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 Post subject: Discussion about rocker, concave and fins...
PostPosted: Sun Jan 06, 2013 10:58 pm 
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Not much action on the forum these days and given the fact that I am planning to prepare a couple of templates this winter and vacuum bag outside this summer, I thought that a discussion on the impact of rocker, concave and fins would help me understand more kiteboard designs and hopefully help me improve the quality of my first boards...

I have some wakeboard boots and would like to build a board that will be adequate to ride in relatively strong conditions (overpowered). So my question is what, between the rocker, the concave and the fins, has the biggest impact on the ability of a rider to keep its edge (and ultimately jump to decent heights).

Worst case scenario I will copy my Cabrinha Custom board but I am still curious of your opinions on this subject.

Thanks for your help.

Dany


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 Post subject: Re: Discussion about rocker, concave and fins...
PostPosted: Mon Jan 07, 2013 12:07 am 
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Joined: Thu Jun 05, 2003 3:37 am
Posts: 2362
Quote:
biggest impact on the ability of a rider to keep its edge
?
The boots, actually, I think :naughty:
Copying the cab might be the best case rather than the worst case.
Skip the channels (they're a PITA, trust me).
You won't get the rocker exactly the same, or the flex, but at least rocker and outline will be in the right neighborhood.
What comes out different, you may like better (hopefully).
If you are going to use boots for sure, make it a little longer than the cab you have. For the outline template, I would make the % enlargement in the long direction a hair more than in the width direction, but be careful not to make it too parallel-railed or carving may suffer unless its a noodle.

A tasty recipe imho for a 75-85kg rider (boots +/-) would be:
138x42, concave 5mm in middle, flat tips.
deck 650gsm e-glass
extra 100-200 patches around inserts
wood core (9-10mm if cedar or poplar, 11-14mm if paulownia or balsa, tapering to 2.5-3.5mm at tips)
bottom 450gsm e-glass, with extra 100-200 between inserts.

If you use cedar, poplar, spruce, larch etc.:
Preserve your rocker: Leave the board on the rocker table a few days to let the resin completely harden.
Preserve your rocker: Leave the board on the rocker table a few days to let the resin completely harden.
Preserve your rocker: Leave the board on the rocker table a few days to let the resin completely harden.
That was not a typo.
Have a method of clamping the board onto the table other than vacuum to facilitate this (search past posts, or think sand, weights, screws etc.).
Ride the board before putting any fancy finish or graphics on.
If board is too stiff, carefully grind into the top deck layer with orbital sander until you are "1/3" into the glass. Re-fill and reassess. Repeat if neccessary.
If board too limp, grind up a viagra and mix with shellac, apply to your face --oops, I mean add another layer of glass to the deck.
When satisfied, decorate if desired.
Have fun.

Re: other issues you mention, use larger fins (35-50mm) in chop/harsh conditions to help with control, boosts and avoid side-skipping, less or no fins in flat water to butter it up etc.
Concave helps set an edge and stay in control especially at the newb level, with boots it is less neccessary.


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 Post subject: Re: Discussion about rocker, concave and fins...
PostPosted: Mon Jan 07, 2013 12:30 am 
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I will make the board with A600 Corecell 10.5mm thick and standard 3 layers of fiberglass top & bottom. I agree that the channels looks like a pain so I don't plan reproducing it!!! I will gradually tamper the nose/tail to 6mm but will not start before 2-3 inches from the boots to make sure that the board is hard enough...

Any other words of wisdom ;-)


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 Post subject: Re: Discussion about rocker, concave and fins...
PostPosted: Mon Jan 07, 2013 12:57 am 
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The foam will save you from worrying about holding the board on the table so long, and from spring-back issues, but it's still good to leave it overnight, even with fast resin imho.
I have not seen many boards 6mm thick at the tips but I don't know maybe that is better with the corecell??
The risk with foam is shear failure and by making the board thicker around the inserts, little is done to reduce this risk.
Generally, thicker = stiffer but not stronger with foam, unless you use REALLY heavy foam.
Even 3" thick surfboards snap, after all.
What does help is adding glass to the deck and patches, and having smooth contours with no step-like shapes or grooves in the transverse direction. Likewise tapering the tips reduces breakage potential by effectively increasing the skin:core ratio.
I don't have any further advice on a recipe for foam boards, because I only have experience making minor repairs to them, not building them. I always build wood core. So far.


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 Post subject: Re: Discussion about rocker, concave and fins...
PostPosted: Mon Jan 07, 2013 1:50 am 
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Joined: Sat Aug 29, 2009 11:25 pm
Posts: 2440
Location: The Naki
Danydan wrote:
. So my question is what, between the rocker, the concave and the fins, has the biggest impact on the ability of a rider to keep its edge (and ultimately jump to decent heights).
Dany


This is how i see it in a basic way as this.

more rocker = more power holding = less upwind ability
retangular shape = better upwind, better pop, better flat water= holds less power, worse in chop
rounded shape = better in chop, holds more power = worse upwind, worse pop
More flex = more power holding = less upwind.
Big Fins = more power holding, better upwind in chop= less slidy feel. worse upwind in butter flat.. drag.
small size = more power holding = less upwind.
less width = more power holding = less upwind

So you have to strike a happy medium between upwind ability and power holding. Also theres a compromise between flat water and chop performance as well.

Of course this is discussing the basics. we haven't touched on tapered tips and concave or variable flex patterns. Certainly theres a lot to think about when making a board.


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 Post subject: Re: Discussion about rocker, concave and fins...
PostPosted: Sun Feb 03, 2013 7:13 pm 
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Joined: Mon Sep 19, 2005 12:49 am
Posts: 1594
Location: scott klandl burlington, VT
forget about fins. theyre useless except when landing a jump or whenever the board is flat. You're edge is your fin.

bottom shape, again, unless you go dramatic (ie mako) its pretty unnoticeable. If your gonna put fins on your board, just work with a flat bottom and not worry about one more detail that does little.

Rocker/outline is everything copy the cab, its a good place to start

flex is everything... dont copy the cab, board is a fracking noodle.
If your using foam, start at 12-15mm thick and go down to at least 4mm in the tip


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 Post subject: Re: Discussion about rocker, concave and fins...
PostPosted: Thu Feb 14, 2013 1:06 am 
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Joined: Tue Jan 02, 2007 11:50 pm
Posts: 271
Location: Ottawa, Canada
Yes, unless you want a flexy board Scott is right you need a bit more stiffness. Going from 10.5 to 12mm (1/2") corecell is one way to do this. Or, you might swap out one of your layers of 6oz plain E glass for "Style 7715" unidirectional E-glass. It is like 2 layers of 6 oz in the lengthwise direction for the weight of one layer. About 80% of the strength in along the long axis. Best stiffness bang for your buck out there at CND $9.50 per yard 38" wide, as long as you don't need high stiffness on both axes.
I just ordered some online from here: http://www.aircraftspruce.ca/catalog/cmpages/rutan.php
or in the US try here for even less http://www.aircraftspruce.com/catalog/cmpages/rutan.php

You could not just use this stuff or you would be lacking in stiffness the other way, but almost all boards I build use a layer of this stuff, and I'm not about to stop using it.

If you want just kind of average stiffness, you can use a layer of 7715 plus one layer of regular 6oz E over 12mm core, but if you have no concave and thin it a lot you might end up flexy.

Peter


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