PLastic plumbing fittings are an economical alternative to more costly bronze or Marelon fittings.Plastic plumbing fittings like these Mushroom Head Barbed Thru-Hull Fittings cost less than similar fittings made of Marelon or Bronze, but are for above-the-waterline use only. Possible uses include overboard discharge for sink and scupper drains.Mushroom-style thru-hull with barb for hoseAbove-the-waterline use only