"Two holes"--- for liquid to get into the sounding pipe.
"flange"-- it is "striking piece" not "flange", it is used to protect the ship's bottom being struck to hole.
"Two holes"--- for liquid to get into the sounding pipe so as one can sound the depth of liquid .
"flange"-- it is called "striking piece" or "strike piece"and not "flange". It is used to protect the ship's bottom being struck to hole.