The Way of the Water .

Which way does water flow on a rail ? ( inside rail )

Up ? As in vertically or somewhat along the length of the board ? At what angle ? or curve ?

Im interested in rails and I can find a lot of information about hold, bite and drag but a lot what is written is in direct conflict with each other and the rest seems elusive in details.

Some say thin rails give bite, others say release, and most of the information seems to be about describing the shape of the rail rather than how it affect water flow. Hence the elusiveness of accurate comments.

Does it attach itself to the rail up to the apex (grey line) and then detach itself ? ( dashed line)

Or does it stick to the entire rail and flow over onto the deck ? ( Blue line)

Anyone know ?


My guess…

It depends on speed and rail shape. The harder/sharper the edge, the lower the speed that water separation will occur at, and it will of course separate/release at the hard edge. The more round/soft the rail, the higher the speed necessary for the separation to take place, and water wraparound can happen. Tucked under edges are a middle ground, wraparound at slower speeds, release at higher speeds. Here’s Tom Wegener’s article, where I first saw this info:


I first thought about how rails were invented, if you are shaping a soft foam block into 2 flattish sides and trying to join the sides, a curve would be the natural join because its easy to shape, doesnt produce any corners which dent, and can be glassed into a smooth shape. Even going back to ancient solid wooden boards, a curved rail is the most natural shape any person would make, its also the simplest and safest way of joining the deck and hull.

But I ask how well does it do what we think it does ?

Following the theory that convex surfaces provide grip, if a 6 foot board with average rocker, roll and vee through the bottom provides grip through having a convex hull.

Then how much grip is provided by the tiny curve of a 1 1/4 in thick rail ? ( assuming that the rail is thinner than the stringer on a domed deck board.)

The outside rail doesnt count,

the front foot of the inside rail isnt in the water,

the last 18 in are clearly sheeting flat water away.

So the only effective rail is between his feet to provide all this grip.

The water appears to be coming off the midrail at about 30 deg so is it coming vertically up the rail as often mentioned and then going back ?

Or is the water constantly travelling along the rail at an angle,

(thereby making the route it travels into a much longer softer useful curve )

and then detaching at the rails apex or thereabouts.

( the coanda effect making it appear as though the water is wrapping around the rails)

Thus making the underside of the rail the only part of the rail that provides hold?


But Coanda isn’t illusory–it’s a fact of fluid flow. It’s happening if that rounded rail is any part of the interface with the water. How much difference could it make? I intuit that it would make a lot, eg if you made the same exact board with perfectly blocked rails all the way, you would sense immediately how much the rounded rail of the WP-back half had been helping, in terms of forgiveness and attachment/release/drag. I had never thought of how prone to dents block rails would be or the various degrees of strength characteristics of the curved rail though. Thx for that as I’m looking at doing something like a Waveskate.

FANTASTIC thought provoking subject. This is now entering new territory and maybe it is outside the realm of current science to give a definitive answer, but you never know if you never go.

Taking a wholistic approach I ask how can we, or should we isolate the various componentary players at work?

Do rails perform in isolation ? One would think not if the laws of physics determine the outcome of frictional interaction and hydrodynamic flow characteristics of the crucial variables of surface area, hull shape, fin dynamics, fulcrum effect of surfers weight distribution all combined with wave flow/ water flow direction at any given time relative to the boards position on the wave face.

Soooo many variables at any given instant. So this is a great starting point to making a breakthrough to possibly learning what are the components at work and what is their role in achieving a desired effect.

Speed, arc length, drive, release, slide…

Can someone with a real brain jion in?


the last 18 in are clearly sheeting flat water away.

So the only effective rail is between his feet to provide all this grip.

I’m too ignorant to contribute here, but I’d like to ask a question about these statements to help me understand what’s going on.

For the last 18" that are sheeting the water away – isn’t the sheeting action proof that that portion of the board is interacting with the water? If so, then there must be friction, so to what extent does friction help grip?

Trying to come to “grips” with a topic way over my head ;>)

The bottom does that. Go run the tap in your kitchen and hang a spoon with the round side next to it, slowly let it move into the water stream, and watch what happens–Coanda suction (EDIT: “suction” is a real imprecise and inaccurate term–the attachment redirects the water until it flows off in a deflected stream–the Newtonian reaction to the redirection of that flow and its mass is “lift” and it is work).

Guess you could try it with a butter knife too. Nothin’. Until you angle it and watch the water deflect off it. That’s the bottom deflecting the sheet off the water surface. (Which is also lift–without Coanda attachment-redirection–I think that it’s thus less expensive)

Again, I completely stand to be corrected on this, but I’ll give it a shot…

“Following the theory that convex surfaces provide grip, if a 6 foot board with average rocker, roll and vee through the bottom provides grip through having a convex hull. Then how much grip is provided by the tiny curve of a 1 1/4 in thick rail ? ( assuming that the rail is thinner than the stringer on a domed deck board.)”

I think the answer to this lies in the rail shape v speeds involved. The big, thick rail will get sucked into and up the wave face more than the one with the smaller radius. As you increase the speed of flow over the thick rail, you potentially get drawn further up the wave face, due to it’s ‘grip’ on the wave. The smaller radius rail releases water more, so as you increase the speed of the flow, it wants to break away easier. I think this begins to explain one reason why boards for smaller, mushier waves tend to have fuller, more rounded rails…

The rail sheeting water away is providing grip, but in combination with the bottom o the board, as there is some bottom in the face of the wave changing the water flow’s direction before the clean release (and hence sheeting effect) off the rail.

"Or is the water constantly travelling along the rail at an angle,

(thereby making the route it travels into a much longer softer useful curve )"

Yes, due to the board’s forward speed, the water only goes across the rail line at a right angle near or at the tail…

“Thus making the underside of the rail the only part of the rail that provides hold?”


Take the rail you drew (the one w/ the red circle in it) and try to see what shape the bottom/rail presents to the flow. As the board is banked ‘on rail’, the shape the bottom and rail present to the water flow change, changing the release point depending on how fast you are going (this is starting to get complex, quickly). This is the same with rocker. As the trim angle a board makes with the water flow changes, it presents a different shape to the water flow. How that shape changes with relation to the trim angle the board is at is why rocker is so important in board design.

So, getting back to the rail. My guess is the tail rails on that board are hard, and release water easily. To keep the tail from slip-sliding away, fins are underneath that hard tail rail section. As the board is banked into a turn, you want the inside rail to grip more as you turn harder. So ahead of the hard tail rail, the rail gets softer. As this softer rail is submerged with the higher bank angles of sharper turns, this softer rail is what provides some of the greater ‘grip’ to sustain a smaller radius turn. As I uderstand it, the fins mainly provide a ‘righting’ forces, which will get the board back to going straight…the hull initiates and carries most of the turning load, with the fins to counteract, preventing the turn from becoming a spin-out…

Again, I stand to be seriously corrected, and would like to be so I can learn more about this…


your last comment laughable and i am sure in jest!! If you know the history of the shapers that have been around you would see that the search for the empirical answers has always been at the fore front sometimes you can throw all the money at something and still not get the desired results.As a point of reference only, Al Merrick has a huge budget now, time will tell if some new paradigm shift will occur and if not, it does not stop from recognizing him from running a good business from a business perspective.Shapers that all of us have never heard of work on things everyday that many of us will never see.this place here has showed a small portion of that.the laws of physics are just what they are and we are governed by them.If you took the best engineers on the planet and asked them to make the best surfboard my bet is on the guys[surfboard shapers who are engineers in degree or degreeless] who have been doing it for a long time.The reason being is that their starting point would be the current surfboard as reference which is a very refined product.Now take the same engineers and ask them to create a craft that you can use in the ocean to do this certain thing without any reference point and the trial and error would begin would it look like what we currently use?You want to waste your time on that much trial and error {missing waves] or use the refined product now.Once read a great quote that said something like this, you can put all your books[I will add computers] down and they will circle the planet and let the mother do her work on them and all will be left is mother/planet.But i will say it make great reading your subject matter, and that some times the answer is right in front of us but we just don’t see it.The multi fin thing is a point of example so many incarnations is it really better? not really some is marketing thrown in with a new feel, better that depends on who you ask.I look forward to the gills breathing apparatus that will be so user friendly that we can go to the bottom of the ocean in my underware and that all of the land lords don’t eat me where no man should be allowed in the first place.But let the search for the Grail go on, its in your mind or on the back of some school kids book cover. Aloha…

I’m not sure you’re asking the correct question.

It isn’t how the water wraps up around the rails, but rather how much is directed under the board (kind of the dual of your question).

As a stab I might say that 50/50 rails take about x% of the water under the board, while boxed out edges “grab” more / less (I can’t decide).

But the effect of rocker cannot be discounted. Rocker is lifting the rail up, out of the way in the back half of the board regardless of concave/convex bottom), so reducing the “grab” potential of rearward rails (and directing water flow out at the angle in your picture). Stand on the tail, eliminate the free exit of water that is provided by the rocker and the hard rails “grab” less (?) water, so the tail sinks (maybe it’s more water, but the sink comes from surface area stuff?).

The water that flows over the top of the rail doesn’t really count, except for the drag it induces on speed and turn recovery). That’s why many rails have a discontinuity on the top edge to aid water breakaway - boxed tail rails vs pinched rails are a case in point.

My $0.02 (%A to USD it’s US$0.018 - woo hoo)

Greg, I agree that Coanda is real but I ask “Is what we believe about rail curves more convenient than factual ?”

And is the effect of Coanda overplayed ?

The area available for effective coanda on the convex bottom of a 6’6" board is about 1000 square inches and on the effective length of rail to provide coanda is a slender strip of 30 square inches at best.

In comparison thats 3%.

3% to divert an effective water flow from its course,

3% to draw functional volumes of water at 90 deg and more away from the face,

and the same 3% provides enough hold to suspend man and board on a vertical face?

I offer that even if the area of 3% was magically enlarged three times by making rails 3 times thicker ,

then the difference in coanda effect would be minimal .

As proof of my point I offer the history of rounded rail shapes going from almost 3 inches thick in the 60’s and 80’s to now almost 1".

If rails were almost 3 times thicker back then, where’s the proof of the multiplied Coanda effect ?

Its not there because the coanda effect is not the effective force upon rail shape.



Greg, I also agree that a square rail would have less “forgiveness” than the rounded rail and this is where I ask “Is the effect of Coanda over-stated ?”

Ply paipos like PHD have almost no rail at all yet hold efficiently in the steepest, fastest waves and modern bodyboards have flat sided angular rails and yet are able to hold a line in a steep tube. Both have no fins and minimal rocker so where does Coanda or other rial forces exist ?

So Coanda is not essential on a rail,

so is it even there at all ?

Certainly the bottom curve of a rail has a lot to do with the subtle release of pressure and this is where I position the crux of the effect of a rail shape, that is, between the horizontal plane of the hull and the distance of the apex of the rail, both in Depth and Width.

Pressure is focussed on the inside rail ( and the available hull area) near the rail and at times that pressure is very high due to the reduced area. How quickly and how measured the release of pressure is controlled is due to the curve or transition between the hull and the rail shape. Up to the apex or thereabouts, IMHO.

The rest of the rail curve means nought in terms of Coanda.

Please forgive my halting, incomplete responses as my server is slow.


i understand what you are saying but I can’t put my answer into written word. I could converse in person with you on the subject/

I dunno, Brett–I believe in curved rails if for no other reasons than the ones you mentioned about durability, strength, and because when you bank, your rail will interact more and at critical angles of lean during committed carves…

it seems like no penalty and some gain to me, but to each his own–I also can’t help myself on the aesthetic of a curved versus a blocked rail, if there’s no penalty

Ive removed the first part here because it was stupid.SF.

A plausible theory becomes generally understood and accepted and then forever onward its never challenged so it becomes law, despite its inaccuracies.

And when it is challenged, its defended by decades of convention.

Greg, Im not advocating square rails just pointing out that in this pic

the water flow is the same for both rail shapes and the only difference is the bottom curve of the rail. And that would indicate that it is the only functional part of the rail, the top curve of the rail is of no consequence to the water flow.

Sorry if this post sounds belligerent, not intended. I’m just asking questions.


ok here goes my feeble attempt, i don’t have an application on this computer where I can draw diagrams like you have, so i am going to refer to the diagram above my last post. when height is increase in relation to the depth and with, the more a board is going to get up and plane. it is going to skid in high force turns rather than grab.

when width is increased in relation to height and depth the more the board is going to be able to sink into the wave as there is less volume.

Increasing depth in relation to height and width is going to make the board sit lower in the water grab the water better as it is nice convex surface. modern hp boards are designed to plane most of the time, there fore having rails with a high apex(above 50% of the full thickness) is not really going to work all that great as hopefully the board won’t have that part in the water. However in a classic noserider rails like that are fine as you want the board to be grabbed by the wave and held stable. Vee raises the apex of the rail making the board more stable. too much v and boards spin out because the actual rals can’t get very engaged, just like if the board has to hard of edges.

"Pressure is focussed on the inside rail ( and the available hull area) near the rail and at times that pressure is very high due to the reduced area. How quickly and how measured the release of pressure is controlled is due to the curve or transition between the hull and the rail shape. Up to the apex or thereabouts, IMHO.

The rest of the rail curve means nought in terms of Coanda."

Yes, as long as speeds are high enough that separation happens at or below the apex of the rail. Also, having a nice curve to meet the lower rail curve does look nice, and is strong too, like an arch or flying buttress…

This image says a lot…

The square rail offers a ‘cleaner’ release, and is drawn less into the wave. Look at the stern/transom of powerboats for an analogy. The rounder rail sucks into the wave due to the delayed separation and change of course of flow…imagine how much a powerboat with a nice rounded transom/hull interface would sink into the water under power…The rounder rail also has more wetted area, hence more drag (read: control)…

By changing the rail profile along the board’s length, you change how the board reacts when that portion of rail is submerged…


Riderofwaves, The pics are made with simple mspaint, all windows machines should have it, then reduce the pics in width so they arent wider than a standard page and then save them as jpeg because its a slimmer format for posting than bmp etc… Pity you havent got something similar but you can download it or paintPLUS.

EDIT: TUX and paintbrush are available for mac users.

I like your explanation about varying the depth/width/height. Brilliant ! What a great way of describing it, thanks.

Does it mean that youre saying the bottom half of the rail curve, as defined by the width and depth to be more important for hold ?


Max, I disagree with “as long as speeds are high enough that seperation happens at or below the apex…” Because seperation occurs at all speeds below the apex.

At slow speeds the water barely makes it up the rail curve and seperates way below the apex.

At higher speeds the water easily seperates before the apex because the forces are too high for the Coanda effect to draw water away from the face. I cant see where theres enough middle ground on such a small curve for effective volumes of water to wrap around. I dont see this happening at all.

The rail is not drawn to the water at all, the rail is forced down onto the wave. And the water is not drawn to the rail in any degree more than a 1/2 curve can overpower the forces of a rising wall of water. Almost nil I’m guessing.

As the speed increases the water is pushed not drawn up the rail and away from the rail at the apex.

I dont doubt the validity of the lower rail curve however I question the application of the Coanda effect in this instance and thus the top rail curve in any effective role.


I run mac.

Hey SF,

Taking the left side of this graphic into account:

I think that the curve becomes more drawn out with the ‘diagonal/oblique’ flow and some wraparound is possible. But I do agree that a flow 90 deg to the rail will have little to no ‘effective volumes of water’ wrapping around…I think…

It goes back to my thought earlier as to why rails are softer ahead of the fin cluster. The flow is not 90 deg there, but more like the diagram above, and hence some wraparound could be happening, but I highly doubt that it gets all the way to the deck, though…(I really don’t know). But at very low speeds (maybe unable to sustain planing), it very well could.

As for the rail being ‘drawn’ to the wave face, all I can think of is the spoon-under-the-faucet experiment. A curved surface can be drawn into a flow if the circumstances are right…

For a flow to ‘stick to’ and follow a surface (and how long it does before separation), the things that have to be taken into account are the surface roughness/smoothness, the curve/shape of the surface, the overall size of the surface, and the speed, density and viscosity of the flow. I haven’t figured that stuff out, so I really can’t say except for guessing…