredible technology we can make a reasonably stiff yet thin bar? After all it's not rocket science