Most boats have at least a few thru-hull fittings. As the name implies, thru-hull fittings are penetrations going through the hull of your boat. They serve many purposes. Thru-hull fittings can provide water for your live bait well, raw water for engine cooling, drainage for the galley sink, or allow a bilge pump to discharge. Many of these fittings are installed below the waterline.
What does a thru-hull fittings look like?
Thru-hull fittings have an external flange that rests against the outside of the hull and use a gasket to help prevent leaking. The fitting has a threaded cylindrical body that goes through the hull, and an internal flanged nut that tightens the fitting to the hull. The interior end of a thru-hull fitting can have a shut-off valve or have a barbed end that accepts a hose held onto it by clamps.
Suggestions for thru-hull fitting maintenance
- Install thru-hull fittings made of corrosion resistant material like brass or nylon. Ensure its construction material is compatible with the hull material.
- Use a quality marine sealant during installation
- Replace existing hose clamps with new corrosion resistant clamps annually. Use two clamps at a time on the thru-hull fitting.
- Inspect thru-hull fittings every few months. Many are hard to see and are not maintained. Exercise attached valves to ensure they are not stuck open.
- It is a good idea to close the valves on thru-hull fittings while a boat is stored for long periods. Don't close valves on above board cockpit drains during storage.
Emergency Actions
Having a valve on the interior of a thru-hull fitting can be very useful in the event of a hose failure. As you can imagine, the failure of a thru-hull fitting installed below the waterline is a true emergency. An amazing amount of water can flow through a small hole down in the water a couple of feet.
Carry tapered wooden plugs sized for the thru-hull fittings used on your boat and put together a simple damage control kit. Some manufacturers even build thru-hulls with external caps that can plug the opening during an emergency. Review the article on Vessel Damage Control for more hints on how to prepare for potential emergencies.
Alan Sorum - Alan Sorum is the Maritime Operations Project Manager for the Prince William Sound Regional Citizens' Advisory Council (PWSRCAC). Alan is ...
