Vented Loops on Below Waterline Heads:
If the Head is installed below the waterline, or may be below the waterline at any angle of heel (virtually always on a sailboat) - properly positioned Vented Loops must be installed.
Vented Loops should be installed on BOTH the Raw-Water Intake Line, between the Intake Seacock and the Head - AND - also on the Head Discharge Line between the Head and the Holding Tank.
ADDITIONALY, a Vented Loop should be installed in the 3/4" Hose between the Toilet Pump and the back of the Bowl. The factory-installed 3/4" Hose must be removed, and a new (longer) Hose c/w Vent installed.
The top of all the Vented Loops must be (about 8") above the waterline at the greatest angle of heel.
Of course, sub-standard installations abound - and most of them work just fine - and havn''t sunk their owner''s boats. So maybe these Loopless schemes are "good enough".
Or you could do it right.