I've had several issues over the years with through-hulls above the waterline. Once, years ago, I found that my bilge pump
was running frequently in a rolly anchorage. I did not have a vented loop on my above the waterline through-hull and my vessel was sipping water
in with each roll. The vented loop became an easy fix. I also find that I'm disturbed by the drip stain and increased algae that can form around these effluents.
My solution was to place a piece of tygon tubing tightly shoved into the outflow tube. I was aware that this decreases the aperature diameter, but I determined that this had no negative effect with the cooling water
flow that I required.
This has worked well for me keeping my waterline clean, but recently, here in the St. Johns River of North Florida
, I've found them missing on both my port and starboard sides. It turned out to be a case of theft! These guys were seen pulling them off!