Re: Adding automatic bilge pump to manual bilge pump
Yeah, no one likes cutting holes in his boat, but this time you should. Each pump needs a completely separate line and its own through-hull. Here's why:
First, let's deal with the question of check valves. All the trash and junk that falls inside the boat that you don't catch ends up in the bilge -- wood splinters, bits of paper, paperclips, whatever it might be. No matter how clean you keep your boat, stuff will end up there. This stuff tends to clog check valves so they no longer close (if you are lucky. It's worse when it clogs in a backflow so it can't open). So no check valves, please.
Now the Y valve. Y-valves ensure only one pump at a time can operate rather than both. That doesn't help you as much.
What about a Y fitting instead? Imagine your Y-connection with no check valves -- you pump from the bilge right back to the bilge.
And finally, don't forget to put a tall loop with vacuum break in your new hose as you install it, The top of the loop should be above the waterline even when well heeled -- otherwise, you will end up with reverse flow carrying water into the bilge when you heel to the through hull side.
T. P. Donnelly
S/V Tranquility Base
1984 Islander 30 Bahama