T&T: Vented loop

[Al Thomason]

I think you are right, but remember, a vented loop is just to prevent 
failure modes which result in siphoning actions through the whole system, (I 
guess siphoning is not really the correct word, as with everything below the 
water line, it just comes in).  e.g.: :  Head/pump below the waterline + 
failed valves / seals inside the pump which allows water to flow  through 
unchecked.  Putting the Vented Loop between the pump and the bowl will 
prevent this action from happening.  However, it is not intended to protect 
from any other failures, e.g. cracked pump housing, leaking pump housing 
seals, failed hoses.

When leaving the boat, the seacock will provide protection from all of these 
failures (and more).  Plus, being in the habit of turning off seacocks when 
leaving the boat also makes sure they get exercised regularly and are 
therefore more likely to function when it comes time to REALLY need them :-)

-al-