Originally Posted by Chrisjs
My friend's sailboat(46ft) has the head seawater inlet below sea level. The system relies totally on the user closing the inlet valve after use, and no leaking/failure of that valve. On more than a few occasions guests leaving the valve open have allowed the toilet to overflow. Seems to me like an accident waiting to happen but I guess sailboaters are kind of seat of the pants guys!!!
an accident waiting to happen...if his toilet is left in the "wet" mode when no one is aboard to notice that their feet are getting wet in the cabin, his boat can sink in its slip.
Your friend he (or whoever installed his toilet, which could have been the builder) should have read the installation instructions...and it's not only sailors who don't think they need to. The installation instructions for every seawater toilet--manual and electric--make it very clear, and even include drawings, that ALL seawater toilets with even part of the bowl below waterline MUST have a vented loop in the intake that's at least 6-8" above waterline at ANY angle of heel...which on most sailboats puts it 3-4 FEET above the bowl. Power boats don't heel much, so 8-10" inches above waterline when the boat is at rest with full tanks (200-300 gallons of fuel and water can raise the waterline several inches) is high enough.
On a manual toilet and any electric toilet that has a short hose connecting the pump to the bowl, the loop has to be installed between the pump and the bowl (putting it in the intake line between the thru-hull and pump will prevent the toilet from priming)...which requires replacing that short piece of hose with two pieces long enough to put the loop where it needs to be.
On electric toilets that don't have any connecting line between the pump and the bowl, the loop has to be in the intake line...which requires an electric solenoid valve in the loop that's wired to the flush button. The valve closes when the flush button is pushed to allow the toilet to prime, opens when the flush button is released to break any siphon.
The first photo is a vented loop with the solenoid valve. The second one shows where vented loops are installed on most sailboats. The small one is the intake loop, the large one is in the discharge line, needed only if the toilet is plumbed to flush directly overboard.
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein