Whitecap Duplex Stainless Steel Thru-Hull w Barb The Whitecap Duplex Stainless Steel Thru-Hull is available in a variety of sizes to fit a wide range of boats and applications. These Thru-Hulls are constructed of durable stainless still that is more dense and has greater rust resistance than traditional 316 stainless steel. Specifications Part No. Hose Size Flange O.D. Thread Length Barb Length Overall Length Cut-Out Max Hull Thickness 6540X 58 1-78 1 1 2-14 1-18 58 6541X 34 1-78 1 1 2-12 1-18 5