Bladders are great when they can be put securely in the bilge. There, they can conform to the (otherwise underutilized) space and not detract from performance or boathandling by being in the way on deck. Bladders out in the open are difficult to secure properly, and are susceptible to puncture, shifting, and leaking from any number of causes. Jerrycans would be easier to carry aboard, secure, (to rails, where they might also act as spray guards) empty, and (possibly) dispose of. In the Cruiser's Forum you suggest placing the bladder by the mast. Forward of the mast would tend to sink the bow, which can be very slow. Aft of the mast would likely put the weight on top of the cabin trunk, (raising your center of gravity and adding to rolling) and interfere with the boom vang. Everything on a boat is a compromise, but a bladder this big, even on a 46' boat, does not seem like the best solution for providing extra fuel if you can't stow it below.