Rough fins versus smooth fins

[Stew]

Hi,

I've put this in general as it's potential around a number of branches of SUP.

So I was out for a 5km timed paddle yesterday - not the normal activity for me.

While paddling, I got to thinking on the fins and drag. I seem to recall years ago seeing with windsurf boards that some people would lightly scratch up the underbelly of the board to improve speed as the scratches would grip water and the friction of water over water was less than water over the board. Now, the board I was on is an inflatable so no scratching going on there but I wondered if abrading the fin would have a positive or negative.

Positive could be less drag for times you're on a race board.
Negative could be less hold on waves for surf?

Totally just guessing here and wondering if anyone has looked at it.

I know there are many factors are fin design and board design and also hard to quantify differences in the real world but this woud be the same fin, same board, etc with the only difference being smooth or rough.

I also do expect the difference for me would be negligible but I have a curious mind.

[sflinux]

Coming from a kitesurfing background, I have the opposite perspective.  If I had a fin that was chewed up (hitting bottom, etc) I would get a noticeable hum when going fast.  If I sanded this fin, the hum would go away.  Feels faster without the hum.  But when it comes to scratches I don't every worry with high grit (i.e. 600-1200) sandpaper, 220 is good enough for me.
I would think you would have a more dramatic effect by changing the fin, then sanding.  For example I have a surf tech 8" fin that has a pretty thick profile.  If I swap to a 8" Need Essential fin, I can feel the speed difference due to the thinner foil. 
I can feel less drag by smaller fin (i.e. 8") versus taller fin (i.e. 10"), less surface area, less drag.   But with that there is a balance, as with a bigger fin you can feel the board track with each pull of the paddle stroke (straight line).
Then you have the option of narrow base fin (race) versus wide base fin. 
And the option of upright versus raked fin. 
Then you have the option of winglets.  And the option of cutouts like the Quobba. 
And the option of single fin, versus twin, versus thruster, quad, etc.... Single fin feels fastest to me, though a well designed finless SUP has been on my wishlist.
But for long paddles, I would think the most dramatic effect would be the narrowness of the board (narrow has less drag) and length (longer has more glide).
With an iSUP, I would imagine a firm board would paddle faster than a soft one.
For long paddles, I try not to think about "board" variables which would improve my speed, and instead just focus on trying to improve my paddle technique.
Going out without a fin is fun to mix things up.

[sflinux]

If I wanted to make a faster fin, I would change the material.  I've built several wood alaia surfboards.  They are unglassed and covered with a skin of oil.  People that ride them swear they are faster than other materials that modern surfboards are made out of (fibreglass, epoxy, plastic, etc).  There is something about the oil/water surface interaction that helps the board slip through the water.
If I wanted to make a faster fin, I would make a thin profile fin wooden fin, sealed with oil.  Tung oil is my favorite.  But linseed oil is another popular choice.  But if you are low budget vegatable oil works too (just doesn't cure & you have to reapply each session).  Typically I wood mix with turpentine in a 1/3 to 1/2 ratio, and do several coats.  When you oil wood, the grains natually come out, which plays on your rough versus smooth hypothesis.
My first SUP was 24" wide 12' old windsurfer where I made a wooden fin for it.  The fin eventually accidentlally broke while porting that heavy board.

[jondrums]

I have a good friend who worked as a hydrodynamic simulation engineer on America's cup boat projects.   He swears that by all accounts, a slightly rougher surface should have less drag according to theory.    However, preparing the race boats it was all hands on deck hand sanding the entirety of the hull to high grit sandpaper as smooth as possible and all the programs do it because it is considered faster.   He was never able to find a theoretical backing for it, but the America's Cup programs do whatever works.

I think we've got enough kite foil racers accounts that going to very high grit sandpaper at least on the front third/half of the foil section makes a difference in drag.

[Stew]

Coming from a kitesurfing background, I have the opposite perspective.  If I had a fin that was chewed up (hitting bottom, etc) I would get a noticeable hum when going fast.  If I sanded this fin, the hum would go away.  Feels faster without the hum.  But when it comes to scratches I don't every worry with high grit (i.e. 600-1200) sandpaper, 220 is good enough for me.
I would think you would have a more dramatic effect by changing the fin, then sanding.  For example I have a surf tech 8" fin that has a pretty thick profile.  If I swap to a 8" Need Essential fin, I can feel the speed difference due to the thinner foil. 
I can feel less drag by smaller fin (i.e. 8") versus taller fin (i.e. 10"), less surface area, less drag.   But with that there is a balance, as with a bigger fin you can feel the board track with each pull of the paddle stroke (straight line).
Then you have the option of narrow base fin (race) versus wide base fin. 
And the option of upright versus raked fin. 
Then you have the option of winglets.  And the option of cutouts like the Quobba. 
And the option of single fin, versus twin, versus thruster, quad, etc.... Single fin feels fastest to me, though a well designed finless SUP has been on my wishlist.
But for long paddles, I would think the most dramatic effect would be the narrowness of the board (narrow has less drag) and length (longer has more glide).
With an iSUP, I would imagine a firm board would paddle faster than a soft one.
For long paddles, I try not to think about "board" variables which would improve my speed, and instead just focus on trying to improve my paddle technique.
Going out without a fin is fun to mix things up.

Yeah, I get all that.  I was curious about the one element - I've seen loads of finology of other parts.

Maybe I'll make a super super thin fin from some bits of titanium I've got kicking round! Definitely not for surf and the potential for getting sliced but flat water......

[Stew]

If I wanted to make a faster fin, I would change the material.  I've built several wood alaia surfboards.  They are unglassed and covered with a skin of oil.  People that ride them swear they are faster than other materials that modern surfboards are made out of (fibreglass, epoxy, plastic, etc).  There is something about the oil/water surface interaction that helps the board slip through the water.
If I wanted to make a faster fin, I would make a thin profile fin wooden fin, sealed with oil.  Tung oil is my favorite.  But linseed oil is another popular choice.  But if you are low budget vegatable oil works too (just doesn't cure & you have to reapply each session).  Typically I wood mix with turpentine in a 1/3 to 1/2 ratio, and do several coats.  When you oil wood, the grains natually come out, which plays on your rough versus smooth hypothesis.
My first SUP was 24" wide 12' old windsurfer where I made a wooden fin for it.  The fin eventually accidentlally broke while porting that heavy board.

My guess is the oil is holding the water so that low water on water friction suggestion.

[Stew]

I have a good friend who worked as a hydrodynamic simulation engineer on America's cup boat projects.   He swears that by all accounts, a slightly rougher surface should have less drag according to theory.    However, preparing the race boats it was all hands on deck hand sanding the entirety of the hull to high grit sandpaper as smooth as possible and all the programs do it because it is considered faster.   He was never able to find a theoretical backing for it, but the America's Cup programs do whatever works.

I think we've got enough kite foil racers accounts that going to very high grit sandpaper at least on the front third/half of the foil section makes a difference in drag.

Interesting to read. Thanks!

I guess for the foil side of things, it's quite a different aspect going on so tricky to compare.

[FRP]

Intriguing question about fins. It perhaps has some relation to the dimples on a golf ball. My simple understanding is that the dimples reduce the turbulence and drag on the low pressure side of the ball. The difference with water turbulence compare to air is likely significant but with a fin in a test tank dimples could be and likely have been tested. When I look at quoba fins they look like they would have more drag but the real world experience says no they are faster. Cheers Bob

[Dusk Patrol]

Hobie made this claim with their all around "ATM" SUPs, and put a 'fine grit' finish on the bottom of their boards. 

Which reminds me of the unspoken truth (in marketing at least) around SUPs, and that is that there is no slower sport