I don't trust centrifugal pumps as my primary pump. The primary (pump at the lowest level of the bilge) doesn't need to be large, but it activates most often, and it IMHO needs to be a self-priming diaphragm pump that's insensitive to dips in the discharge line, and not stopped by mud, strands of hair, and other debris that collect there in concentration. There's nothing more discouraging than hearing your pump whirring away, discharging your battery, with no water coming out the discharge. And it's important to be able to actually see the discharge so you know it's working! Therefore, the discharge should be someplace that's visible at any degree of heel and without standing on your head. That eliminates the stern and both sides. There are only two places left: the engine port and the cockpit.
My primary pump is a Whale Bilge IC Automatic Remote Mounted Bilge Kit. It is expensive, for its GPH rating it uses more current than a centrifugal pump, and pumps only 300 GPH -- but it's reliable (your heart is a form of diaphragm pump). The intake/sensor head is at the lowest point in the bilge. That pump keeps the water level in my bilge to an amount that I can completely remove with three squeezes from a large sponge. The discharge is into the cockpit. Why? That's the shortest length from my mounting point just inside the starboard locker and -- it's impossible to not notice it's running from the cockpit. Water in the cockpit cannot drain back into the cabin because the pump has one-way valves (essential for its operation - just like inside your heart). Yeah, sometimes my shoes and pant legs get wet. I don't have an audible alarm on this pump because it activates frequently and an alarm freaks out my passengers (and disturbs my sleep if a little rain gets inside). I have a timer, a flashing LED, and a timer reset switch. I have seen many implementations of counters on bilge pumps. I have never understood why anyone would use a counter instead of a timer. Did the pump run once for one minute, or once for a week? There's a huge difference!
I have a small leak from the engine compartment into the cabin that I have spent many hours trying to locate (any clues anyone?) that admits water whenever the water level reaches the top of the engine port - which is just about anytime I was underway with good speed. So, I added a small centrifugal pump in the little well (bilge?) forward of the engine port. It's screwed down to a brick. It too stays very busy. That pump, and my two backup pumps in the bilge, discharge to the engine port. The backup pumps are a Rule 2000 and Rule 3700 pump, that activate progressively as the water level rises. Both have a float switch and an electronic Water Witch switch wired in parallel, and both have wake-the-dead loud alarms. They have never activated except during testing. I strongly recommend testing centrifugal pumps with a garden hose into the bilge -- the slightest dip in the discharge hose or strand of hair will prevent them from working.
My Ariel has only three through-hulls: the two cockpit drains and the tube for the rudder shaft. I'd eliminate those if I could figure out how. My depth sounder/water speed sensor is mounted to an oak plank that extends from the engine port. The two illegal head through-hulls have been removed and glassed in -- one of the nifty expensive bronze ball valves leaked around the ball seam and nearly sank my boat one windy night in the middle of the Bay - thank goodness for my wetsuit and plumber's epoxy putty that I had to stuff in from the outside. The sink hose has also been sealed since I discovered, after getting pooped offshore, that a full cockpit turns the sink into a secondary cockpit drain, i.e., water shoots into the cabin from the sink.
Through-hulls below the water line are evil and imperil the boat, and adding them for bilge pump discharges seems like a contradiction in objectives.
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