Designing a single fin box for 3D printing

General Discussion
1

Reading through old threads about fin design on Swaylocks, it has just dawned on me that my attempts to 3D print large single fins are hampered by the standard fin box size.

Kayu mentioned a few years ago that he thinks the fin boxes should be about 1.5 times wider to allow for better foiling of fins.

Many experienced shapers insist that thicker foiled fins work better, at least in some conditions.

Fins of marginal strength break off just above the entry into the 9mm wide standard longboard fin box, at just the line where the foil thickness is suddenly reduced to accommodate for the narrow size of standard fin boxes.

A fin box with a wider slot would make it much easier to make thick fins that are less prone to snapping off. This would apply to 3D printing as well as wooden fin making and all sorts of hybrid methods.

I have leaned a fair bit about 3D printing with polycarbonate based materials and nylon materials during my failed attempts to print a large fin; based on this experience I’m confident that I could print a sturdy single fin box with an acceptable amount of warping. The warping would likely be inconsequential because it will get sanded off, or it might even fit in nicely with the tail rocker of the board.

Nylon may be unsuitable because resin is less likely to bond well with it. I’ll test resin adhesion to Polymaker PC Max and Taulman Nylon once I finished typing this.

Questions:

  1. How wide do you think the slot in the box should be? I suggest 15mm to 20mm.
  2. How long should it be? Same length as existing boxes would probably make sense.
  3. Does anyone have a CAD file or STL of an existing single fin box? Many links on the web claim they offer fin box files, but they are usually adapter inserts, not actual fin boxes.
2

I agree there are limits to the standard box. And I think you can answer your own questions by looking at what applications you are going to use it in.

Starting with what fin you want on your board. Deep fin with a large base? How are you going to make the fin? Fiberglass panel? CNC Router? 3d Printing? Want to move it around in the box or sit in one position?

Once you know the base of your fin…multiply it by anything from 10-15%. That will give you a the thickest part of your fin. Then you can build you box based on that number.

If I had the same problems you have with the width of the base I would spend time designing a new fin system. Start with the box width you want for yourself based on the fin you want. Design it, make it and surf it. After you do that design an adapter so people can use your new system with their favorite “soon-to-be outdated” fin. New fin box means new fins.

Looking at the standard box it appears to be make in two parts and somehow glued together. I’m assuming that the main part is made in an injection mold, popped out and then the bottom is added. Just a guess…

3

Thanks Dave,
I guess I’m trying to get opinions so we can maybe agree on a new “crowd designed” single fin box size.
For the fin I’m toying with I get 120mm base length x 10% =12mm; 120mm x 15% = 18mm. So a 12 to 18mm wide box would work for that and my 15mm to 20mm guesstimate is in the right ballpark.
I do definitely want the fin to be able to move back and forth in the box. I’m thinking classic single fin box setup. No multi-fin stuff for me, too complicated and not needed IMHO.
The other important factor for me is better safety than standard fins: The fin should come out of the box if you fall on it or run over someone or something. So it needs to be ‘pinless’ forward and aft. I have had some success using ball spring plungers for this purpose, and affordably priced sizes of stainless steel BSPs could also play a role in determining the width of the box and fins.
For now, I have ordered some 12.9mm long BSPs, they would work with a 13mm to 18mm fin. I think the 15mm version would be better, but for reasons unknown to myself, they cost a lot more.

Anyway, maybe 15mm BSPs will be available later on or somewhere else for less money, so I think 15mm fin tab width it is for now.

648×596 47.4 KB

4

The work has already been dome to come up with a ball spring system specifically for surfing. I have worked with this business for several years and absolutely love their product. They are very kind and very responsive. For the entire time I have been using them I have never received a complaint about failure or rust.

http://www.kumanosystem.com/

I would say there are equal benefits and drawbacks to installing the system in the box vs. fin.

Dave

5

Keep in mind that the wider the channel in a box, more drag and turbulence may be an unwelcome byproduct.

6

I agree.
I will have to come up with some solution for that, maybe some telescopic extensions to the fin base.

7

Their shopping cart does not let me select anything other than ‘United States’ as destination.

8

Contact Michiaki via the email listed. Explain what you are doing. If no luck PM me and I’ll help.

Dave

9

I’ve had my earliest wood fins break just above the box, but put it down to not enough fiberglass up the sides of the fin in that location.

I’ve not yet added substantial thickness to a Single fin above the box, which would create a weak spot right at the leverage point.

A new box to accept a wider thicker Single fin standard would be awesome, but I will not be routing out my existing boxes to accept them. I know I can make a thick fin strong enough not to break right at the base of the fin at the box, but it certainly is not easy to get the overhang/mating area perfect, and if one fin was customized to fit perfect in one board, it will likely not fit the next.

I have found my custom fins I fit in a certain boards box tightly with no wiggle, will not fit inside a newer board’s box, so each board tends to have a fin to fit just it. I hate a wobbly fin and want the whole fin base to exert pressure equally on the whole of the box that it touches at the hull and dep in the base of the finbox, and not rely on the finscrew or roll pin to give it the appearance of tight. Years ago the appearance of ‘tight’ was enough for me.

If it is just the roll pin and the screw/plate holding the fin without wobble, then the pin and plate exert much more pressure on much less surface area of the box, and then the fin eventually becomes wobbly within the box anyway, possibly stretching it, perhaps breaking the box resin bond and then the box relies in the shape of the cured resin around it to hold it in place. Such boxes weep salt, and might be ticking time bombs waiting to fail on the best conditions in years.

I attempt to get my fins to fit snugly with no roll pin or plate. This can take a lot of time, especially when one goes too far and needs to apply more resin to fin base. Also a stretched finbox on a well used board requires a lot of effort to get the fin to be Snug without the roll pin or screw/plate. Some of mine allow for only 1/2 inch adjustment before getting too tight, or too wobbly, So I need to know where within the box I want the fin to fit best.

My finboxes are routed into cedar, not just a stringer and some foam. An Ideal single fin box, In my mind, would be designed so the box could not stretch, so the middle of the box was reinforced.

Also The sharp square ends of the box always seem to eventually cause the glass to break/crack there. A smooth tapering to zero would be nice, even if not production friendly. Kind of like why deck patches are oval, or diamond shaped, not square.

Look into why the original de Havilland Comet was taken out of service after much loss of life. Square windows. Any modern fin systems use square corners? None but the FU box standard.

http://www.bbc.com/future/story/20140414-crashes-that-changed-plane-design

10

Excellent ideas, wrcsixeight, thanks for that!
Rounded external surfaces are also important to prevent warping during 3D printing.
I agree it can be very difficult to make a good tight fitting fin base. Maybe that’s just because it is easy to detect the imperfection, and in reality it is just as hard to get a perfect foil on the rest of the fin though.
Ball spring plungers (BSPs) can help to make fins fit in more boxes, and I have a little plan to test fins in the water under single or double blind conditions. So the fins must be able to be switched between different boards quickly out in the water.
The fin base I designed has BSPs set at different depths in the base, so that some expand into the groove in the bottom of the fin to hold it at a specific depth, while others remain compressed between the walls of the box to reduce wobble.
Larger diameter balls in the BSPs (or maybe nylon balls) might be better long term, because they should cause less wear on the box. With a wider fin box, I might not have to insert stainless steel rods, and there would be more space for more BSPs.
The no longer available ‘Effect System’ from Japan http://community.magicseaweed.com/viewtopic.php?f=13&t=6429&start=1110#p387153) used BSPs and interchangeable plastic cylinders of slightly differing height to adjust for different fin boxes. It sounds like the Kumano System recommended by Dave is also using something like it, but I’m not sure, have not held one of their fins in hand. I assume that the Kumano-ai part are very similar to what Effect Systems used. http://www.kumanosystem.com/kumano-ai-1

861×556 54.8 KB

375×576 30.4 KB


11

No doubt some of my fin box ‘stretch’ is from repeated insertion removal cycles abrading the interior of the box itself.

What i always found weird is how hard a fin could be to slide forward or backwards in a box, yet the addition of a Piece of waxpaper, whose added thickness on both sides of fin base, should make it even harder to move roll pin to edge of box, but it has the opposite effect.

This lead me to an even more time consuming process by wetsanding the fin base with 320 grit glued to a hard block, when trying to get it to fit as snugly as possible.

If sandpaper is not glued to the block, edges get rounded, and one seeming enters into a never ending chase of high spots, only to then find the fin is too loose and wobbles and gets to start over after applying more resin to fin base.

I like the idea of more BSP’s. At what point/number does removing the fin from the box become exceedingly difficult?

How about a fin left in a board for a year? Do the Salt crusteez inhibit the ball from retracting? It would suck to compromise board to finbox seal trying to get a fin in/out. Damage to a finbox is much more satisfying when actually surfing, but not nearly as satisfying as hitting something with no damage to board at all.