I’ve wondered about this for many years. I’ve surfed a square rail as an experiment but it ended up as a bit of a dog. I tried a diamond ‘stealth’ rail too which ended up as a pig. Now as last resort in the experiment derpartment I have built a board with concave rails and well I wonder what more experienced hands might have to contribute as to whether or not this could actually work or become just another toad destined to the bucket list from hell. 
What’s that croaking sound?
**Disclaimer **I don’t claim to be experienced !!( though I did stay at a holiday inn express last night)
But I 'm wondering what are you trying to accomplish with this rail design?
Rail shapes change down the length of the board for a reason.
They tend to be more round in the front to allow water to wrap around it creating hold.
The bigger the radius the easier it is for the water to wrap around. ( I think it’s called the coanda effect)
Then they tend to sharpen towards the back allowing the water to separate from the board to lessen the planning drag.
To keep the same shape all the way down the length doesn’t seem practical.
If I was to guess how the rail profile that you drew would act I would say that it would have a tiny bit of hold created by the little radius on the bottom,
but mostly it would just release as it got to the sharper area above the radius.
The concave area above that would probably just be a turbulent mess that would cause drag without much benefit.
In my limited experience I have found that water doesn’t like to conform to complicated shapes .
But that’s just my 2 cents and I could be totally wrong.
If you decide to build it I’d love to hear how it goes.
I say try it, Quanta. Sounds interesting. If you don’t want to make a new board to try, you could always sand in the rails on an existing board and lay a glass tape layer inside the rails.
wasn’t there a morey boogie that had rails like that?
Oh it’s going to be tried. We are just waiting for the paint to dry.
My second reply becuase the 3 images I tried to post were all upside down. Is that becuase I’m on the other side of the Planet? HE he. OK I give up … I flipped 'em and they still come out upside down… Oh well they are in the pipe. Just stand on ya head to have good look at em. LOL.!!!
I’ve got one in the shed I’ve kept from about 5 years ago. I wasn’t a fan but it might take more than one shot to dial it in right. What I found was that ur eliminating the bottom half of the rail as well as taking away 1\ 2 in off the width all over.
I’ll post a pic if ur interested.
I’m very interested SF. I wouldn’t have invested my time and money otherwise. A buddy of mine will be surf testing this tomorrow [surf permitting] but it’s looking good here on the Gold Coast mid winter.
Love to see your pic.
Cheers!
Hope you have a great surf on it !
My cunning plan was that it would introduce a second rail edge that might increase grip during a turn but it did the opposite. I learnt that water loves curves. And the water completely ignored the sharp edges of the concave. However my concave was a constant 3/4 inch high from nose to tail so totally different to yours.
Freeze frame on the DVS black carbon board with a swallow tail flyer that Dave Rastovich was riding in the opening sequence of Blue Horizon. The tail rail after the flyer has what looks like an inverse curve to a sharp edge.
Your rail will work after the fins and probably up to 16in from the tail. The mid section shouldn’t hold on a steep face.
Have fun
Thanks for your thoughts WW. The board got surfed today with a quad fin set up in small beach break conditions and the report is in. Down the line it’s fine but it’s hard to turn. My read is that it tracks. Over the next few days we will go to a twin fin configuration and then to a single fin and finally no fins at all or maybe a teeny weeny one. I contend, the concave rail is acting like a fin but it is early days. I’ll keep you posted and yes we ARE having fun. 
The comcave rail is disaster. hard to turn and basically a no go Zone.
Thanks for trying and keeping us informed. What you do not try, you will never know if it works or not.
“The mind is like a parachute: it only works when opened.”
Could it be similar to a flute on a flyer but directly on the apex of the rail?
DR surfed one of my colaberations with Chris Garret some time ago. The rail does not work but don’t worry , there’s more in the works. Watch this space.
Balsa. Your encouragement is appreciated. life is an envelope of paper waiting to be folded not unlike a rail.
Ah Felix … you are soo close with the flute anology but at what piont is the ‘APEX’ of the rail? You mean it’s widest point?
Yep widest point of the board. If you had a 50/50 rail it should at the furthest point of the board where the under curve meets the deck curve.
