Autopsy of a failed thru-hull fitting

Overboard is to the left, hose connects on the right

A thru-hull is a fitting that is used to connect the outside of the boat to a hose on the inside.  They are usually used for water intake and overboard discharge (In and Out).  Bronze is used below the waterline because a failure there could be catastrophic.  Plastic is usually used above the waterline on fiberglass boats.  Tortuguita has 25 fittings above the waterline and 15 below.

In order to prepare the boat for gelcoat work, all the plastic thru-hulls were removed and will replaced with new ones when the work is done.  

I really didn't suspect there were any problems but had quite a surprise and can now explain some mystery water leaks.

This is a thru-hull that is for the forward shower drain pump.  Since we don't use these showers, we didn't notice any significant leaking.  However, when a wave would splash up, it was apparently leaking in through the crack and had been seeping into the boat.

By the yellowing of plastic, these were old cracks

On the other forward shower drain, I found a broken 'tail' under the hose clamp and a crack at the outer hull.  Privilege likes to hide things like thru-hulls in hidden compartments that require quite a bit of work to access.  Since I never needed to replace one of these hidden thru-hulls, I had no reason to dig into these false compartments.

Many of the failures are from over-tightening the big nut on the back.  This cracks the plastic at the outside edge.  This particular thru-hull was also cracked at the inside edge which is usually caused by vibration or pressure fatigue.

New supply of 25 TruDesign fittings

Cheap thru-hulls like the ones that were just removed are made of nylon.  My research led me to only 2 brands of decent replacements.  Forespar (US) makes a product out of a proprietary plastic called Marelon.  TruDesign (NZ) makes their thru-hulls out of fiberglass reinforced nylon.  In this part of the world, it made more sense to go with the New Zealand brand.


Besides having a boat that is not leaking, the really cool thing about this whole project is now I know of more secret compartments to store things.