The best way to do it depends on a couple of things...whether the
boat is a sailboat or a powerboat and whether it's an uphill run from the toilet to the tank.
The vented loop needs to be at least 6-8" above waterline AT ANY ANGLE OF HEEL...which on most sailboats puts it 2-3' above the bowl...In which case, the best place for it is immediately after the toilet. Whether it's sailboat or not, if it's an uphill run to the tank, putting a loop immediately after the toilet that's at least slightly higher than the top of the tank means you'd only have to flush long enough to push the bowl contents over the top of the loop...gravity will get it the rest of the way to the tank. You can put the y-valve anywhere in the line coming down from the loop that's accessible and keeps it well above the waterline.
If it's a sea
water toilet, you'll also need a vented loop in the flush water inlet ...NOT in the line between the thru-hull and the
pump, but in the line connecting the
pump to the bowl...which means replacing the short piece of hose the mfr uses to connect 'em with lines long enough to put it the required 6-8" above the waterline at any angle of heel, which means that on a sailboat it also will be 2-3' above the bowl. The attached
photo shows the typical
installation for both loops on most sailboats.
Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein