Not very efficient, but you may find it useful for some experiments. I perused the code from the link you provided (kuba's), although there are better alternatives in the answers.

    ClearAll[spiral, genTri, mp];
    spiral[n_?OddQ] := Nest[
       With[{d = Length@#, l = #[[-1, -1]]},
         Composition[
           Insert[#, l + 3 d + 2 + Range[d + 2], -1] &,
           Insert[#\[Transpose], l + 2 d + 1 + Range[d + 1], 1]\[Transpose] &,
           Insert[#, l + d + Range[d + 1, 1, -1], 1] &,
           Insert[#\[Transpose], l + Range[d, 1, -1], -1]\[Transpose] &
           ][#]] &,
       {{1}},
       (n - 1)/2];
    genTri[n_] := genTri[n] = IntegerQ[(-1 + Sqrt[1 + 8 #])/2] & /@ Range@n
    mp[n_] := mp[n] = Image[Unitize[spiral[n] /.    
                                 Thread[Flatten@Position[genTri[n^2], True] -> 0]]]

    Erosion[mp[601], 1]

![Mathematica graphics](https://i.sstatic.net/xPwHy.png)

There they are, your 17 arms