Need for sanding fins?

[pjgazele]

So I was watching a fiction movie recently (Breath) where some sort of surf guru was using what appeared to be a sanding cloth on their surfboard fin. Here’s a question… generally, unless you’ve got some nic on your fin or damage causing a sharp protrusion, is this ever necessary or recommended on used fins? If so, when? For only certain types of board styles and or fin styles? And if so, any recommend types of sanding cloth? Or is this all hogwash?

[KitesurferAU]

Having a very sharp edge on a fin won't make a difference for you unless you are pro surfer but it will make a huge difference when you kick out, your board flips and you land your butt straight on your fin.

[waikikikichan]

If it was an older movie and the guru surfer had glassed on fins, I can understand. Nowadays you don’t hear too much about fins “Humming”. Back when I had these glassed on fins, at a certain speed or angle, the fins would make this huge HUMMING noise. Like really loud.
I don’t think he was sanding to sharpen but more to dull down the edge to lessen cavitation.

[dtc]

Breath is set in the 1970s so it might be as wkk said. Or it might have just been something for the actor to do in the scene

[oldmansurfer]

I have yet to surf any removable fin board at the speeds I used to get surfing with my single glassed in fin back in the 70's but if I glassed the fin in my board might hum and vibrate quite a lot more the greater the error on my part. If my shaper glassed the fin in then it would make a kind of zippering noise at high speeds like "zzzzzzzzzzzzzzzzzzzzzzzzzzzz". However if I recall the thing with sanding the fin was for speed which I don't think it helps much unless there was some rough areas that needed smoothing.

[pmcaero]

I don’t think he was sanding to sharpen but more to dull down the edge to lessen cavitation.

I don't think cavitation is a problem for most surfers, and I doubt dulling down the edge would make a difference.
https://www.surfer.com/blogs/design-for ... -to-speed/

[waikikikichan]

Sorry if I used the wrong scientific term. But here is something from the Futures Fins website about sanding and humming of fins.

Q: Whats the reason why fins sometimes hum?
A:
The short answer:
Pressure differences = Vortices or Turbulence = Vibration = Humming Fins

The long answer:
As water flows around a fin there are different pressures on each side of the fin. Water flows meet up at the trailing edge of the fin, and in cases with very different pressures, vortices or turbulence can be generated. These vortices push and pull on the trailing edge of the fin, causing it to vibrate. This vibration creates pressure waves or sound. Think of it as your voice when you blow air across your vocal cords.

Have you noticed that your fins hum at certain points while you are surfing, for example during a bottom turn on a fast wave? The vibration hum can be amplified or louder in cases of resonance. In fin terms resonance happens when the pushing and pulling from the vortices causes the fins trailing edge to move back and forth how it wants to move naturally. Or more technically, resonance can be defined as the circumstance when the force applied to the system is equal to the damping or resistance of the system at the systems natural frequency.

How do you fix it?:
Sand the sides of the trailing edge with a 100-150 grit sand paper (give it a few passes) and then surf it, if it still hums then give it a few more passes with the sand paper. The idea is to minimize the pressure difference at the trailing edge by sanding off any imperfections that may have been left from the production process. Make sure you don't sand too much, the more you sand, the sharper and more dangerous the trailing edge gets.

[pmcaero]

Sorry if I used the wrong scientific term. But here is something from the Futures Fins website about sanding and humming of fins.

Ah, it makes sense now. Thank you for the detailed explanation.

[RichardSmithe]

I had no idea that it can even be sanded... Thanks for all the advice.

[Naeco78]

It could also be the shaper that was glassing in the fin, for an almost finished board, depending on the scene. But ya never know with Hollywood.

[RichardSmithe]

Cool video, he makes it look easy.

[RichardSmithe]

I have done the research and I saw that it can be sanded and the easiest way was to get a sander and do it. For some reason I thought that these sanders are expensive, but it turns out that you can get one for only 40 bucks.

[Naeco78]

Hey RS, was it mostly for sanding down the chipped edges that would happen with fiberglass fins? That would make sense why it's less common now, with so many removable and composite fins.

[RichardSmithe]

I have done the research and I saw that it can be sanded and the easiest way was to get a sander and do it. For some reason I thought that these sanders are expensive, but it turns out that you can get one for only 40 bucks.

That is a great price and I decided to order one. I will have to learn how to do the sanding first because I am new to all this. Before getting a sander you should learn which one will serve your needs. I learned this by reading an article about the best sanders. I for example do not plan to use one everyday so I got a simple small one that will be good enough when I need it.