I am looking at this vintage Wenonah Saranac Royalex boat. Any idea what might be causing the discoloration?
The owner said it was coated with epoxy but I’m not sure that is the cause. Unless it is. But I’ve not seen this before. I know the substrate under the vinyl is not this color, so it doesn’t appear that the vinyl is totally compromised.
Mold? If that’s not a craiglist photo, and you have access to the boat, I’d try water, a little bleach and a rag and see what happens.
Looks like sooty mold. 
It’s a fungi which grows on honeydew (aphid secretions). Was a problem for a couple of summers at my place, blackening furniture, siding, boats and everything else that was uncovered, including rocks in my garden, walkways, docks. Everything.
I used hot water and Dawn liquid detergent on the one boat affected (the other was covered) and Zinsser Jomax house cleaner mildew killer on everything else.
I originally suggested bleach, but, yeah, Dawn is an amazing cleaner, and it’s certainly safer.
I’m at a loss why the Royalex was coated with epoxy. That’s not OE
I’ve seen RX boats with that discoloration usually they were wrapped in a tarp
Could be mildew or tannin if left under trees. Dawn will tell you quickly.
Turns out the boat was beginning to show wear, so the prior owner had literally poured epoxy down the keel from the bow to about 1/3 of the way down the boat. It appears that before the epoxy could cure, it fell into the dirt! So there was literally dirt cured into the epoxy. Thus, the discoloring. I have sanded it, properly fixed all compromises in the vinyl, and added skid plates, and painted it. This is what it looks like now:
@g.a.anaya64@gmail.com said:
Turns out the boat was beginning to show wear, so the prior owner had literally poured epoxy down the keel from the bow to about 1/3 of the way down the boat. It appears that before the epoxy could cure, it fell into the dirt! So there was literally dirt cured into the epoxy. Thus, the discoloring. I have sanded it, properly fixed all compromises in the vinyl, and added skid plates, and painted it. This is what it looks like now:
Looks good!