worn through hull on boot stripe

Inspecting nylon through hulls

One of the common problems that we find in a marine survey, on both sailboats and powerboats, is worn nylon through hull fittings (often also abbreviated as “thru-hulls”). First, plastic fittings should only be found above the waterline, never below the water where the only through hulls we want to see should be constructed of a good marine grade bronze. Despite their ubiquitous presence these nylon (plastic) through hulls can suffer greatly from ultraviolet degradation after only a few years of exposure to the sun. As a Fort Pierce, Florida marine surveyor, seeing the ravages of the sun on boat materials is a regular event. For this reason it’s critical to inspect nylon through hull fittings annually and replaced at the first sign of wear. Sun damage can make these fittings brittle to the point where they will crack easily, and the stress placed on the thru-hull by an unsupported hose can also cause failure, with the weight of the hose acting like a lever as the boat bounces around while underway. Nylon through-hulls typically fail where the body of the fitting joins the outer flange, which can result in a crack or even total failure. Once the flange shears off there is nothing left to keep the through-hull in place, meaning it will eventually be pulled inboard, leaving a gaping hole in its place. As you can see in the photo below, that can leave a significant sized hole in the vessel and often very close to the waterline.

missing through hulls
Missing through hull fittings near the waterline

All of a vessel’s through hulls should be inspected annually, and if you see signs of UV damage or cracking in one it’s a good idea to replace all of the other through hulls of the same age. Notice the location (inches above the waterline) of the sun-degraded through hull in the photo below. Also notice how you can see that some material has already been lost from the flange portion.

worn through hull
Sun damaged nylon through hull (thru-hull)

Another problem that we sometimes see on boat surveys is when through hulls are placed in cored hulls where the hole is drilled and the core is not sealed. Over time as the bedding sealant breaks down a gap will form behind the flange portion and allow water to penetrate into the coring. Even in a properly installed through hull one of the inspection points we want to carefully look for is any gap between the flange and the hull. The sun can breakdown the bedding compound as well as the nylon fitting itself leaving the noticeable 1/8″ gap which you can see below.

gap behind through hull fitting
Gap behind through hull flange

Remember the hoses

The hoses attached to the through hulls need attention also. Make sure the hose clamps are secure and the weight of the hoses is adequately supported and not being held by the through hull itself. Just as the sun can degrade the nylon fittings, the rubber hoses will break down with time and also become brittle and/or cracked. So remember when inspecting nylon through hulls to also inspect all of your hoses annually and replace at the first sign of degradation, before they become a problem.

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