said, bilge pumps are low pressure. If you put your pump in a bucket of water
and point the hose up, you will get a few inches of bubbling water like an old fashioned water fountain.
The pump in your system will move some water up the sink hose until the weight of the water offsets the pressure the pump creates. Then the flow goes out the thru hull
. Flowing water seeks the path of least resistance.
That said, having salvaged and repaired many large boats, I prefer to have bilge pumps on their own system and thru hull
, a proper vented loop, and no restricting check valves. Plumbing
out of sight tends to get no routine maintenance
. Thru hulls below the water line, open but rarely used, can get blocking marine