All About Bilge Pumps
Those Essential Devices for Keeping Your Boat Off the Bottom
by David H. Pascoe, Marine surveyor
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Total Capacity - GPH
|16 - 20||2||2500|
|21 - 26||2||3000 - 3500|
|27 - 35||3||3500 - 4500|
|36 - 42||3||6000|
|43 - 49||3 - 4||8000|
|50 - 59||4 - 5||9000 - 10,000|
|60 - 60||4 - 5||10,000+|
Evaluate the Number of Compartments
While the table above gives us a general idea of how many pumps are needed, it can't take into account how many compartments there are in the hull that need to have pumps. Every hull is different, so you have to evaluate your boat from the standpoint of the number of compartments that need to be fitted with pumps, as well as the best location to have redundancy. To evaluate the number of pumps you need, take a look at the hull and determine where the low point in the bilge is. Water will accumulate at the lowest point, but you need to know where that is. Next, determine the number of water tight compartments or hull dividers such as bulkheads or high floor frames that prevent free flow of water from one section to the next. That means determining whether there are limber holes in those dividers or bulkheads.
As a general rule, every compartment that doesn't allow free flow of water from one to the other needs to have a bilge pump. At some point, water can rise in this compartment until it finds a way to flow through the bulkhead (such as all those holes for wiring and plumbing) or over the frame into the next. And while this may not sink the boat if this happens, rising water in a compartment can cause a tremendous amount of water damage. This is actually more of a problem in small boats than large ones. That's because small boats often have very shallow bilges where a small amount of water in the bilge can end up flooding the cabin sole and cause damage. Yet this can also be a problem for shallow bilge sailboats. Especially for planing power boats, keep in mind that bilge water will flow to the stern while underway if there is free communication, or it will be stopped at water tight compartments. That's why you need to evaluate carefully the location pumps need to be installed.
Determining the Number of Pumps
Now that you know the number of compartments that need pumps, we next relate this to where the water goes when the vessel is at rest, and while underway. For sailboats, that's pretty easy because the fore and aft trim doesn't change much, so the center bilge is usually the target area. For most powerboats, the water will accumulate in the mid section at rest and aft while underway. Based on that, you will need the redundancy at these two locations. Any other compartments can get by with only one pump, of a size and capacity needed for normal dewatering.
For any twin engine powerboat over 35 feet (generally excluding trawler types), having four pumps is a good idea; you want the back ups at both points where water will accumulate. For outboard or stern drive boats with the engines aft, the water will always run aft, so the back ups are only needed at this location. For sailboats with a keel sump, this is the only location where redundancy is needed, except for larger boats with a dedicated engine room that definitely should have dual pumps because of the potential for plumbing system failures, a damaged stuffing box, exhaust system and the like.
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 deplete, 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
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 impeller 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. Continued to page three.
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- What Makes for an Adequate System?
- Evaluate the Number of Compartments
- Determining the Number of Pumps
- Outboards and Stern Drives
- Capacity of Pumps
- What Brand?
- Pump Installation
- Float Switches
- Open Versus Covered Switches
- Doing It the Right Way
- The Discharge Outlet
- Emergency Pumps - Who Should Have Them and Why
- Battery Power
- Wiring Pumps
Posted November 10, 1998 (First posted 6/10/98 at www.yachtsurvey.com. Page design changed for this site.)