This unit fits through the slop well behind and under your outboard through the transom and then overboard. It is meant for fiberglass boats. You will need to bore a 25mm hole from the very bottom of the slop well, facing slightly downwards (which will make the job easier anyway) and through the outside (transom) side of the hull. This drain is about 55mm long but the outer ring seal needs about 7-8mm of this, so the effective gap (between the inner wall and the outer wall) needs to be 45mm or less. The outer ring and inner sleeve all need to be sealed with Silastic or some similar sealing agent. Generally 2 of these are fitted, one for each side of the well.