The 36 volt battery and the 36 volt alarm bell are with you in the boat. Probably, go with what voltage your trolling motor uses.
Below the horizontal water line is wood 2 inch X 2 inch handle to hold cross piece of 4 inch long copper nails staggarded in two rows of nails about 3 inches between nails on a braced wood cross piece (the many vertical switches at the bottom).
It looks like a big copper toothed rake to drag on the bottom.
At bottom, two prongs on rake make contact and circuit to ring bell in boat. Now, use magnet on device to raise your metal "catch".
