The vent line on ANY tank serves two purposes: 1. to provide an escape for air in the tank displaced by incoming contents. If no air can escape, the tank--and system--will become pressurized, creating backpressure that prevents any more water, fuel or waste from being added to the tank. 2. to provide a source of air to replace contents as they're drawn out. If the vent becomes blocked, once the pump--water pump, fuel pump, dock pumpout or macerator pump--uses up what air is above the surface in the tank, the pump will pull a vacuum that prevents it from pulling out any more. If a partial blockage or anything that significantly reduces the diameter of the vent line ( a buildup of waste on the walls due to heeling or overfilling the tank, or a partially blocked thru-hull) doesn't allow enough air to pass through the vent to replace the the contents at the same rate they're being pulled out, it can slow the rate at which the pump can pull them out. Which is why it's so important to maintain your tank vents.
It's very rare for this to occur in fuel and water tank vents, but it can happen, so inspect ALL your vent thru-hulls regularly and clean them out if necessary...critters have been known to set up housekeeping in 'em. It's an all too common occurrence in waste tank vents...and while you obviously can't backflush fuel and water tank vents, it's the best and easiest way to prevent waste tank vent blockages--and prevention is ALWAYS easier and cheaper than cure!...Just stick a hose nozzle up against it every time you wash the boat. If your vent thru-hull is a design that won't let you do that, replace the thru-hull--ONLY the waste vent thru-hull!!--with one that does.
