Re: live bait well
I have had 2 types.
First boat had a top-side fill, top-side overflow, and bottom-side drain. Only used a fill pump. Drain was through the side of the hull since the tank was mounted against it on one side; overflow was via a transom hole.
Problem was that it was a very large tank running fore and aft for a boat that wasn't all that big and when you let it fill up, it tilted the boat to that side and if you put power to the boat, water came out all over the deck where you were sitting.
A standpipe of suitable length (about half the tank height) worked just fine. Similar to DJ's comments, I put a rt angle at the bottom of the pipe and stuck it in the drain hole......sorry, but I don't remember what I used to stick in the drain hole which used the same type tube/plug as the bilge does on most small boats. When it was time to drain the tank, I just pulled it out and it drained normally. If I didn't want to use it, I just left it out and installed the drain plug. Never had a problem with overfilling with this.
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Current boat had a factory installed system where there was a similar situation (tank was centered and cross mounted in the boat) with the same problem.....overflow was too high....but at least the tank was crosswise, not longitudional to the boat.
I rigged up a second pump on this one plumbed to the overflow with a float switch at my desired height....this tank was much larger than the hatch and it wasn't easy to get to the holes like on the previous boat.
Problem with this is that there is a race situation between the two pumps so you have to keep your eye on what's happening. The drain really needs to be a higher capacity than the fill. Other thing is that the pump is a centrifugal pump (like the pump that cools your auto engine, if you ever saw one). Since it is of open construction, water, through siphon action, will come out the overflow from (through) the pump and drain your tank.......even if your overflow pump happens to be off. So I had to put a shutoff valve in the line so that when I was ready to bring live fish home I could shut off the overflow line and retain the lake water in the tank..... 1" pvc ball valve.....couple of bucks.
Seems to work pretty good now.
HTH
Mark