Join Date: Feb 2000
Location: Annapolis, Md
Thanked 216 Times in 167 Posts
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I have had bladder type holding tanks on two separate boats now, with the bladders made by two different manufacturers and removed both after a couple years. The problem was not with the tank itself leaking fluids (although there was a problem with the locking nut loosening on one over and over again and leaking a little) but with the fact that in both cases the odors get embedded in the fabric and permiated through to the surface and pretty soon the whole boat started to smell like sewage.
In both cases, I went to rigid plastic tanks and the smell went away. Ronco Plastics makes tanks in almost any shape that you can conceive of, at much cheaper prices than the higher quality bladders and will custom place the intake, vent, and pump outs to suit your installation.
As to the venting issue, the vent plays three roles, It 1. allows the tank to exhale gases, 2. Provides air so the microbes don't go anerobic, 3 acts as an overflow relief and over-full warning system if the tank accidently is filled beyond capacity. (BTW I have heard of bladders being over pressurized and busting. Just because millitary spec blatters survive being tossed from planes, it does not mean that boat holding tanks are built to Millitary spec.)
Even with a Bladder you want a vented tank since the bladder will fill with gasses over time and reduce the capacity of the bladder as the gasses displace the fluids.
On any kind of tank, the vent and fill tubes should be located on the top of the tank and the pump out connection should reach the bottom of the tank. With a bladder this means that the pumpout connection is on the bottom of the tank. With a rigid tank you can get fittings that enter the top of the tank but have a tube that reach the bottom of the tank. In an ideal world all of the connections should enter the tank from the top so that the tank does not need to be pumped dry if the discharge tube gets blocked.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay