Rail Shape for a flatwater race board?

[peterwSUPr]

Hi everyone, I am building a 14’ carbon race board which will be paddled mostly for fun, and in a few rec races.  Most of my paddling is done on glass or almost glass from my back yard.  I can get in a few downwinders with small wind waves on the river, like maybe a foot to 18” at the most.  I don’t plan to paddle a lot in wind because I windsurf and kiteboard a lot, and we don’t get as much wind as I’d like as it is.  I’m definitely looking for something that will use the full 14’ waterline with just the right rocker for my weight.  Not ideal for downwinders, but I’m OK with that.

Anyway, I’ve got my outline and rocker pretty much figured out, but am not quite sure about rail shapes.  There is very little supping going on around where I live, so I don’t ever get to see all the different boards that are on the market.   It’s also tough to get rail shapes out of most top-view online pictures.   I’m just wondering, are boards like I describe still going with pretty boxy rails with vertical sidewalls?  (I’m referring to the front ¾ of the board and know rails get harder at the tail)

Based on another recent discussion with the group, I think it is clear that for a given width, there is more stability in a boxy rail than in a round rail, so there is max flotation as far out from the centerline as possible.  But, would there be other benefits from being slightly wider and more round?  A friend was pointing out to me that race canoes and kayaks are not boxy, so I wonder why we’re different?  They certainly are more tippy too.

I’m not looking to try to re-invent the wheel, and assume the big board companies have done more R&D than I have, so while explanations and theories are certainly interesting, I’d be interested to know what current boards on the market like I’m describing are looking like these days.  Anyone seeing what the latest of these boards look like?

Thanks, Peter

p.s. ice thicknesses are currently in the 20 to 40 inch range in my parts, so I have a bit of time to think about this!

[PonoBill]

I've been going in circles about this. I'm working on something new, but I don't know if it is nonsense or not, so I'll just tell you what I'm reasonably sure of. The boxy rail thing is for initial stability and decent flow along the template. I think it's actually nonsense, caused in part by some de facto limitations that production boards have--namely if they are racing boards they're either 14 or 12'6".  There are no major production unlimited boards. Within that template designers have some tough choices. Initial stability is vital because that's what people will feel immediately. Narrow boards are also important. If you have a curved rail with a narrow board almost everyone who gets on it for ten minutes will find it "too tippy" so it won't sell. 

It probably makes far more sense to abandon the notion of "rails" completely, and make race board bottoms curved, or at least have a curve that ends with a near 90 degree flat at the top with an increasing radius until it meets the flat bottom. This will yield a board that has a less wetted area when there is less weight on it. Initial stability will be nearly nonexistent, but the board will get wider between the centerline and the waterline as it tips, so it will have a great deal of secondary stability. In other words it will have a rail like a canoe or surfski.

[DavidJohn]

IMO the boxy rails are very necessary .. Not just to reduce tippyness and to help float a heavy paddler.. but mostly to reduced drag by stopping the water flowing over the deck and tail area.. Check out my recent post showing the new flat water Fanatic.. I think it's the latest and greatest in design.. here.. http://www.standupzone.com/forum/index.php/topic,23632.0.html

[supuk]

from what i have found from all the boards i have personally tested and built the boxy rails are fine in the flat but as soon as you get them in any sort of cross chop or wind the become a pig and it pushes the board around so much you loose any advantage in stability they bring almost straight away And if you have  a large side area the bigger the board the harder it will get to control in side winds and the more time and energy you will spend trying to keep on track rather than going forward.

[Kaihoe]

My new custom DC is heading down Ponos thinking.  It's how Travis Grants boards are going.

Round bottom, pinching to a soft vee behind the fin. With soft rails, still has box rails about the same size as the fanatic but nowhere near as hard..... Busted my ribs surfing so it's been sitting on the wall tempting me since it arrived

[supuk]

the old first javelin had round transition in the rails and was know for how tippy it was and the same with the 2013 falcon. I think the key in stability in not actually the stability its self but in how easy it is to control the pitching and the way it reacts to rider input so it more a sum of two things put together.

[PonoBill]

If a board trims properly it should never have water coming over the back. Unfortunately it's hard to build a 14 board that will be narrow enough to be reasonably fast that will trim flat for a 220+ pound paddler. I'm pretty certain the boxy rail thing is a way to build a board that suits the widest range of racers. I don't see a performance benefit in terms of drag. Skin drag is simply a factor of wetted surface times coefficient of drag. Boxed rails go the wrong way with skin friction.

Canoes and sculls have been optimizing their hull forms for a hundred years, with sophisticated development programs that SUP has never had. For SUP it's by guess and by golly, and most of the shapes by guys who have no idea what a Froude number might be.  For Sculls it's the hydro tank at MIT and a bunch of really smart dudes slapping slide rules 50 years ago.

[peterwSUPr]

Thanks for all the input guys.  Lots of info since my recent question was asked.  Interesting pics of the Fanatic.  Although it says "flatwater" in big letters on it, it looks like it is built to pierce and shed some chop up front.  I've been standing on boards all my life and have good balance, but 24" sounds a bit narrow. That might be nice and fast but i think I'd like to try one out that narrow before I make one and risk finding it too narrow.

One thing about board development, much like the other things I do like windsurfing and kiteboarding, is that there has never been huge scientific analysis of the sport.  It is so much cheaper to just experiment than to try and analyse it. 

Thx,
Peter

[Pierre]

thinking about reducing drag and wetted surface, best it no rails at all... semi-elliptical shape works best. by using deep and stiff fins it remains stable.

as posted previously,
http://www.standupzone.com/forum/index.php/topic,22634.45.html

[Area 10]

I think most people would agree that soft, gently arching rails (like e.g. on the SIC X14) are theoretically faster in calm water than hard, boxy rails (like on the 2013 Naish Glides). Any kind of edge is going to catch water one way or another at some point and so create drag. The problem is of course as already pointed out, that there is a substantial loss of primary stability with soft rails. This necessitates the board being wider for a given level of stability, and since narrower boards are easier to paddle and provide less drag, there is always going to be a trade-off between what rail shape you use and what width you want. This is presumably why some of the 2014 Starboards (e.g. All Star) are narrow but have reasonably hard rails and flat or concave bottoms. I'm sure they'd be faster with softer rails and (perhaps) convex bottoms. But probably few people would be able to stand on them.

The other issue with very soft rails is that if you have side-on chop and wind, you have no edge to "bite" and it can be very hard to stop the board from being a weathervane, constantly turning downwind. This is the counterpoint problem to that raised by supuk about boxy rails. As he says, tall boxy sides/rails present a flat surface for the chop and wind to hit and so this affects stability, creating side-to-side rocking, and sometimes a "hammock swing" in confused waters. This can completely negate any speed gains that the inherent stability of the design (in calm water) can give. But on the other hand, although very soft curved rails present less surface area to side-wind and chop, if the rails at the bottom are very soft, it is easier for the wind and chop to push you off course. Their inherent slipperiness becomes a disadvantage. So you have to find a compromise, I reckon, if you want your board to be tractable in a wide range of conditions..

But if as the OP says, the board is only going to be used in glass, then the narrowest, pointiest, shape with the least sharp edges that you can stand on will probably be fastest. Just don't take it out into the ocean...and take cover if the wind gets up. But I have yet to ride any board that is fast in all conditions. I'm no expert, but I don't think the laws of physics allow it.

[PonoBill]

for a deeply curved profile the waterline width of the board is a function of weight, not deck width. You can have a 26" wide deck that has a 22" waterline with 200# on it or 20" with 150#, like Stuey Campbell's Penetrator. If you looked at the shape of his board underwater (we did) it was a narrow, long curve pointed at both ends, even though the tail was square. The shape of the bottom determines the waterline dimensions. boxed rails do the opposite--they have the same waterline shape regardless of weight, unless they have rocker.

Boards with that shape are not difficult to stand on once you get used to their constant rolling, it was actually very stable and self-righting. But it took days to get comfortable with it, which is something no ordinary customer buying an expensive racing board would consider.

[Area 10]

I think you have to distinguish between stability and the perception of stability. They are probably not the same.

Those of us who paddle in the sea have probably all encountered the situation where the water is glassy but there is a slow undulating swell to it. Often these conditions, although the water is glassy, can make boards feel surprisingly tippy - often it can feel more tippy than if you were in sizeable chop. This doesn't make sense in physical terms because of course where a board is being pushed around violently, the rider will need to work harder and faster to compensate for these board movements, so it should feel like greater "tippiness".

What is probably occurring here is that the perception of stability depends on the signals that the brain is receiving from the proprioceptive senses. Chop and hard rails send big clear signals to the brain and so it is easy for riders to compensate. But gentle rocking and slow movements send subtle hard-to-detect signals and so we find it hard to compensate, and the result is a perception of lack of stability.

I think soft rails probably send more gentle (i.e. harder to detect) signals to the brain, which is compounded by the increased roll. So they feel a lot tippier. IMO it isn't so much about width at the waterline - although of course that is important - it is more about the quality and nature of the signals relating to movement that the board is feeding you through your feet/ankles.

Incidentally, this is also probably why light boards "feel" faster than heavier ones, even though the evidence (in flat calm conditions, and over a sizeable distance, not 200m sprints) is distinctly lacking. Our bodies are very good at detecting changes in speed and direction, especially acceleration. This is why roller coaster and other fairground rides are exciting even though the speeds involved are less than in the average family car. It is easier to accelerate a light board so it feels faster. The problem is that it decelerates faster too. For the same input, the heavier board will accelerate slower but will decelerate slower too. This lack of changes in speed makes the board feel sluggish - and our brains interpret this as slowness.

Now, no doubt a lighter board for a given thrust is slightly faster than a heavier one. After all, a mass has to be moved from one place to another, and that is going to take energy. But probably over distances of a mile or more this difference is tiny, when the difference in the weights of the board as a proportion of the overall weight of the board+rider is so small (e.g. 1-3% or so). However our perceptions of the difference between two identical boards in two constructions (e.g. carbon vs. AST) can be that the lighter board is much faster. It is probably largely our heads messing with us.

Well, that's my theory, anyway.

[55NSup]

Interesting insights. My experience in kayak design and sailboat design, which is all book and design study,  says rounder rails the entire length are faster. In calm water, glass,  all the wind and chop issues don't exist.
I'm finishing up my 16-5 with round rails,  low deck and rocker, 26 wide. Designed for my 80kg to have nice waterline and prismatic coefficient.
Spring and summer will tell me if it is too tippy.

[55NSup]

Cad file is not exact to final shape. Made bow fuller.

[aranysup]

If box rail could make the sup narrow than make box i hink narow is fast and flat water sould be 25 if its 14 you get used to it