I would also check on what type of valve is being used for a 'seacock'. Is
it a true seacock or just some kinda valve??

If it causes any obstruction to the flow (globe valve) or angle valve, you
will always have problems with clogging.

It should be a plug type or ball type valve so there is not obstruction or
tendency to collect dirt in the valve seat area.

In many of my used boat purchases, I have found totally inappropriate valves
in such drain lines.


--
My opinion and experience. FWIW

Steve
s/v Good Intentions