Join Date: Feb 2006
Location: Florida, US northeast
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Here's commentary from a plastic engineer. At a molecular level plastics are long chains of molecules like ethylene and propylene, thus poly-ethylene, etc. Sort of like a molecular dish of spaghetti. UV radiation breaks these chains and, eventually, returns them to their original state. In polyethylene, for example, this is a gas. In reality it never gets this far but as the chains get shorter the stuff gets weaker. Think the thin plastic webbing on cheap lawn chairs. A couple of years in the sun and they are gone.
There are two ways to minimize this damage, UV absorbers and bulk. UV absorbers "eat" the UV before it gets to the plastic molecule but, eventually, these get used up. Bulk keeps the damage on the surface, with the damaged layer protecting those underneath. That faded stuff is degraded plastic. In theory, a thick poly hull could last for a long time, although it would look bad.
Boat manufacturers aren't going to put any more material in a hull than they need for strength, so there goes bulk. UV absorbers are expensive and there won't be any more than is absolutely necessary. Anyway, they get consumed when doing their job so it's only a matter of time before they stop working.
I'd consider a plastic boat as a disposable item. When it gets old it's used up. I'd call a very faded boat old. Like a rotted wood boat, repair is expensive and impractical. Even worse, with inexpensive plastics, like PE and PP, repair with paint or adhesives isn't even possible. Nothing sticks. It's not for nothing that PP and PE are used for lab ware. Nothing touches them.
How long that takes depends. Only taken out of the garage when used a couple of times a year? Ten years is probably OK. Left out in the sun on a Florida beach? Three years is probably too many. Scratch the surface with your fingernail. If anything comes off, it's way too late. If it's badly faded, it's well on it's way out.