Outboards and Stern Drives These boats require special attention to pumping systems because of the weight of the engines. Any water in the bilge runs aft and it requires very little water to sink them, particularly when they have self bailing cockpits. A back up pump should be considered a necessity. The pumps should not be located under the engine where you can't see or reach it. If it is, move it forward to where you can reach it.
The problem with most of these boats is that they have no battery charger, so as soon as the batteries depletes, the pumps don't work. That's another reason so many of them sink. The only reasonable option is to install a marine charger and shore power system. Adding larger batteries will help, but somehow you have to keep them charged up.
Capacity of Pumps I will start here with a word about those little 4" square boxes that companies that make them call bilge pumps. Yep, I'm talking about the Rule 500 and 800 pumps. Only a fool would believe that one of those things could pump 500 gallons per hour; they can't and they don't, not even in a horizontal direction, yet alone vertically. I am absolutely adamant that those things should never be used as a primary bilge pump. Not only is the capacity inadequate for just about any boat except a dinghy, all it takes is a bit of string or hair tangled in the impeller to bring it to a halt. They're okay for use for dewatering small areas where water might accumulate -- like outboard of stringers, but never as a primary pump.
Except for those little buggers, there's no doubt in my mind that Rule makes the best pumps so I'm going to use these as examples. The most common sizes are the 1500 and 2000 pumps, with big leaps up to 3700 and 5000. We've tested many of these pumps and the one thing to be aware of is that they do not pump at those rates. As near as I can tell, those numbers are for pumping water horizontally, but when you have to pump the water up and out (called static head) those numbers will drop dramatically, by 50% or more when you're moving water up 3 to 4 feet.
My concept of the ideal pumping arrangement is to have two pumps at the one or two points where the water accumulates, at rest and underway. Let's say you have a 40 foot power boat. In that case I would choose the Rule 2000 and 3700, two of each, using the 2000 as the primary pump and the 3700 as the back up. Why not the other way around? Mainly because the smaller pump has a lower power demand which is more desirable for normal dewatering. No need to be activating the high capacity pump for everyday needs. The 3700 serves as both a back up AND an emergency pump. The 3700 has a 19 amp draw, which can deplete batteries fast; in an emergency situation, you will run the engine to keep the batteries charged.
For sailboats, you really have to pay attention to how high the water is being pumped. Needless to say, a weak, a low capacity pump is not the way to go. For a 40 foot sailboat, pumping the water up 3 feet or more, I'd consider two 3700's the best choice. I have seen 2000 pumps four feet down in the keel with only a small stream of water dribbling out the side. Don't forget that resistance in the discharge plumbing also retards the flow.
What Brand? After several decades of seeing these pumps in service, I have no qualms about recommending Rule pumps; they're the best. They are, of course, centrifugal impellor pumps that will not pull the last 1-1/2" of water out of the bilge. If you want a dry bilge, the only way to get one is with a diaphragm pump, and your option there are the PAR pumps (Short for Peters And Russell, now ITT Jabsco). They are less reliable, but they have the advantage of being repairable, whereas Rule pumps are not. I don't recommend PAR pumps as anything but secondary pumps for dewatering as their capacity is very low, 6 gpm or less. These pumps should only be mounted in a dry, dry, dry location. Neoprene impeller pumps are also available, but I don't recommend them unless you know how to use them. They will burn up if they run dry, so you can't turn it on and walk away from it. If you use either of these types, you MUST install an inline filter to prevent debris damage to the pump.
Pump Installation Considering the need for redundancy, there are two ways to install back up pumps . You can install both at the same level in the bilge and locate the float switch for the reserve pump up higher, say 6 - 10" so that it will be activated when the primary pump fails or can't keep up. The alternative, which I prefer, is to mount the switch and back up pump itself up higher (Illustration below). The reason for this is the tendency of debris in the bilge to foul the impeller over time; mounting it higher up precludes this. In either case, the installation should be arranged so that the back up pump takes over at a predetermined water height. Preferably this should be at a level before water rises above the cabin sole (or any equipment in the bilge like batteries) and causes damage.
Float Switches Those wonderful little buggers. Doncha love 'em? Yes they suffer a high rate of failure and you're always wondering why someone can't invent a better one. Well, devising a better switch would be easy. Problem is, you wouldn't pay the cost of the thing, so we have to suffer with what we got. Actually, most switches fail not because of lousy switch design, but because of thoughtless installation or lack of maintenance. These are not self-cleaning devices. There are four things you need to consider for reliable switch installation: (1) no debris in bilge, (2) nothing should interfere with the rise and fall of the switch, (3) it must be wired properly, and (4) it must be protected from the surge of water in the bilge.
Open Versus Covered Switches. The enclosed float switch would seem like the ideal solution to switch fouling problems except for one thing: you can't see or test the switch. Further, the enclosed switch is just as likely to become clogged with sludge and things like hair in the bilge as the open switch. Only now you can't even see it. The only problem they really solve is water surge damage. The open switch is the better choice as long as you clean it once in a while, and locate it so that its protected from water surge.
Note: Sludge is formed when oil in bilge water adheres to surfaces and then collects dirt. Eventually it becomes a tar-like substance that will prevent the float switch from moving.
The switch can easily be protected from surge by simply locating it within 3" of a bulkhead with the flapper facing AFT. Always AFT. See my point? If surging water catches the flapper from the front, it tears the flapper off its hinges. Okay, now that problem is solved. The next one is that you have to keep your bilge clean. Nothing, but nothing is going to survive a bilge with sludge and debris in it. Finally, all your wires and hoses have to be secured to that they don't move and end up sitting on top of the switch. Don't forget boats bounce around a lot; those things have to be well secured.
It would be my guess that well over 50% of all pump failures are caused by water getting at wire connections and causing corrosion and high resistance. People just don't realize that corroded connections cause a power loss that can either cause the pump to burn up, or the wire connections to overheat and terminate all power flow. That's why its imperative that the wire connections be made as high above the bilge as possible, and that they be protected against getting wet from any other source, like water dripping from above.
Doing It the Right Way I recommend that the connections be made using a covered, plastic junction box, the one hole type (Such as the Carlon boxes you can get for a few bucks at Home Depot), mounted on the nearest available vertical surface. Obtain a small terminal block, preferably with brass terminals. If you can't find small ones, cut a larger one in half; they're made to be cut. Attach ring terminals on the wires, wire it to the terminal block, and put the terminal block inside the junction box and install the cover. You can leave the terminal block loose inside the box so you can pull it out to check or repair connections. Be sure to mount the box with the wire hole at the BOTTOM, not the top! This is a particularly good way to install pumps in open boats and under cockpits where leaking and condensation sweating is a constant problem. Forget about butt connectors, electrical tape and silicone and heat shrink; none of these solves the water problem.
The Discharge Outlet Its amazing that after all these years, so many boat builders still do not know how to properly install the discharge plumbing. You'd think any fool would realize that you can't just pump it out through a hole in the hull a couple inches above the water line without the water flowing back in. But they don't.
The discharge outlet is usually placed near the water line because the splash from the discharge goes up on the hull side and makes a mess. So the motivation here is no splash. Fine, but you have to do something to prevent water from coming back in. That something is called a riser loop. The riser loop extends the discharge hose well above the water line to prevent this. Of course, if for any reason that discharge should go below the water line, you are right back to the reverse siphoning problem again. Unfortunately, there's no good way to deal with this short of raising the discharge higher up. Syphon breaks and check valves are notoriously unreliable because of their tendency to get clogged.
