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 Post subject: convex or concave sidecut?
PostPosted: Wed Apr 23, 2008 12:09 am 
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Location: San Mateo, CA USA
Building a finless lightwind plywood board. Convex or concave sidecut? What are the advantages and disadvantages of each? Snowboards use concave, surfboards use convex. Both turn good so will either one work on water?


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 Post subject: Re: convex or concave sidecut?
PostPosted: Wed Apr 23, 2008 1:24 am 
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From what I can understand, a board with convex edges will work better without fins. I'm guessing it has got something to do with less displacement in the centre and more at the tips?

On a lightwind board I'd say the absence of fins is a good thing.


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 Post subject: Re: convex or concave sidecut?
PostPosted: Wed Apr 23, 2008 3:34 pm 
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lightwind boards are all about early planing and thus depend on a lot of surface area and as little drag as possible. Therefore, you need a wide board with large tips (= large area) and very little rocker (= low drag).
The outline of the board has less influence on lightwind characteristics than on upwind performance and handling.
For upwind performance you want either a lot of lift from large fins (surfboard) or as long a rail as possible in the water. That's were the concave sidecut comes into play. It allows for more rail in the water when edging and the board will generate less drift downwind.
That was the simple part.... now to more complex issues:
Flex: for lightwind board it is best not to have any. Flexible boards tend to increase rocker when subject to load. You'll have to finetune the flex to your prefference, very stiff boards are hard on your ancle and knees, very soft boards are better in chop but not so good for jumping (flex kills pop).
Concave: the main effect concaves have is to reduce rocker (concave only between pads, not all the way to the tips). Flat bottom boards glide earlier than concave boards, but tend to vibrate a lot in rippled water which causes ancle swelling if user with very tight straps.
if you wand a good lightwind performer and not sacrifice other characteristics, I would suggest to have straight (parallel) rails between centers of your pads, 2 to 2.5cm three stage rocker (flat in the center), stiff in the middle and progressively flexy towards the tips. Tips somewhat narrower than the center, hard rails close to tips, softer (about 8mm diameter) tucked rails in the center.
I would not go any wider than 44 and not longer than 140. This will give you a good lightwind board without sacrificing parformance in chop and mushy waves, decent upwind performance and some pop. Wide tips, as in concave outlines, generate a lot or unconfortable spray.


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 Post subject: Re: convex or concave sidecut?
PostPosted: Thu Apr 24, 2008 7:49 pm 
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Location: San Mateo, CA USA
Thanks for the info. I think the Spleene Door outline is the right idea. Concave sidecut, but slightly narrowed tips for less spray.


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 Post subject: Re: convex or concave sidecut?
PostPosted: Fri Apr 25, 2008 6:15 pm 
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If a dude is a 200 pounder, would you go longer than 140 in length?

I haven't glassed my ply yet, so I still have a chance to shape it. I already have my fabric (design) laminated to surface of the ply. Can you suggest a way to create a slight underside concave between the footstrap area? I was thinking about glassing a couple rolls of glass, close to each side, before my final glassing. Would this suffice? Any suggestions?

Thanks,
David :o


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 Post subject: Re: convex or concave sidecut?
PostPosted: Tue Jun 10, 2008 1:46 pm 
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Location: scott klandl burlington, VT
other things to keep in mind

performance vs. pure application

I'll agree with Aebrl entirely in theory and Practicality

but...

concave outlines provide a really crappy ride. its unforgiving in the slighest chop and all it wants to do is go upwind. they are very rough on the knees

the bigger tips are bulky, which makes them heavy and cumbersome, both in the air and when turning in the water

only skimboards equal these shapes for LW riding, so the question you gotta ask is how bad do you want to mow the lawn?

IMO the north phantom is the best way to lessen the evils of big boards


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