anchor, grapnel
碇 centers on a heavy device used to hold a vessel in place: an anchor or grapnel. The idea is something that fixes a ship firmly to the seabed.
碇 combines 石 (stone) with 定, which likely contributes the sound and the idea of fixing or stabilizing. The character originally referred to a stone used as an anchor.
A stone 石 that fixes 定 a ship in place: that's an anchor. Picture a heavy stone tied to a rope, holding a boat steady.
For テイ, imagine a ship's anchor made of a giant T-shaped stone: the T shape cues テイ, and the stone anchor keeps the ship from drifting.