On Aug 31, 9:11*pm, wf3h wrote:
a few years ago there was the 'princess of poop' (i think her name was
peggy) who knew ALOT about marine heads...

anyhow, i have an electric head on the boat which has a 3 gallon
tank. *it's filled with water and deodorizer. *when the head is
flushed this gets recirc'd through the tank. no connection to raw
water is made. however, there's a direct connection from the rear of
the toilet to a pump out port on the boat but i'm not sure if the tank
on the head can take the pressure of the pump out, so i want to
install a *holding tank.

i'm looking at a 6 gallon polypro waste tank with 2 connections and a
vent connection. it looks like these waste tanks are USCG certified
(since there will be no sea connection).

so do i NEED a tank? is the 3 gallon holding thank on the head going
to take the pressure of a pumpout without blowing up?

if i install a polypro tank with no sea discharge is that legal for
USCG cert? (it seems it is).

any hints to ensure no unpleasantness?

I'm having a hard time picturing your system. Your description makes
it sound like you have a closed loop, with the 3 gallon tank acting as
both the flush water and the waste repository. But that can't be
right, so I'll assume you have a more traditional arrangement:

Raw water intake -- toilet - 3G vented holding tank - pumpout port.

The volume of the tank is unrelated to the amount of pressure (or
amount of vacuum, to be more correct) the tank can take, although, a
smaller tank can have thinner sides than a larger tank rated for the
same vacuum.

To answer your question, in order to pump-out, you must make sure your
tank is vented. As the effluent is evacuated, an equal volume of air
must be allowed to enter the tank. If the tank is adequately vented,
the amount of vacuum generated by the pump-out station will be
irrelevant, as no vacuum will build up in the tank. If it is not
adequately vented, or if the vent is clogged, damage to either the
tank (even a stout one), the toilet, or the pump will occur. The real
limitation is the amount of rise from the tank to the pump-out port,
and the rating of the hose connecting the tank to the port (i.e. you
don't want the hose collapsing because of the weight of the effluent
being sucked up). And of course, your vent must be big enough to let
in air at the necessary rate.

Laws notwithstanding, having a means to evacuate the tank in the
absence of a pump-out facility can really save the day -- just
something to think about.

Hints? Yes: make sure the tank is empty before disconnecting
hoses. ;-) More seriously, buy a heat gun, and use it to heat the
hose ends -- makes the job of getting hoses on and off much easier.

Good luck!

Adrian.