Many thanks for your advices my friends.
OK but would'nt be P200 SkyShark better for light winds (more flexible and lighter) ?
Sure - shaving weight will always extend the lower wind range. However, the difference in advertised weight for full length (32.5") SS P200's and P300's is 2.5 grams. You will use 4 spars for the spreaders (2 for each spreader) which means the maximum weight savings is 10 grams. Cutting them down a whisker will mean that the weight savings will actually be a hair less. If saving 10 grams is important to you for the extended lower end wind range, by all means - use the P200. However, I prefer to use the P300 for the extension of the comparable upper
wind range between the two because I think that 10 grams compared to the weight of the entire finished kite will be / is such a small percentage of the overall weight that the savings would be basically negligible.
For an example of the wind range of mine (built with 0.75 oz Ripstop Nylon, by the way) - I just took it down after having it anchored out front for the last 4 hours. When I launched it, there was just above a whisper of wind (about 5 mph). I had loads of fun playing with it, watching it hang there with the bridles all but slack, and even had it thermal straight up a time or two. Then I anchored it and went about working in the yard.
Fast forward to about a half an hour ago when I took it down. The wind had picked up to 17 mph with gusts over 20. Honestly. It would climb and ride out "gusts" at the top of the wind window and seemed happy to be there. Sure, the spreaders were swaying back under heavy wind load, but it sure handled the wind just fine.
I have read that ferruled spars often broke and that is was better to frame the Rokkaku with full lenght : 200 cm for the spine and 2 x 160 cm for the cross spreaders. What do you think about this ?
And for the spine : 32.5" SkyShark in the middle + 2 x external little spars is not better ?
And what do you use please for the ferrule : internal SkyShark ferrule or external ?
P.s. : nice Rok Fore Check !
True - a ferruled connection is a stress point and often where a break will occur. So, if you can use a full length spar without a ferrule this is a good practice. But, see my remarks above. Heavy load on the SkyShark spars with no problem at all ferruled in the center of the span. A key tip is to use a bit longer of a ferrule than what you'd think the bare minimum is. For instance, a "standard" SkyShark ferrule is a 3" length of solid carbon rod as an internal ferrule (1.5" of rod inside the end of each tube for the ferrule.) You can use 4" or 5" for added stability of the ferruled connection.
Besides, on the spreaders, if you put the full length spar centered on the spine with two pieces on either end so that you don't have a ferrule in the middle, you will add more weight in ferrules than you will save going with P200's over P300's.
Regarding the spine - if you can find a full length spar that is 200+ cm long in carbon or wrapped carbon, please, let us know!
You can probably find one in wood or perhaps fiberglass, but I prefer the lighter and stiffer carbon (like pultruded Avia) or wrapped carbon (like SkyShark.)
For the ferrule, as I mention above, SkyShark is ferruled internally with lengths of 0.2400" OD solid carbon rod. 3" is considered a standard length. I buy full length (48") carbon rods and cut them for ferrules myself.