Mainsail: I wanted to ask you about having the discharge close to the waterline. I will be installing a Whale pump that is designed for shallow bilges. I'd like to 't' into the sink discharge through hull which is only about 4 feet from where the pump will be and I'll avoid having to create another hull opening. Will I still need to put high loop in the discharge hose even if the pump has a check valve?
I never like to tee into a below waterline anything for a bilge pump and I also never recommend a sink.
Below waterline fittings can become clogged and or have the flow reduced by sea growth.
When you leave the boat one should always close the seacocks. If you close the seacock the bilge pump will just fill the sink and over flow back into the boat.
Sink drains especially can become clogged with food, hair, tootpaste and other gooey items that when mixed with aquatic life begin to reduce the ID of your plumbing and reduce flow or increase head pressure.
I never advise the use of tee's. Always use a Y adapter if you absolutely, positively must share a line. I hate to ever see a bilge pump on a shared discharge.
I stand by what I said about check valves they do nothing but add a potential failure point.. They are notorious for not opening and causing the pump to cavitate, not move any water and killing batteries..
It's your boat but please do think through all the possibilities.. and yes you still need a high vented loop!!!