Groco Thru-Hull Installation Wrench. Slide the thru-hull fitting, flanged end first, over the tool. (It will stop at the proper step. ) Tie a length of monofilament to the eye and place it into the already-cut hole in the hull of the boat. Apply caulking to the flange of the fitting, and pull the fitting through the hole and against the hull from inside the boat. Thread the monofilament through the locknut or seacock. With the fitting pulled firmly against the hull, the locknut or the seacock may be screwed onto the fitting. - Designed for Groco Thru Hull fittings from 1/2 inch up to 3 inches. This tool may not be compatible with fittings from other sources.