Rail shapes for finless hold.

[krusher74]

OK so i'm thinking about my next board and have been researching rails

My last finless board had a soft edged lower 45degree chine on it (bodyboard lower half rail) This worked very well.

I know the basics, that a hard edge lets water sheer off (loose) and a curved edge water can stick too.

So if instead of a soft edged lower 45 degree chine I just had a 50/50 round rail, how would have affect hold and speed? Does what he's saying in this video from 6.48 about the rounded rail sliding out make and sense https://www.youtube.com/watch?v=Wq_Y3GOWM40

[nomastomas]

The 60/40 tucked-edge rail is the standard. It combines some of the release offered by the down rail and with some of the hold afforded by the 50/50 rail. Modern surfboard rails are transitional in that they begin as soft 50/50 in the nose, gaining a tucked edge by mid-point and turning down, hard in the last 16" or so of the tail. Prone craft typically have very short rail lines, so the transition would happen very quickly, or not at all. Consider how much of the rail line is in the water, and go from there. Now, with a finless shape you may want to sacrifice some speed for better hold, in which case a softer/rounder rail, with no tucked edge may be the ticket. I prefer thin or pinched rails over thick, round rails, so that I get better penetration. But, that's just my take...

[soulglider]

krusher, instead of round 50/50 i use a pointy 50/50. sometimes the whole length, that is to the 12-16" from the tail. and some the 50/50 sharpy first third of the board to a 60/40 sharpy for the next bit until the 12-16" from the tail and ones that really work great (depending on how you ride) 50/50 sharpy the full length. i say, experiment, you might find something you really dig outside the box, look at Mr. Mikes boards...

[nomastomas]

Whether "Pointy" or "pinched", low volume rails rule!

The Aku software has a neat way of quantifying rail thickness. It provides cross-sectional thickness at 1/2" in from rail apex, 1" in from rail and 2" in from rail apex. Anyone can make the same measurements on an existing board using a set of calipers. Using these reference points on the TBG4, I like 1-1/4":1-9/16":1/15/16" at the mid-point of the board. For comparison, a standard 6-2 x 18.5" x 2.375" SB is 1-5/16":1-9/16":1-3/4".

[krusher74]

I totally agree with the thoughts on rail volume, I find especially on bodyboards a voluminous rail will want to cork out of the wave face and be hard to hold traction on (but I guess all bodyboard rails are very high volume )

The rail on my first paipo board was very unusual, big 1 ,1/2" 45 degree chine up rail with a little top curve into the deck transitioning into a pinched 90/10 up rail at the front. Works great though.

But I am wanting to experiment on the new board (looking for more speed/glide), was thinking of a 50/50 pinched low volume front 3rd rail transitioning into a 40/60 up rail in the rear 3rd with a 45degree chine on the lower 60. I worry about losing the chine totally as it seem to be the thing to put on finless boards like wegners seaglass tuna.

Any feedback on my idea? does less chine area = less hold, or is it just less of any shape rail = less hold?

[nomastomas]

In general yes, less of any rail = "hold".

There's nothing particularly magical about the chined rail, other than volume reduction. Someone has already mentioned that it is a by-product of the manufacturing process on soft foam body boards. A "hot wire" is used on a CNC machine to cut the foam, and you can't cut a curve in the "Z" axis with a straight wire. (well, not economically. You could approximate a curve by doing hundreds of 1/16" rail bands, but that would take too much tool time.)

I'm not sure why you want to use a 60/40 up-rail (40/60 rail would be more correct) in the tail unless you don't want water to release easily? Water wants to wrap around curved surfaces, and this creates drag. A tucked edge would serve your design goals better. The question would be "how much edge?" on a fin-less board. You want to maximize the hold generated by the round rail for as long as possible, waiting until the last minute to offer a release point. If it were me, I'd start with a bottom edge 6" up from the tail and across the tailblock.

Don't forget the impact of bottom contour on rail shape. I place double open concave in the last 12" of my shape specifically to alter the shape of the rail so that it has more of a "hook" shape when viewed in cross-section. This has proven to be very effective in generating hold (but not as effective as the properly-sized fins). Here's my take on fin-less design http://mypaipoboards.org/forum3/viewtopic.php?f=4&t=560 if you're interested.

[nomastomas]

For illustration: Bottom contours impacting rail shape

[krusher74]

I guess what I am scared of is that everything I have surfed has had a large block flat chine rail and my brain equates that to hold.

I will be doing my usual single concave in the rear half of the board (with tail release) rolled into slight outer V, (this is what the wegener videos told be create hold on the alaias)

So then if I go to the following rail picture top most of the board with the lower last 6" for release. you think that will hold as much as my previous chine (given similar volume)

Thomas can you explain to me why wegener uses the chine rail on his seaglass as its not for the easy of manufacture like a bodyboard. Does it actually have less hold and is really makes the board faster , the bottom giving the real hold?

[nomastomas]

Wegener states he doesn't' mind occasional side-slipping or what he calls "la la-ing", so for him the "vacuum rail" works.

Try not to look at a single aspect, but consider the whole. Wegener touts the beveled rail, but look how the bottom concave shapes the rail into a hook for better hold. Looking at the outline, see how the tailblock is wider than the middle of the board, creating a fin-like protrusion in the outline at the tail. Watching film of the Tuna, cut-back turns appear to be more controlled slides than full-rail carves, although the board does seem to hold pretty well, even in the upper third of the wave. Also remember that water flows up the face of the wave and at an angle to the rail as you move across the face. That flowing water is redirected by his concave bottom, creating upward lift. This upward lift may help to counter the downward force of the rider, keeping the rail from slipping out. By contrast, convex or belly bottoms allows the water flow to go around, allowing the hull to sink. Lastly, rail length plays a part because the more rail line interfacing with the wave, the more hold. Taken to the extreme you have a rail that holds well but doesn't turn well. Wegener has settled on a lngth of 5-3 for lighter riders and 6-0 for heavier. Bottom line: Wegener has done a great job of "balancing" design aspects to achieve his desired performance goals, namely finless surfing, with easier paddling.

An interesting experiment would be the "hourglass" outline shrunk down to 48", with 50 degree rail bevel, 1/4" V at the rail in the tail and a deep, 3/4 length bottom concave. Width at the tailblock would be width of rider's hips plus 2", with middle width an inch or so narrower. Adjust thickness to yield desired volume, and crown deck to maintain thin rails. I add 2-1/2" of nose rocker because its easier to adjust pitch when standing then when laying prone. Just a thought...

[CHRISPI]

Decide and design by numbers. All fish and mammals that speed through water finish their structures with a knife edge so the rail shape is dictated by do you want to plough or knife through the water, remember the inside rail runs under water. These slab sided boards don’t make sense. Body boards have to be slab sided to stop them folding, glass or ridged boards allow an undetermined number of variables have fun deciding. Compare skim board wakes when they are tracking on a rail to just about any other wave craft has got to decide it .Try visualising wakes from underwater.

[nomastomas]

"These slab sided boards don’t make sense." -chrisp
No, they don't, and they shouldn't work as well as they do, damn it!

[JJJ]

Great timing on this post as I am in the middle of a build using 1/2" marine plywood.

The board is 50" long, has a cornered-off nose width of 14" (was 18" before I chopped the nose), a tail width of 24" and a wide point of 28" (12" from the tail).

Was planning on knifing the rails to form an equal edge - I guess you'd call 50/50? Now I've changed my mind and am thinking of shaving the edge of the deck to make the rail edge even to the bottom of the board; sort of a right triangle from the wide point to wide point through the tail and leaving the rails as-is north of the wide point.

The first picture is my crude drawing of the rail shape..

The second picture is my rough outline of the board - it's not perfect, but but close enough for me.

I will eventually glue a strip of the plywood to the nose and grind out a scoop/spoon.

Has anybody had any experience with this type of rail shape?

[soulglider]

jjj, i have on my plypo's. works good, but i didnt find it works better than just barely rounding off the corners with a light sanding. your rail looks better aesthetically but performance wise...i dont think so. interesting outline, wonder how that will work.

[krusher74]

Sorry, I dont know what you say about the 1/2" plypo rails!

"These slab sided boards don’t make sense." -chrisp
No, they don't, and they shouldn't work as well as they do, damn it!

I guess i'm going to have to make one without the "magic chine" in the myth-busters style!

So much choice! i'm liking the look of G right now.

[nomastomas]

Nice chart. "G" is pretty much a 50/50 with flat deck & bottom, and a tucked edge. How thick will the rail be? Tuck is measured as the distance from the rail apex to the edge on the bottom, typically about 3/4". How much tuck do you plan to use?

[soulglider]

S. I choose S! not sure I would for a standup board but for my kneeboards and my paipo/bellyboards its a yes. only, with a bit more pointynessness to it. all of the others are viable for sure but for me S is the one. did i mention, i like S?

[nomastomas]

So..."S", right?

[soulglider]

i knew i would be confusing but i like S. but some of the others are nice too. especially the hawaiian ones, but S is best. i think.

[krusher74]

Rail "S"! you guys are hodads!!

Anyway, took some pics and was comparing rails of my paipo and a my "longboard" bodyboard. Its interesting when you take a close look and see the what they are really like compared to what your memory tells you.

The pics are from the tail, but on both boards the rail does not change significantly on either board in the back half of the rail.

I can now see that my small wave loving,powerful wave handful! has a much larger rail volume. And certainly feels very corky in the rail , it suffers from lack of hold in powerful waves and struggles to submerge and let the board turn sharply.

My paipo rail has maybe half the volume , love 1ft slop to 10ft barrels and will hold a hard a bottom turn as i can force on it (but will also break loose for 360's and grab easily again)

Also looking at the bottom edge of the back of the rail on my paipo at no place is it totally sharp,it's maybe a 1/8" round over. (Tail end of board is sharp) I though it had at least 6" of sharp edge at the back of the rail but it does not.

I guess from the pictures my rail is an upside down "Q"

Maybe I got ahead of myself picking rail G as a possible as I don't really understand what that's give me

"How much tuck do you plan to use?"

what does tuck do? ( I understand what it looks like) I my chine just a great big flat tuck?

[bgreen]

Krusher,

Two photos of the rail from the board that you surfed. Blue is more mid rail, red is closer to the tail.

MM_midRail.jpg (79.67 KiB) Viewed 6224 times

MM_neartail.jpg (68.52 KiB) Viewed 6224 times

I'll post some photos of other boards separately,

Bob