I THINK I know happened...
))
That is, if it only happened the first time you tried to use it (or maybe you didn't have the nerve to try it more than once): The tank became pressurized...when you flushed, you opened the "trapdoor" in the bottom of the bowl, releasing the pressure inside the tank...instead of the bowl contents going down, the outward blast of air from the tank sent it everywhere. If the portapotty isn't a permanently installed pumpable model, it was prob'ly just due to gasses built up in it while it sat for a while. It can happen again, especially in hot weather, so I suggest you leave the tank out after you've dumped and washed it out. If it IS a permantly installed pumpable model, it has a vent that goes to a thru-hull...and it's pretty good bet that the vent is blocked, prob'ly at the thru-hull. Dirt daubers love to build nests in thru-hulls...they're especially attracted to holding tank vents...or, if the tank has ever been allowed to overflow out the vent, there could be "stuff" in the vent line. You need to keep an eye on all the vents and vent thru-hulls on your boat. A blocked vent can do more than make a mess! At least you can be thankful you weren't bending it over it when it blew...<lol>