TT rails

[Bille]

I told an aircraft engineer, what the common lay-Up for a tt in the
132 X 40 size would be, if done in woven E-glass ; he did the math and gave
me a schedule that should work if using S-glass uni lengthwise, along with E-glass
uni at +-45-deg , with an 1/8th" cell 1/4" thick Nomex core ,
((which is Much higher compression strength than Rohacell or
Divinycell (or any kind of other foam core)).

HERE IS THE QUESTION :

What should my rails (profile) look like , on both the center, and tapering
towards the tips ; figure 4 to 5mm thick, for the full length and ends of the
board ??
Could someone make a drawing of the rail profile please ?

Thanks : Bille

[dirk8037]

just meassured roughly my old north xride and it has 6.5 in the middle and 3.5 mm in the tips tapered very gradual.
But its stepped down from ca 30m from the rail since the core is considerably bigger.
I do not know if that is an option using Nomex.
Last Nov I rented a Core Fusion i liked qite a lot, which had considerbly thicker rails though still slightly stepped down .
I did not had the impression that it would cut harder through the water.

I also remember a post of someone in an other forum who took part in a test of similar boards with different rails (hard and tucked under the edge).
He was absolutely baffeld how much different it made, hard edge much better upwind.
The core was almost rectanguar and for my average capabilities I did not felt any limitations.

Just one question, since you are the first one I hear from in over 15y to use Nomex.
I always thought is is made for more static loads and not for flexiong like a TT.
Did your aircraft eng said anything about that?
How are you going to do the tapering?

Cheers Dirk

[Trent hink]

A hard edge is the most efficient.

As you soften the edge, there is more drag. But along with the drag comes a feeling that rail “grips” better and has better control.

[Bille]

...
Just one question, since you are the first one I hear from in over 15y to use Nomex.
I always thought is is made for more static loads and not for flexiong like a TT.
Did your aircraft eng said anything about that?
How are you going to do the tapering?

Cheers Dirk

Flex , (in sandwich construction) is strictly a function of keeping the
bottom lamination, at a specific distance from the Top lamination.
The , "Thicker" , (or Height) the distance from top to bottom ; then
stiffness increases by the , (^3) or cube. Adding more lamination
on both sides, will also stiffen the flex, at the penality of added weight.
Foam core will keep the laminations on both side , at a specific distance
but the core is soft so the distance changes ; with Nomex , the core
stays stiff and the laminations stay the same distance at all times.

I can drive my van, over a sandwich of 6oz carbon, Nomex, and 6oz
carbon ; the core will survive , where as a foam core will not.

Nomex is a Bitch to work with ; learn the rules on how to use it, and
it functions at a WAY better rate than any foam core ,at a much lighter
weight.

[Bille]

Strictly speaking from a ease of lay-Up point of view ; the hard
rail on the bottom Left, would be the easiest to make . I could
also just square it off ; or even a tad concave if that would
work better ?

Bille

[Trent hink]

On a tt, the top shape of the rail does not make much difference, the key area is the bottom shape, because that is where the water moving beneath the board releases. Water is not just moving from the nose of the board toward the tail, it is also moving outwards from the rail, and you can see this happening easily in any "nice spray" photo. Essentially the same thing is happening on submerged rail, except the water is hitting more water, instead of air.

When I used to make boards, I liked to use a solid material for the rails, that way I could start with a very hard edge and grind it a bit softer if I felt I needed more control.

I found I liked to have my boards with hard edges around the tail and up near the fins, and then gradually soften the edge up toward the center of the board, but I don't know if that is the best way, and its probably just personal preference.

I never liked soft rails all around the board, or too soft near the tail, but I also never liked to ride "lit"

...but a hard rail all around the board works great, and you can get better control by adjusting the fin placement and fin size.

[BWD]

I vote for softer between feet, hardening to more square at tips. Gives good precision feel, release and speed when riding fast off back foot, good grip/stability when edging hard, carving with full rail engaged, or slamming though chop.

[Trent hink]

If BWD and I are in agreement....

The best option, from your image Bille, would be: lower left near the tail, transitioning into top right at the center of the board.

This can be a thing where subtle changes make a big difference....

Going with a nomex core, I doubt you plan to wrap the rails, and instead have chosen the easy and rational choice of closing out the edges of the Nomex core with some other material.

If so, it will be easy to make the bottom edge hard all around, and decide to soften parts of it up later on, if you choose to...

Back in my day, the cool kids were using abs plastic to close out the rails, but I never saw anyone use Nomex, and I never actually tried abs myself.

BWD has been at it a long time, and I feel pretty sure that he probably helped me out when I was first learning about board building... maybe more than 20 years ago...

I find it interesting that BWD and I seem to have independently arrived at similar conclusions... as far as what we like best for the general idea of the rail shape in a twintip.

I have some pretty strict ideas about the ideal bottom shape, so it is possible that BWD and I disagree on some of the finer points, and it is true that there are finer points to consider.

[rynhardt]

I build my boards with ABS rails and a round profile, as I like to think it gives more grip/control (i.e. is less prone to skipping out in rough chop).
Since I generally ride in powered conditions I'm not too concerned with upwind ability.

So.. the rail profile depends on what conditions you're designing for.

As for Nomex, I'd be very interested in following your build process. I use vacuum bagging with core cell, which allows me to control the resin volume fraction.

I'm guessing you'd need to use some kind of pre-preg and an autoclave to avoid resin pooling with a traditional wet-layup or vacuum bagging.

[bragnouff]

Foam core will keep the laminations on both side , at a specific distance
but the core is soft so the distance changes ; with Nomex , the core
stays stiff and the laminations stay the same distance at all times.

I can drive my van, over a sandwich of 6oz carbon, Nomex, and 6oz
carbon ; the core will survive , where as a foam core will not.

Nomex is a Bitch to work with ; learn the rules on how to use it, and
it functions at a WAY better rate than any foam core, at a much lighter
weight.

I get that Nomex has some great properties, but isn't that overkill for a kiteboard? What is ideal for an aircraft structure isn't necessarily ideal for our application. Specially if it's significantly harder to work with than typical PVC foam cores. If Nomex is lighter than Airex or Divinycell, does it make a significant difference in the end since the core is typically pretty thin?
Granted, I've never tried to drive my van over my boards, but 3 of my boards are or were full PVC foam core, and are pretty much indestructible through conventional means. (with a 5 year warranty to back that up). And if damaged, are easy to repair, since the foam is closed cells and will not suck water in. Shaped surf-like rails are thick, refined, grippy and trustworthy no matter how much pressure or angle is applied.

So maybe Nomex is the best material out there in terms of properties, but wouldn't it be good enough to use cores with Good Enough™ properties for the design specs?
If your core properties limit you or prevent you from putting some proper shape into the rails, or tapering the volume towards the tips, then it's negating every possible benefit it was supposed to bring to your design. Stiffness is one thing but take into account the questions of flex patterns, which are a desirable property of your board, whereas most likely aircrafts are designed to not flex under normal load, or with just enough flex to avoid destruction.

Unless you're absolutely rooting to have a Nomex Inside™ sticker on your board, the best material doesn't necessarily make the best kiteboard.

With that being said, I'd love to see your results! Good luck!