
Thimbles are indeed made of mild steel and then galvanised. Galvanizing is a process of applying a zinc coating to steel, typically via hot-dip galvanization, to protect it from rust and corrosion. Mild steel is a common base material for thimbles due to its strength and affordability.
The K3 thimble, a type of rope thimble, is indeed used in aquaculture mooring to protect wire, fiber, or synthetic ropes from damage at the connection point of a mooring line.
Thimbles like the K3 provide a solid, protective element that prevents the rope loop from crushing and helps distribute the load.
Rope K3 Thimble HDG Diagram:

Specification of Rope K3 Thimble HDG:
| Size | SPEC(mm) | Weight(kg) | A (mm) | B(mm) | C(mm) | D(mm) | E(mm) | F(mm) | G(mm) | H(mm) | L(mm) |
| 2 1/4” | 57 | 0.08 | 57 | 42 | 47 | 26 | 22 | 16 | 8 | 3 | 3 |
| 2 1/2“ | 64 | 0.14 | 64 | 48 | 55 | 32 | 26 | 18 | 9 | 4 | 4 |
| 3” | 76 | 0.19 | 76 | 58 | 64 | 38 | 28 | 20 | 10 | 4 | 4 |
| 3 1/4“ | 82 | 0.26 | 82 | 60 | 67 | 39 | 30 | 22 | 11 | 4 | 4 |
| 3 1/2” | 89 | 0.28 | 89 | 74 | 75 | 42 | 34 | 26 | 13 | 4 | 4 |
| 4“ | 102 | 0.52 | 102 | 78 | 80 | 50 | 42 | 32 | 16 | 5 | 5 |
| 5ʺ | 127 | 0.97 | 127 | 95 | 110 | 65 | 42 | 40 | 20 | 6 | 6 |
| 6ʺ | 152 | 1.5 | 152 | 115 | 120 | 75 | 62 | 48 | 24 | 7 | 7 |
| 7ʺ | 178 | 2.1 | 178 | 140 | 140 | 85 | 70 | 50 | 28 | 7 | 7 |
| 8ʺ | 203 | 32. | 203 | 160 | 160 | 105 | 70 | 56 | 32 | 8 | 8 |
| 9ʺ | 229 | 5 | 229 | 175 | 180 | 110 | 92 | 72 | 36 | 10 | 10 |
| 10ʺ | 254 | 6.4 | 254 | 190 | 200 | 120 | 100 | 80 | 40 | 10 | 10 |
| 11ʺ | 279 | 9 | 279 | 210 | 220 | 145 | 114 | 90 | 45 | 12 | 12 |
| 12ʺ | 305 | 13.2 | 305 | 230 | 245 | 150 | 130 | 102 | 51 | 14 | 14 |
| 14ʺ | 356 | 17.2 | 356 | 290 | 300 | 170 | 143 | 115 | 58 | 14 | 14 |
| 16ʺ | 406 | - | 406 | 310 | 320 | 185 | - | 130 | - | - | - |
| 18ʺ | 350 | - | 350 | 340 | 350 | 205 | - | 150 | - | - | - |