Its good that the Gun Bag seems waterproof. I went on a 5 week trip last year with an Aerotech sail bag on the roof, and it was'nt waterproof, luckerly it did'nt rain much otherwise it would of meant drying the sails out or face the mildew monster. I wanted to buy the Gun bag at the time but they were all sold out

Another thing to consider is fuel consumption, with a board or 2 and sail bag full of sails on the roof, full consumptions takes a huge hit.