Most boats have thru hull drains that come from sinks, bilge pumps and other things that are used to get water dumped overboard. Often times they get looking like this, and it's hard to get this crap off the side of the hull. I did this four years ago when I bought Beachcomber but some of them came out and I ended up with the stains you see. The larger hole still has the plastic piece I cut and there's no stain..
Somewhere I read about a very easy mod you can make to your thru hull drains that helps to prevent this from happening.
If you go to your local hardware store and buy a foot or so of the clear plastic tubing that is the same diameter as your thru hull that's the key.
Cut the piece off square on one end and cut the other end to form a little spout or lip.
Stuff the piece of plastic tubing into the hole, leaving the spout sticking out a bit, but not more than 3/8" or so. To make sure the plastic piece stays in place I added a drop of Gorilla Glue to the side.
Here's what the finished product looks like...
Now that I have all my drains done I shouldn't have any more of those ugly stains.