Originally Posted by pressuredrop
i understand the need for a raised loop (as with bilge
pumps that have thruhulls near the waterline) well above the waterline but why must it be vented? im finishing up my engineering degree but im scratching my head on this one...
it seems that a simple rasied loop would be sufficient to keep the boat from flooding itself? would it not?, please enlighten me?
I mostly go along with what you say. It seems to me most unlikely that the low head loss side of the siphon (which is the sea side due to valves, etc on the boat side of the loop) would not just drain down without the vent. That especially so for the larger diameter, typically 1-1/2 inch discharge loop such as on marine
toilets - in fact Jabsco
in their current
instructions only mention the use of a vent in the discharge loop as being optional.
However, I would consider it good practice to fit the vents even though they can be a pain if they leak (especially the pooey side one for the toilet
). What I try to do is to remove the vent cap and innards and use small stainless steel
pipe fittings with hose tails, etc to pipe the cap and innards to a wet area where leakage is not a problem - for example, in my own boat's case that was done for the toilet ones so that any leakage would drain to the shower
tray. This is easy to do with the likes of the simple Forespar duckbill vents in their loops.
In some instances I don't consider even a loop is required if backflow can be reliably prevented in another way. A common area is for showers where the tray is usually below the waterline and any sump and pump in the bilge. If the sump pump is one whose valving prevents back flow (such as the multiport diaphragm
ones) and backed up with an industrial quality non return valve I have no problem with that going straight to the sea without a loop, although, as always, one should close the sea cock if the boat is unattended for more than the same day. If the sump pump is left on that provides even more security
Dribbles from the vents can be problem on a loop in the raw water
circuits of the engine
if they are exposed to exhaust
pulsations as may be so if with a wet exhaust
system. Routing the vent away to a safe area in the above way is useful then too.