Replace with a rigid tank
Although bladder tanks are sometimes used for sewage holding--more on sailboats than powerboats--they are not a good choice for a waste tank...for several reasons: Bladders are invariably installed in an area of the boat that’s inaccessible to install a rigid tank—stuffed down any opening into a place big enough to contain it. And only rarely are the bladders properly secured to prevent any movement. Since sailboats are typically so much more “active” than houseboats or cruisers, heeling side to side, bladders move and chafe till they leak. Fittings must be owner-installed in some bladders, and because the tank is in an inaccessible place, it is almost impossible to install the fittings correctly. Rarely, if ever, is any holding tank checked or maintained, so it isn’t at all uncommon for a bladder to blow out its fittings at the first sign of a clog—even more common in bladders that aren’t vented (which should never be used for waste holding in any event, because waste produces expanding gasses that can blow out fittings or even burst the tank, and because methane is flammable). And because bladders "hug" the contents, preventing odor out the vent from a flexible tank is all but impossible. I suspect that your odor is due to leaking fittings connection in your bladder tank and also hoses that have become permeated with odor. So yes...I think the boat needs a new rigid tank AND new hoses. Your best source for a new tank is Ronco Plastics (http://www.ronco-plastics.com) They have more than 400 shapes and sizes, including over 100 nonrectangular, for a very reasonable price. And depending on the age/make/model of the toilet, it may be time to replace it too. Odor issues are not deal breakers, but they are excellent bargaining points. So if you're any kind of a negotiator, you should be able to get the seller to adjust the price enough cover at least half the cost, if not all of it. The link below should be useful to you if you buy the boat.
