RAIL curve, parabolic, how does it work?

[krusher74]

Hi, was thinking of designs of a paipo today as was looking at alaia's for inspiration

Could anyone explain how the parabolic or standard outline curve/rail line work hydrodynamically?

[nomastomas]

I'll take a stab at it. In surfboard design, I find rails to have a very complicated impact on performance. On surfcraft, the rail is another control surface. Control surfaces always induced drag. That's how they work. In a kayak, dragging a paddle along the side causes the yak to turn towards that side. The more that the rail is engaged in the wave face the more drag it causes and the more control it exerts. With the wave-side rail engaged and the beach-side rail now up, and out of the water, more drag is created on the wave side. This drag "differential" causes the board to pivot in the direction of the drag. On an alai'a with a concave outline curve, that little flare of the outline creates a protuberance at the tail, which digs into the wave face, as pressure is applied on the wave-side rail and the board rolls over. The extra drag it creates may be used to keep the board attached to the wave face, maintain a straight line and/or cause the board to turn better. When weight is lifted off the rail, and the board rolls back onto the bottom (disengaging the rail from the wave face), drag is reduced and the control gained by the rail is lost. The board drifts more-or-less down the wave face and settles into a position where the lateral drag reaches equilibrium, i.e. drag is uniform side-to-side. An outline that is straight out the tail offer less potential drag and consequently less control (but more speed). Pulling in the outline at the tail, reduces drag even more. It also allows the rear of the tail to better conform to the shape of the wave deep in the pocket. Thus, the board can be ridden on rail, deeper in the pocket with less drag. Anyway, that's my take...

[rodndtube]

The ancient Hawaiians used convex bottoms instead of concave bottoms. Something to consider. Same basic principal that Tomas outlined above -- increased drag from the rolled bottoms helps to keep the rail in the wave.

My buddy just started experimenting with small fins on a 48x18x1.5 wood board -- he said he felt increased control and the natural feel of being finless.

[soulglider]

"The ancient Hawaiians used convex bottoms instead of concave bottoms. Something to consider." aaaaaaand now we're back to displacement hulls.

[krusher74]

I know a straighter rail is faster and a curves rail turns better.

But reverse curve I"ll have a think on the concepts you guys have mentioned and do some googling!

[bgreen]

The ancient Hawaiians used convex bottoms instead of concave bottoms.

Rod,

Larry mentioned that you were going to the Bishop museum. Did you get to see the boards there?

Bob

[GeoffreyLevens]

The ancient Hawaiians used convex bottoms instead of concave bottoms. Something to consider.

Doesn't the concave come from Tom Wegener's experiments over the past few years? He started by directly copying the Bishop boards then found that adding the concave worked even better for most uses?

[OG-AZN]

You have to remember that the concave on Wegener's boards was developed to address controlling a thin finless board ridden stand up. Prone boards where the riders legs and fins are in contact with the wave don't have as many control issues. On thin wood prone boards, I haven't found concaves making enough difference to justify the time to shape them. Flat or slight convex is easier and works just fine. Might be more useful on a thicker finless foam & glass board or bodyboard. I had a bodyboard (Jack Lindholm designed "North Shore")when I was kid that had a variant of a concave bottom with a square rather than crescent tail. It was very paipo/alaia like when I think about it - wide nose/high wide point, fairly narrow square tail. That was a great board.

[GeoffreyLevens]

Thanks OG. Earlier, from reading this thread and other stuff, I was wondering about comparative effects of rounded/convex bottom vs the more complex rounded near rails and concave down the center

[rodndtube]

The ancient Hawaiians used convex bottoms instead of concave bottoms.

Rod,

Larry mentioned that you were going to the Bishop museum. Did you get to see the boards there?

Bob

I only saw the boards on display -- three in all -- and those were not set up to see much (could barely make out the bottom sides). The one board was a Duke, so not representative of the ancients' boards, and also a foot surfing board. The other two were alaia/paipo-sized and a very small paipo that bordered on hand-board size. The alaia/paipo was very thin and in a corner display case so hard to see anything. The very small paipo (looked like 24 inches or so) initially appeared to be concave but upon second examination was very convex with a hollowed out deck.

[soulglider]

yeah O.G. is right. slight and shallow concaves are a beautiful esthetic but dont really work as advertised in a plywood board. i have used them and they actually add a controlled longitudinal flex to the board, the type of flex that actually works when one pulls up on the outside rail while in a bottom turn. convex bottoms are faster and more "down in the water" for that oh sooooo good feeling. thinking i wanted a bonzer paipo about 6 years ago, i talked to mike eaton. he said, "these boards only come alive when its big" and that was according to his "unnamed, swamis local" customer (he knows who he is) of his that has been surfing them since the 70's.

[krusher74]

To push this back on topic it was rail curve I wanted to talk about not bottom shape

Maybe I should use the terms, curved rail, straight rail and reverse curve rail

[nomastomas]

Prone boards where the riders legs and fins are in contact with the wave don't have as many control issues. On thin wood prone boards, I haven't found concaves making enough difference to justify the time to shape them. Flat or slight convex is easier and works just fine. Might be more useful on a thicker finless foam & glass board or bodyboard.

Hits the nail on the head...Not as many control issues, but, top speed is constrained by drag. It occurred to me the other day, that despite all my efforts to produce a hydrodynamic-ally efficient hull, at 45" of length, usually my thighs, but sometimes my knees, calves, ankles and feet (with fins) are dragging in the water! All the bottom contours, e.g. concaves, channels, belly, etc in the world are not going to counteract that drag. At the end of the day, I'm body surfing with a really big handplane. Which can be fun, but not the game I'm after. There are moments on my 45" when I pull myself towards the nose an inch or two and I experience a burst of speed. I use to think that it was because the changes in position made the board plane more efficiently, but now I think it was due to the fact that my legs were now completely out of the water and not acting like a sea anchor. I'm looking for a ride experience more similar to kneeboarding than body surfing. Back to the drawing board...

[rodndtube]

For me, 50 inches is the magic mark: just below my chest, at the sternum. At that length the board is not so long to keep me from effectively arm paddling and swim fin kicking.

[krusher74]

ok after some googling and reading It would seam the term i was looking for was parabolic rail (edited thread title)

From some Wegener online info, it seems as nomasthomas said the outward kick at the back of a parabolic rail give extra rail bite (like a fin) usefull on a stand up alaia, but not needed on a paipo, and parabolic being slower than a normal curved or straight rail



"The parabolic rail holds the board into the wave better than the straight or regular curved rail. The deep concave lets the board grab into the wave more. The benefit is you get a lot more control and ability to hang in a steep face. They tube ride well. The down side is they are a little slower and do not ride prone like the Finley shape. I make these from 5”5’ to 7’0” and 16’ to 17” wide. The deep concave makes them more prone to cracking than the other boards so I am especially careful with the wood I use. I have found certain cuts that flex more and crack less than others." Tom Wegener http://www.tomwegenersurfboards.com/history/alaia-story