I’m looking to make an all around DW SUP and I will be using the Armstrong Downwind MKIII (7’3" 105L) as a guide. I have 10" future tracks but wondering if I should consider going with the 15" chinook tracks (heavy) to help with mast placement. How critical is it to have the longer tracks?
I would go with the longer if you have never built that specific style of board before.
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Its also not very hard to determine where the mast tracks will be. I put the tip of the wing an inch in front of the center of buoyancy on the board. Has worked perfect on the last couple boards. Make sure to measure the COB when it is just shaped foam and no inserts anywhere.
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Not worth the weight IMHO.
Your tracks have to be placed according to tve Center of Buoyancy of the board.This has never failed me.I even build my boards with single screw strap inserts,less weight.
10in tracks should fit most foils in the market today unless you ride something really weird.
FOILBox Position:
Mark the CB, measure 30.5cm towards tail and that is the middle of your foil box position.
STRAP POSITION:
FrontFoot BackScrew: CB+15.5 (FrontFoot FrontScrew 50* 18.5cm)
BackFoot FrontScrew: CB-20 (BackFoot BackScrew 17.4cm)
-Your floating and foiling foot position should be the same.They should also be as Forward as possible to reduce nose inertia and lenght of board in front of the foil (less touchdowns).The forward limit is what you can float and pump up on foil comfortably.
-This floating foot position is defined by the Center of Buoyancy of the board (pretty much the same as te Center of Gravity of the shaped blank with no boxes or inserts).
-So the Foil box position is defined by the Center of Buoyancy.
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100% Agree. Here was the 4th board I built. Riding Axis Advanced gear (Not Advanced+ at the time). I set the boxes 11" back from the Center of Buoyancy or 28 cm.
250 session later still loving the board.
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With the Axis Fireballs and the Advanced + fuselages I have been pushing my gear to the front of the boxes. Not the end of the world but I figured I might center things further, while retaining some ability to use older fuselages. My latest build I am moving things forward just another 2 centimeters.
Here are my measurements for the latest 9’ 7" x 17.5" @ 128 liters:
Using a Laser to estimate CG (Center of Gravity) vs Center of Buoyancy (CB) with the Box installed. Note: CG usually ends up 4 cm behind the CB and the CG doesn’t move much at all after full lamination and build.
CG now 8.6" or 22 cm in front of Middle of Tracks (26 cm from CB).
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I have ridden my latest SUPfoil build with Takuma Kujira V1 1095 and Duotone Glide 2.0 900.
Tbe Takuma goes 2cm behind middle of tracks,which is typical for Takuma Kujira V1.
The Duotone goes smack in the middle 
I find it funny how when most of us started foiling we were on big boards with Gen 1 wings. With the new set of wing generation the boards are similar to the gen 1 stuff but the foils are now mounted towards the center of the board instead of the back 1/3.