I've installed a second bilge pump on our 22, in the floor depression between the fuel tanks. Here are a few particulars that might be of use to others:
Forget mounting screws, brackets, etc. Just use 4200 to bond the bottom of the pump's plastic strainer basket to the floor. Has worked for two years, and never come loose. The pump comes loose from the plastic bottom whenever I need to clean the metal strainer.
There's no way you're ever going to pump out all the water from the rear sump. Two factors are at work. First, the pump will start drawing air, and quit pumping, while there's still some water in the sump. There's nothing you can do about this. Second, once the water in the discharge line stops flowing overboard, all the water that's left in the "up" leg of the line will flow back into the sump. The only way to handle backflow, absent a check valve, is to reduce the amount of water in the line. There are three things you can do here. First, keep the line as short as possible. Cut the length of the "up" leg in half, and you've cut the amount of backflow in half. Second, get a pump with the smallest outlet port available. That way, you use smaller diameter hose, and have less water in the hose, waiting to flow back into the sump. Reducing the hose diameter by 1/2 reduces the amount of water by a factor of 4. Finally, as a variant of the second approach, use an adaptor/reducer to reduce the size of the hose. Either way, you're trading off flow rate for volume of return water. That's OK with me, since my purpose in installing the pump in the first place was to get rid of a small amount of water. If I want a backup, high volume, emergency pump, I'll install that separately.
I've wired a bilge pump "kill switch" for the second pump, at the helm station. That way, if the "kill" is engaged, throwing the bilge switch on the main switch panel will only activate the main pump, at the aft of the cabin. With the "kill" disengaged, throwing the main switch powers both pumps.
As somewhat of an aside, after years of getting annoyed at the water that accumulates in the two sumps when the boat is on the trailer, I took a lesson from my dad, and put a small strip (1 inch by 12 inches) of rag in each wet area, and use it to wick the water out. The main bilge sump wicks up onto the deck, and then drains toward the rear sump. The rear wick wets in the rear sump, and goes on through the drain plug hole, with a tail hanging down outside the boat. You have to be careful that the wicks don't end up plugging the drain hole, but so long as you're careful on sizing, and secure the ends, these wicks keep the sumps bone dry when you're on the hard.
