It is a 15 gallon bladder. The brand is Nauta, made in France. I keep it in the V berth in the V-shaped space under the port bunk. It is not screwed down -- the V shape holds it naturally, and I have a latched door above it to prevent it from rolling out in case of a knock down. When full, it fills only 1/3 of the volume in that space.
The bladder was on the boat when I bought it. I see no evidence of any other kind of holding tank predecessor, so I must conclude that Tarwathie either had no holding tank or she had a bladder when first built in 1975. She is a factory built boat -- maybe Bud can recall what holding tank options were offered.
The bladder is probably the same brand that Bob reported failed on him. Why did his fail and mine didn't? My best speculation is that when at sea, I always pump the bladder dry, and leave the Y valve in the sea position. Therefore, heavy weather and a full bladder never coincide. Indeed, if I was making an ocean crossing, I would leave the bladder empty and fill up that V-berth space with other stuff.
The advantages of a bladder became apparent this winter here in Marathon. Someone announced a new enzyme product on the cruisers net. It was said to dissolve holding sludge that causes odors. The sludge won't pump out so it gradually reduces the effective capacity of the tank. Someone else suggested that cruisers get together to make a bulk purchase of a case of 12 bottles. It mushroomed, and within a week they had to order 15 cases for the boats in the harbor. Evidently, odors and reduced capacity are routine p
roblems for conventional rigid tanks. Not so for my bladder.
Disadvantages include vulnerability to any product containing chlorine, and overfilling. If overfilled (and unvented), it pressurizes like a balloon. Then, when the deck fitting is unscrewed for pump out, it
results in a geyser of black water -- yuck. Never happened to me yet.
Here's a
picture of my installation at a time when the bladder is only 20% full. You can see head, pump-out and vent connections, but the vent hose is plugged (no vent is needed). My view was standing in the V-berth, holding the cushion and cover open and pointing the camera down and port.