Cut lengths, build marks & predicted strength for 12-strand hollow HMPE (Amsteel Blue). One engine — fid 21·d, full bury 72·d.
Standing length is the finished line below the eye (set to 0 for just the spliced end).
Assumes two equal pieces joined in the middle. You lose one bury length (72·d) of overlap.
The finished size depends on how hard you dress the knot — the model's baseline assumes a firmly-set knot. Tell it your style, then dial it in exactly from one test build.
Rough starting points. Tightening harder pulls more rope into the knot, so the finished shackle comes out shorter unless you cut longer.
Measured at the diameter currently selected above. Calibration is stored on this device and applies to every size.
Video break-tests & technique comparisons: HowNOT2 · L-36 testing
Length: one geometric engine. Every 12-strand splice scales off the fid length = 21·d (Samson standard) and a full bury = 72·d (≈ 3.5 fid). Brion Toss, Evans Starzinger and Samson independently converge on 72·d for HMPE.
Strength: capstan / constriction mechanics. A buried tail is a Chinese-finger-trap — under load the hollow braid constricts on the tail, so grip is self-energizing and a 72·d bury hits 90–95% efficiency with no knot. A taper adds ~20% by removing the abrupt stiffness step.
Strength is a range, not a number. Published break tests scatter a lot (knot efficiency is genuinely a random variable), so this tool shows a band and sizes the working load off the conservative end:
Lengths round to 1/8″ (or mm). Final dimensions depend on how tightly you dress the knot — use Calibrate to your technique to lock it in from one measured build.
Sources: Splicing Dyneema (Sailing Mag) · Samson 12-Strand Class II Eye Splice · L-36 break-test data · HowNOT2 break tests · Cordage Institute CI-1500