Aft bilge pump lets water in?!

A discharge under the w/line for a bilge pump is a silly installation, which I wouldn't expect from any respectable builder.
Did you ever see that on a Cabo?

All well and good, but that's something different from having an u/w discharge - as often happens for the genset for instance, for noise reasons that don't make any sense with bilge pumps.
Anyhow, I'd rather not have valves on the discharge line (as also others said) on any boat, regardless of how prone they can be to sea water backpressure.
If anything, on a boat where for whatever reason is impossible to install a high enough loop to prevent backflow, I'd expect the builder to fit a good diaphragm pump that doesn't need any valve.
Also because most submersible pumps from the usual suspects like Rule, Jabsco, etc. are only mediocre at best, anyway.

I'm starting to notice some trends on the forum, a question is asked and we all try to help, the OP never comments again.
Bad breath, Poor dress code?
What's up?

ie; This re-opened 7 year old thread and the NFG on those transfer relays in another post..

Whenever you find sea water in the bilge, do you also find some in the fish box?
With that arrangement, if sea water runs back into the bilge but not in the fish box, the only plausible reason is that the macerator pumps restrict the backflow. So, you could just replace the current rule bilge pump with a similar one.

In your left picture, is that a leak from your trim tab?
In-between those just long enough screws?
Is one of those screws in the hose?

Was this a left coast or east coast built boat?

Could it be time to get a set of eyes down there,
back-down and maneuver some to make the leak happen real time, see where the water is coming from?

Sadly, the later Cabo's were built, not built, built, then scraped by Hatteras and the new Hatteras corp.

Why I had to ask, We know the left coast builds were pretty well made. One of our YF members made them.
Wish I remembered his handle, I'd invite him to comment.

Yep, time for a bilge monkey.

Precisely what I meant.
The only real reason for not using one of those macerator pumps also for the bilge is that the Rule thing is cheaper.
So, you could save yourself the hassle of finding another workaround, and just replace it with a macerator pump, job done.

BTW, I suppose you must have some other Rule submersible bilge pumps elsewhere, so you could just keep it as a spare.
Using it to replace a faulty one will only be a matter of when, not if...

As an aside, instead of replacing those two self-tapping screws with shorter ones, considering that you can reach them, I'd replace them with bolts.

Why, haven't you ever seen a diaphragm pump capable of 2000 gph?
When I suggested to replace the Rule with one of those, I meant one of that type, not the exact same model/size used for the fish box, obviously.

Yep, Thank you our friend.

OP, there is your original build expert. Private Mail MBY. Ask him to join us on this thread.

A marine (small boat <20M), bilge diaphragm pump, 2000GPH? Please post such a device.

Problem is, per your picture, on that pad, there may always be 1/2" to 3/4" of water back there.
Those Rule pumps are not capable of picking up low water.
And that pad has them higher than the bilge skin.

I can see in your picture the redundancy.
The second pump will only come on if the first pump fails, then the high water alarm to let you know the second pump is not making it, then you know there is a problem.
IMO, a little late by that time.
I would hope an alarm is sounding when that second pump comes on, then a second (louder siren) sounds when that high water float trips.