What I generally recommend, if you want to do the bilge pump setup properly is the following:
First, install a small "maintenance pump" as low in the bilge as possible, using relatively small diameter hose. This pump's job is to keep the bilge relatively dry from the water that routinely gets into it—the stuff that comes down a keel-stepped mast, the water from the packing gland on the prop shaft, etc. This should have a bilge pump counter on it, so that you know how often it is kicking off... if the number of times it is kicking off per day increases dramatically, you have a problem.
The reason you want a small diameter hose for this is to reduce the amount of water that backwashes when the pump shuts down. BTW, a diaphragm pump would also work, and wouldn't backwash.
Put a loop in the drain hose for this to reduce the backwash.. the high point should be as close to the pump side and as far from the output side as possible.
Not a big fan of check valves. If it bothers you that much, get a diaphragm pump instead of an impeller one.
The next bilge pump should be a high-capacity one, with a high-water alarm, and it should be mounted an inch or two higher than the first pump. This is a dewatering pump...
I have read quite a bit on this issue on previous threads but I'm still confused...
Here it is:
This recently purchased O'Day 30 had an electric bilge pump with no float.
So, I decided to replace with an automatic one (rule-mate 750).
The problem is regarding the backflow...
The outlet hose has to go about 4 feet up and 7 feet back on the way out.
So, when the float senses no water, the pump stops, everything in the hose makes its way back in the bilge (which is narrow, not much surface area) and the whole process obviously starts again and never ends until the battery dies.
The easy solution is to raise the pump quite a lot to be above the constant remaining water, but then I have constant remaining water, and quite a bit of it.
My thoughts were to put in a non-return valve just past the pump outlet, but some people on this board advise against that.
What is the reason why such valve is a bad idea??
And if so, what is the solution.
I read about the two pumps set up, but I don't believe that will help my problem considering the elevation issue.