Wayne.B wrote:

It sounds to me like the tank is either not vented properly or the
vent has become plugged up in some way. It should definitely not flex
at any time.

You'd be surprised at how much a particularly strong pumpout can cause a
thin-walled tank to flex...and by how much a full tank can cause the
walls to bulge, which can contribute to flexing.

But your point is still well taken. A strong enough pumpout can crack
even a thick-walled tank if the vent is blocked...and since most OEM
tanks are thin-walled, that makes it doubly important to make sure the
vent never becomes blocked.

Check the thru-hull regularly...never allow the tank to overflow out the
vent...backflush it each time you wash the boat and/or pump out. And
watch for signs that the vent may be blocked--the pumpout or macerator
only pulls out a gallon or two when you know the tank is full...a
hissing sound in the toilet during pumpout (if the tank isn't getting
enough air through the vent, it'll try to get it wherever it can,
including the toilet).

NEVER just hand over the pumpout to a dockhand and walk away...either do
it yourself and watch the sight glass to make sure the tank is being
completely pumped out, or stand there and watch the sight glass while he
does it. If the pumpout stops pulling out anything before the tank is
empty, cease pumpout immediately or you can risk a cracked tank.

A blocked vent can also result in a pressurized tank when the toilet is
flushed...so watch for signs--increasing backpressure when pumping a
manual toilet...sluggish discharge and backflow in both electric and
manual toilets...a toilet that "burps". If you experience any of these
symptoms, do not use the toilet again or attempt to pumpout until you're
sure the vent is clear. A pressurized tank can burst...or if you're
lucky, only erupt back through the toilet.

--
Peggie
----------
Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://www.seaworthy.com/html/get_ri...oat_odors.html