If a seacock is used, it is usually to seal a through-hull water inlet (or outlet) close to, or, below the waterline. The failure of these small metallic helpers can lead to water enterin the boat and so everyone is advised to deal with the condition of their seacocks before something breaks. We bought "Five Senses" used and it was not possible to find out how old the valves were, what material they were made of and how carefully they were exchanged, if at all. However, one thing was certain: the visual inspection alone (see picture above) made it clear that corrosion had spread here. There are no clear guidelines on when to replace such valves, but once they do look like this, the time is pretty close. So if the boat is already on land, the thinking was, why not exchange the seacocks?
If the seacocks need to be replaced, there is also the question of the new material. You have to choose between plastic and metal and then there are different possibilities for the metals. To be honest, when it comes to a seacocks, I am very conservative and I trust a plastic part much less than solid metal. Maybe I'm not right here, but the much larger volume of the plastic valves alone shows me that metal needs less material, so it is more stable. In this case, weight is not an issue for our area of application, but security is. And we need a solution that is suitable for salt water.
As is so often the case, there is a lot of good information to be found online, here is a small collection of links: