Every time I go by my boat which is stored outside there are more and more black streaks on it. What is causing these, and how can I prevent them from forming?

Mine sits inside or on a covered lift so I don't normally get the "streaks", but the times that it has been in the rain I've gotten them. I always thought it was dust / dirt and maybe dye coming from the canvas, the rain "washes" it out and it runs down the sides.
It washes off, or if you have a good coat of wax, it wipes right off.
I think the only way to prevent it is to get a full cover that goes to the rub rail.

I also believe it to be dust and dirt rinsing off the cover. Mine didn't do that until I waterproofed my old cover once and the water started beading up and running off like it was supposed to. Then all of a sudden, I had black streaks!
Wash and dry the cover per the manufacturers procedures and that should help. Then, make sure you boat has a good coat of wax and be prepared to hose/wash it off from time to time.

Happens all the time. Just part of owning a boat. You can wash the cover's, but it will only last a short time. Just plan on washing the streaks off each time you take out the boat. If it bothers you. No way to avoid it.

Guess that's the price of living in the midwest with coal dust?

The dust we have is blown up north from Illinois and Gary Indiana.

Ours comes from Iowa.

Yep...just dust gathering in your cover and then getting washed down the side of the boat in the rain. Only way to avoid this is not to store the boat outdoors.

Sometimes they can be worse right below the snaps of the cover. Just keep it waxed, best way to reduce them...impossible to eliminate.

Depending on the wind, we get Oklahoma passing thru, or Nebraska.

From Okla, you get orange streaks.

Wal Mart sells "Black Streak Remover" in the RV department. Pre-treat the streaks before you wash the boat and they will be gone after you wash it. Stuff is awesome.

Doesn't remove wax?

I dunno. It might. Not sure how to tell. I do get lots of compliments on my boat though...

Star Brite black streak remover says "Does NOT remove wax"....found that impressive.
Stuff works weel though

Does the water still bead up after you use it though? I'm a little skeptical...

Sorry so slow to reply. The water does indeed bead up after using it.