[Puddin'_Tain]

From a safety standpoint having the backwater discharge above the waterline makes sense. I've seen commercial boats with such a set up. However, those were large enough that folks on deck weren't likely to get a whiff of any residual sewage once the pump-out was over and done. On a small sailboat that might not be the case.

I don't think you have to worry about having the backwater discharge forward of any intakes as long as they are on opposite sides of the boat. The discharge on our boat is just below the waterline on the starboard side, while the cooling water intake for the motor is about 8 or 9 feet aft of the discharge, but on the port side. Not much chance of the backwater slipping under the V of the hull, or the keel, and getting sucked up on the opposite side. Intake for the head is just forward of the backwater discharge, and inboard by a couple of feet, so as long as the boat is moving there isn't likely to be a problem there either.

 

[Puddin'_Tain]

... a hose screwing into the deck fitting then twist locking to the pump and a fixed 5 foot discharge hose with a weighted end to keep it pointed down and use it to pump the tanks when needed not every time you use the head

My boat partner and I once considered using a similar solution. However, we figured that pressurizing the holding tank (albeit briefly) wasn't a very good idea. Just too many potential disasters. We also were a bit concerned about where/how to store the dirty discharge hose. Storage on our Ericson 27 is at a premium as it is, without trying to find somewhere to keep a poopy length of hose.

 

[Puddin'_Tain]

Actually suction not pressure and with the caps on there is no mess how do you think they pump you at the docks

Well, we figured that we would avoid having the suction from a pump-out station pulling the waste through a pump (and to avoid having another pump all together). To do so, we thought that we might have a valve on the vent, and then pressurize the tank a bit via the toilet's waste pump. With the vent closed, the only place for the pressure to go would be via the deck plate and attached hose. Once the flow got going the pressure needed would be pretty minimal due to the siphon action as the blackwater flowed back down and out the overboard waste hose. --- Yes, it was a pretty hair-brained idea. Luckily we realized that, and stuck with the conventional thru-hull discharge and y-valve. And, we don't have 5 or 6 feet of poopy hose to store.

 

[Puddin'_Tain]

The deck pumpout should never be plumbed through a pump. It should ideally be through a dip tube through the tank top. Second choice is a dedicated outlet at or near the tank bottom.

Right. But if we were going to attach a hose to the deck fitting to discharge the waste overboard we would have to get raise the blackwater 4 feet or so, either with a pump or by pressurizing the tank an bit. If we kept the pump, I suppose that we could have valved it off in its own loop. But that would have been a bit complicated.