No outside vents keeps salt water and bugs out of the tanks. It is the "correct" way of plumbing them, a lot of boats are done this way. They usually end the vent hose as high as possible, but depending on the tank location that may not be high enough. Just be more careful filling the tanks.
Thanks for your reply, Cap, but seriously, in my 42 yrs of owning and having the opportunity to cruise on many boats, the only "loose" interior vent hose I ever saw was the one I installed on my Herreshoff Meadowlark 37 leeboard ketch that ran to near the trunk cabin ceiling, and would still make a mess of things if you were distracted filling at the deck-level fill.
I would call this method more squeezing nickles by the builder than "correct", at least in cruising sailboats. A screened vent opening facing about 45 deg. aft hasn't caused any detectable fouling on my previous boats, and a wet locker's spider/bug density and the open line end access definitely exceeds the exterior fitting's possibilities.
Swan did it very elegantly by leading the vents to one or more sink spouts, and the cross section is small enough to cause a distinct fill-plate overflow, whereas the Cat. 30's 3/4" fill hose and a 3/4" vent gives absolutely no indication on my boat at the filler outside that the tank is full, and worse, my x-ray vision seems to have failed me...
I'm considering putting a common vent in the aft end of the anchor locker and tee'ing the stbd locker's tank vent line into the fwd tank's line - easy to see the overflow there - and both would fill through the big tank's fill. But I haven't checked out getting the hose through the various bulkheads to the V-berth locker yet.
Pete
