The vents for the holding tanks on my SM (Hull # 335) are part of the
deck fitting for the holding tanks. If you look at the aft portion of
the deck fitting you will see that is composed of a black plastic
spacer and the metal threaded cap. Look at the aft side of the plastic
spacer and you will see two holes drilled horizontally that face aft.
This is the vent. The vent is at the top of the tank (literally). If
you remove the cap labeled "WC" you will see that there is a standpipe
welded to the inside of the metal cap assembly. This standpipe ends
near the bottom of the holding tank. Thus when doing a pump out and
suction is applied at the holding tank deck fitting it draws the black
water from the bottom of the tank while air enters at the top of the
tank through the vent openings. The vent opening also acts as an over-
flow for the holding tanks. If you overfill your holding tanks the
black water will come out of the vents and run down the deck. Don't
ask me how I know this.
Regards, Gary Silver Amel SM 2000 Hull #335
--- In firstname.lastname@example.org, "eric freedman"