Originally Posted by FPNC
... create a siphon through the raw water pump
(which it can) and fill your exhaust and subsequently your engine (s)...
For most of us with rubber impeller raw water pumps a siphon can't
be created through the pump unless the impeller is damaged
. The vented loop is backup security
in the event you don't inspect your impeller often enough.
The impeller pumps are positive displacement
, in normal operation there should be at least two full vanes (6-vane impeller, more for greater number of vanes on the impeller) between the saltwater inlet and the pump
outlet. The front and back of the impeller seal to the pump body and the cover plate. If these seals
are not good pump flow decreases. If everything is in working order the sealed impeller faces and the vanes prevent any siphon from forming. Of course, if your impeller or pump faces are damaged then you can siphon past the damaged areas.
If you do install a vented loop then it is pretty easy to remove any valve at the vent and route
a very small diameter tube from the vent to a discharge outside the boat
. This is very much like the telltale/peehole on an outboard
. The amount of water that comes out the telltale reduces total flow through the cooling
system, so you use a pretty small tube to restrict flow to a small amount. Then you can use the telltale to let you know if anything is clogged, or if there's a problem with raw water flow.