Through-hull fittings should be inspected periodically. Different compositions (i.e., bronze, stainless steel or fiberglass reinforced plastic types) require different attention. Bronze and stainless steel fittings are subject to corrosion due to electrolysis, whereas plastic fittings are subject to cracking due to age and hardening. All doublers in way of such through-hull fittings should also be inspected for integrity. Through-hull fittings no longer in use, should be properly capped (or removed during the following haul out, when the holes can be professionally sealed by the yard). Make sure you familiarize yourself with fittings that are not easily seen due to poor access.