Rail Coanda effect

What profile does the forward rails of a longboard need to have to prevent being “sucked” into the wave face by what is referred to as a “coanda” effect instead of being lifted/pushed out from the wave face (a planing effect): thin (pinched); rounded; flat(squarish); short radius from bottom up to apex and larger radius above apex to deck, or the opposite; a chine. The problem occurs mostly when (slight) angle paddling onto a wave or just after being picked up by the wave with the forward rail penetrating the wave, and just before the pop-up.

I’m no expert, but my understanding of the Coanda effect is that flowing water follows curves, leading to a force down towards the water. Usually, people talk about hard vs soft rails, with a soft rail (like the 50/50 on a classic log) providing more hold, and a hard rail ( like the rail near the tail on most high performance board) providing less drag. So, I suspect the more gradual the curve, or the softer the rails, the more hold you get.

So, for your question, I’d think you would put hard rails near the nose, as well as the tail… the Greek in Huntington Beach does that on his shorty model, the downside I’ve heard of is it can make the board trickier to ride… the theory on that I think is that if the nose touches the water it’ll slice in easily, which makes some sense to me… I suspect rail volume/ buoyancy could be introduced to counteract that tendency.

I hope that helps, and if I’m wrong I hope someone corrects me :slight_smile:

The Coanda Effect occurs because of the low pressure created when a rapidly moving fluid stream/jet flows over a (convex) curved surface.
Think of rail profile as the leading edge of a wing (foil) profile.

Thanks TeeK for the reply.
The problem I have described has been present to a greater or lesser degree on all my boards that I have designed using Shape 3D and had the blank shaped by machine. I have tried blended edges from the bottom into the rail profile and also a “soft” edge along the bottom of the nose section rail. The bottom surfaces in the nose section have all been slightly convex (belly), with an approximately 65/35 rail profile. The soft “edge” seems only to delay the “suck” rather than prevent it once the forward rail has penetrated the wave face and water flows over the rail apex up onto the deck surface. Once up and planing there seems to be no problem even when riding on the nose, and so the problem may also be related to speed which changes the flow angle around the apex.
The solution to me seems that water flow around/over the rail above the apex needs to be minimised. So, I was thinking maybe a chine or higher apex (30/70 profile), or both, might achieve that. Also, changing the profile of the apex itself to more flat-ish might restrict flow from below to above the apex. A pinched rail might also achieve the same effect due to the much shorter radius of the rail profile curve, which would restrict water flow up and over the apex but may not be possible due to the thickness of the rails on my board design. Perhaps a sharp hard edge in the nose section could achieve complete separation of flow at the edge and prevent any flow above the edge, similar to the tail of a board with a sharp edge hard rail.
Due to a chronic shoulder injury I need to kneel paddle my boards, which requires more volume than for a prone paddler, so my boards are thick (3.5” max just ahead of the middle of the board) and the decks have minimal curve out to about 2.5 inches from the rails to provide that volume. So: almost no dome out to where the deck blends into the rail profile curve.
I’m hoping to have another board shaped later this year and would like to change the rail design to overcome this problem.

Thanks for this Stoneburner.
From my observation, most rail profile slices have an aerofoil-like shape, but water flow is angled aft by board forward motion and the path probably is not the same as across an aircraft wing. Also, the two mediums are very different in that water is effectively incompressible, and the water/air interface complicates the overall effect because the air is compressible. So, what I’m seeking is a way via rail profile design to minimise whatever Coanda Effect results from flow over the rail apex to the deck surface in the nose section of the board, whilst having minimal effect on control when nose-riding.

Common design opinion is that a hard edge on the bottom rail tuck allows the water to release. But I’m not a pro-shaper — just a retired scientist.
If you cut a diagonal cross section, starting at the rail 30-40% up from the tail and continuing to the middle of the tail (on most surfboards), it will likely look like the cross-section of an airplane wing. Tail section remains submersed.
BTW NACA foil profiles have been effectively used for hydrofoils (link below). Water still has to move faster over a convex top surface than over the flat bottom surface (creating a pressure gradient?).
The cross-section of a dolphin’s pectoral fin looks very much like a NACA wing profile.
Just my $0.02 of observations.
Pogo Foil



Thanks again for your thoughts.
The surfboard cross section you describe may look like an aerofoil (or hydrofoil for that matter) but barely more than half of the cross-section is actually subject to continuous water flow when riding a wave, and the flow is certainly not parallel to the chord of the section. Surfboards are obviously designed to maximise planing with the minimum area in contact with the water surface when catching and riding a wave. NACA aerofoils do work well on hydrofoils, as they also do on yacht fins and rudders and on surfboard fins, but that’s because the whole foil section is in the water and the flow across the foil is essentially parallel to the chord of the foil as it would be for an aeroplane wing section. The forces generated by water flow across a hydrofoil are probably also a prime example of the Coanda Effect because water for our practical purpose is incompressible, and Newtonian mechanics are explained as more applicable to the forces generated by water flow rather than Bernoulli physics which seems to relate more to compressible fluids in a closed system.
However, my problem remains: how to stop or minimise flow above the surfboard rail apex in the nose section at speeds above paddling speed to reduce or prevent any Coanda effect by using rail foil profile design. I’m becoming inclined to think that a narrow flat chine surface angled from the bottom direct to the apex with minimal blending into the apex may provide the desired effect on flow. Possibly a bit like a bow section on a dinghy yacht. Maybe someone has already tried and rejected such a design for a longboard because I have never seen such a feature on a longboard? I am aware that almost square rails were used successfully on early versions of surf skis and on toothpick surfboards, but possibly mainly for construction strength reasons.

Lots of misconceptions about lift. Bernoulli, Newton and Coanda are commonly oversimplified explanations (some incorrect).


Apparently optimum hydrofoil lift is at a 3-4 degree angle of attack.
Except at the rails, flow is parallel to the surfboard long axis for sure. Perhaps somewhat off topic, but the video below is interesting.
I don’t think there is a simple explanation for your question/problem.

BTW I’m convinced that Dick Brewer used NACA leading edge profiles for his rail designs.