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do fins really need to be so sharp?

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si-fly
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Re: do fins really need to be so sharp?

Postby si-fly » Sun Oct 03, 2010 9:29 am

NO they dont ! if I remember correctly from windsurfing days, as long as you file the sharp back edge square, about 1mm across, the corners release the water just like a sharp knife edge. If you round the trailing edge you get drag!

I have always done this to all my fins and I can still sail faster than the windsurfers and kiters at my local beach.

Water does not flow the same as air which would need a sharp knife edge to reduce drag.

Si.

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kitezilla
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Re: do fins really need to be so sharp?

Postby kitezilla » Sun Oct 03, 2010 3:09 pm

www.kiteforum.com/viewtopic.php?f=1&t=2321970

Here is a picture of what we are talking about from the above thread:
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kf razor_blade_fin_986.jpg
kf razor_blade_fin_986.jpg (120.61 KiB) Viewed 849 times

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Peter_Frank
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Re: do fins really need to be so sharp?

Postby Peter_Frank » Sun Oct 03, 2010 7:04 pm

si-fly1 wrote:NO they dont ! if I remember correctly from windsurfing days, as long as you file the sharp back edge square, about 1mm across, the corners release the water just like a sharp knife edge. If you round the trailing edge you get drag!

I have always done this to all my fins and I can still sail faster than the windsurfers and kiters at my local beach.

Water does not flow the same as air which would need a sharp knife edge to reduce drag.

Si.
Correct !

Having a square trailing edge is way better (less drag) than a round one :thumb:

But you can still cut yourself a bit, when they are "squared" off clean....

Way better than razor sharp though 8)

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Re: do fins really need to be so sharp?

Postby flyrob » Sun Oct 03, 2010 8:18 pm

Sharp leading and trailing edges on surf and kite boards are hydrodynamically irrelevant in the environment they operate in.

A sharp leading edge helps on high speed jet wings when they are in cruise and have almost no angle of attack.

The trailing edge is again only relevant when the "wing" (or fin in this case) is perfectly aligned with the water flow.

Our fins are pretty much always operating at very high angles of attack relative to the water flow. They are functioning as flat plates, rather than a wing would in air. This angle of attack causes a huge low pressure and turbulent flow area on the unloaded side of the fin. Probably with a lot of cavitation. This turbulent flow causes WAY more drag that any difference in the sharpness of the edges.

I never thought of the safety aspects of these sharp edges before this thread so thanks for raising the question. I think blunting them would provide some protection in relatively low energy interactions with flesh such as contact with a foot when kicking. I think if the fin or board hits your unhelmeted head when your board leash recoils it's gonna leave a mark, sharp or not!

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Re: do fins really need to be so sharp?

Postby twah » Sun Oct 03, 2010 9:37 pm

yeah totally don't have this problem cause i ride fins no bigger than 1" that are 6-8" long and not sharp at all or i ride finless.

it's these gay 2.5" fins people need to "grip" with straps on their flat planks. max you need is 1.5" for a little bit of grip and drive (especially in foothooks)

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Toby
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Re: do fins really need to be so sharp?

Postby Toby » Sun Oct 03, 2010 11:30 pm

yes, it is a fin of a North board.

No Peter, still too young for waves ;-) So yes, it is on a TT.

I did a one footer...but could easily happen on a grab too...or if you crash...so not limited to boardoffers only. Recall those crashes, where you hit the water with the board on your feet and it flies to the front of you...you get janked again and fly directly onto the board...the cut will be long and deep if you hit such a sharp fin.

Same thing if a surfer gets washed in a wave...do they have such sharp fins?

park1
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Re: do fins really need to be so sharp?

Postby park1 » Sun Oct 03, 2010 11:40 pm

Whilst the leading edge on surfboard fins are generally not that sharp, the trailing edge can be. Check out Joel Parkinson's foot :o

http://magicseaweed.com/Parko-Out-of-J- ... tent/2559/

longwhitecloud
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Re: do fins really need to be so sharp?

Postby longwhitecloud » Mon Oct 04, 2010 1:01 am

yep, those slicing the paper are the slovak fins.

SBBeachbum
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Re: do fins really need to be so sharp?

Postby SBBeachbum » Mon Oct 04, 2010 2:36 am

twah wrote:yeah totally don't have this problem cause i ride fins no bigger than 1" that are 6-8" long and not sharp at all or i ride finless.

it's these gay 2.5" fins people need to "grip" with straps on their flat planks. max you need is 1.5" for a little bit of grip and drive (especially in foothooks)
You seem to have other issues. Please don't skip school .........

longwhitecloud
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Re: do fins really need to be so sharp?

Postby longwhitecloud » Mon Oct 04, 2010 3:07 am

This issue is not exclusive to 1.5" fins plus. Off the top of my head I think this manufacturer goes down to 0.75".

Just post the vid of u ripping and stop trolling lol


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