After reading your article on proper holding tank venting, it was pretty obvious Hunter had broken every rule in the book for the '99 H310. The tank vent is 3/4" dia., about six feet long and four feet of it is vertical - chances of any ventilation are "0%" or less. I have seen strong men faint when we flushed the head. After much wrestling with the tank, I removed it and had an additional fitting welded into the opposite wall of the tank, opposite the original vent fitting. The tank was then re-installed and another 3/4" vent line (I know it's too small) was run to the rear of the hull, a distance of about 15' (I know it's too long). The reason for the long run is for structural reasons. The line is well sloped to avoid a water seal and gently rises to the height of the deck to avoid taking on excess sea water in heavy seas. I then installed a chromed brass fuel tank vent cover which is shaped like a small air scoop. The scoop was pointed straight forward. The original holding tank vent cover, which is of the same type, was removed and aligned so that it pointed straight back. You can now see that regardless of which direction the air travels along the hull, forward or backward, one scoop will be catching the air to give a positive pressure and the other will be cutting the air to produce a negative pressure ensuring a positive circulation. For the first few days, our eyes watered and stomachs turned and there was no doubt air was moving through the holding tank. After a few days as the bacteria realigned themselves to an aerobic majority, the smell became less offensive. Now after a couple of weeks, odourless. Although this design is somewhat in violation of the rules of good venting practice, the air circulation is positive with the slightest breeze and we are now odour free. s/v island hunter