How can I make fins where the inside of the fin is concave instead of flat?

General Discussion
1

I have a pair of Gerry Lopez fins by Futures where** the inside of the fin is concave** as opposed to flat like most fins.

I want to make my own fins with concave insides and I was wondering if anyone has made fins like this and how you did it?

If I used a wood core, would you shape the concave into the wood core and then lay the cloth? Or shape the inside of the fin flat like normal but add extra layers of cloth on that side and shape the concave out of the cloth?

Thanks

2

Hi magentawave…

One way is to find some sort of a convex glass or metal surface on which you laminate at least 10-15 layers of cloth.  Wax that surface with a couple of coats of automotive paste wax as a ‘mold release.’  I usually lay some balsa sheets right over the glass layers, cover with a sheet of plastic drop cloth material and vacuum or weigh it down with bags of water, sand, or whatever.

Once cured, pop off the glass/balsa panels, cut your outline and foil before ‘capping’ the outside with 4-6 additional layers of fiberglass.  If the fins are going to be used in a box system, be prepared for some fiddling on the tabs to get them to fit right.

Some of the surfaces I’ve used include old car windows or a sheet metal ‘dish’ that was originally designed for catching hot ashes under a barbeque.  I found the metal dish in a dumpster behind the local hardware store.

Carry the concept a bit farther and you enter the realm of hooked fins like FCS CRVS or the custom fins that Les Waddell has been known to make.

3

yep , curved perspex , too …

 

" wildy" here , did his with sanding curves into wood , then glassing .

4

What if you took an existing fin, waxed it, then use that as the mold for your fiberglass laminate?

5

Fin panel and a grinder is how I would do it…

6

I’d use finFoil to calculate the contours. Unfortunately the currently released finfoil builds have some issues with calculating negative contours, but it works with some trying as shown in the screenshot. The next version that will be released soon has fixed this issue.

After calculating the contours, I would grind a layered material (plywood or fibreglass panel with colored layers) to match the calculated contours as close as possible.

7

Hans, as an avid fin designer I am very keen to use your Finfoil program but I only have iOS products. If Finfoil can do standard NACA and custom foils I’ll be trying new foils on it every day.

Once I’ve sorted out a foil shape, what do I need to do to get it onto a physical fin ? Does Finfoil generate a file I can send to a 3-D printer ?

8

Hi Surffoils,

finFoil can use any foil you like, since you can drag image files on the edit screens as is described in this post: http://www.swaylocks.com/forums/finfoil-v10-development-preview-4

finFoil exists for apple products, check the download page.

For now finFoil generates contour lines that function as guidelines for foiling a layered material. But in the near future I will release some tools to export the fins to STL files.

Hans

9

Thanks for that Hans,  I’m looking for a shortcut to make heaps of prototypes so an STL file option will help me develop all sorts of fin ideas. I’ll start at the downloads page and go from there…

10

yup

by hand or machine or cnc or molded

doesn’t matter

come of my fins have the craziest of foil

especially them wavy ones i did but hand

try it its really easy to screw up a perfectly flat surface into anything you want

11

Thank you. The inside surface of my Lopez Futures are concave from leading edge to trailing edge as opposed to the kind of foil shown in your drawing below. So it is a more simple foil which should make it easier to grind the concave.

I think I saw somewhere that your FinFoil program can work on a Mac without a virtual machine. Do you have detailed non-nerdy instructions for how to do that?

 

12

Sure, download the .dmg file from here: finFoil - Browse Files at SourceForge.net

There is a Mac download link on the homepage: http://hrobeers.github.io/finFoil/

13

Thanks for all of your ideas guys!

I have a few more questions about making thruster/quad outer fins with a CONCAVE inside please…

a) If I was to make the fins out of solid glass, would you grind a piece of wood with the desired curve and then glue a 1/8" thick piece of perspex/acrylic to the wood?

b) A curved perspex form like the one described above would NOT work for fins with wood cores, right?

b) I was thinking of using wood cores to keep the weight down because this board will have 3 to 5 fins. How would YOU make a wood core fin with the inside having a concave surface like the Lopez Futures? Would you shape the wood core to the desired size with the concave shaped into the back and then lay the glass on top of it?

c) How many layers of 6 oz E cloth would use on both sides of the wood core? Again, these are outer fins for a thruster or quad.

d) Would a Porter Cable random orbital sander with a **6" disc with sticky backed sandpaper **work for shaping fins? If so, how do you keep the edge of the pad from getting torn up while shaping small sharp surfaces like fins?

 

 

 

 

14

Thanks for the Mention John!  I still make my Curved Carbon Cutaway (3C) fins!  Mostly all I use now.

For Magentawave - I used XPS as a mold covered with 4oz glass.  Then tape Stretch-Tite over the mold, lay progressively shorter layers of carbon fiber then balsa sheets and stick it in a VacBag.  when the curved rectangle comes out I cut the fin shape then foil it and glass two layers of S-2 glass over the decorated balsa to seal it.  Then mold the base.

Les 

15

No.  Resize them down smaller than your computer screen, then you can attach them as a file when posting.

16

Any consider the stalling at high angles of attack that comes with the concave?

There is something to be said for side fins with convex, similar to those diagrams above, or the RedX 80/20, on the “inside” keeping the laminar flow from shearing off.

I suppose adding tubercles (sp?) to the leading edge of the concaved fins would work too.  Oddly, such a simple improvement hasn’t caught on…  Must look to kookish… Ha!

17

I’ve foiled many fins, mostly baltic birch ply, using that exact tool. Try to keep the edge from catching on sharp edges to keep the disc in good shape. But, yah, they get beat up some over the years.  Still able to use them. I have a 5 and 6 inch disc. I can hold that sander in one hand and the fin in the other (wear gloves!) to fine tune them, too. Mike