Holding tank - pump out

[Sootydog]

You In the heads there is a sticker that says something like "Open seacocks before puming out" . Which seacocks would this be referring to? I assume it's to relieve the vacuum build up in the tank but I don't know how the whole pump out thing works.  Any advice appreciated.

[tiger79]

There was a sticker like that in my Cruiser 37 when it was delivered.  However, the holding tank is a gravity-emptying design, so opening the seacocks would mean the tank is empty before any pump-out started!  I took the sticker off.

[Symphony]

The sticker is ambiguous and really refers to a pumped system that used to be fitted to some models.

To empty the tank if it is the more common gravity type you open the outlet seacock, and to use it as a holding tank you close the seacock. If you want to use a shoreside pump out facility they will use the access point on the deck. There is no need to open anything else as the tank is automatically vented to atmosphere.

[umu]

I was wondering about this sticker as well and thought they might mean that you make sure the inlet seacock is open when pumping out.
Because in case the waste tank vent would be blocked and the inlet closed as well it might implode the waste tank

[Symphony]

There is no connection between the inlet and the tank if the joker valve in the loo pump is sound.

[Sootydog]

So when you use shore pump out, does the suction pipe reach the bottom of the tank ?  I've read that the suction hose screws into the deck fitting which implies it needs an airtight seal and in this case how does it compensate for different tank depths?  I can understand it if it's just a small (ish) diameter pipe poked into the deck fitting but I am also concerned that excess negative pressure might make the tank implode.   I have cleared the vent pipes but when I bought the boat the vent was thoroughly blocked with ancient paper fibres and took quite a bit of effort to shift, needless to say with a blocked vent the tank didn't want to empty due to gravity.  I wonder what would happen if you tried to pump out with a blocked vent?

So, out of interest, and just in case it all goes wrong sometime in the future does anyone know how it's all supposed to happen.

Thanks.

[Symphony]

There is a tube from the pump out access point that goes near to the bottom of the tank that picks up the sewage. The system is fool proof if the vent is clear.

If by any chance the vent is blocked then you can always undo the inspection hatch which will relieve any vacuum in the tank.

Understanding how the tank works and maintaining it properly should avoid any problems. Key things are not to fill it with large amounts of solids and leave it full. Empty it as soon as you go to sea and leave it empty. Use plenty of water to flush it through. Do not put paper through it unless it is the type that dissolves quickly.

[tiger79]

There is no connection between the inlet and the tank if the joker valve in the loo pump is sound.

In the event of vacuum in the tank, the joker valve will allow free movement of air or water into the tank.  If it's a locked Twist'n'Lock toilet, the base valve would frustrate such movement.