Tecktalk - Bilge Pumps

February 2000 Telltale Page 3 - by Tom Winlow and Marcel Laroche.

(thanks to David Pascoe for this information )

After the integrity of the hull, the integrity of the bilge pumping system comes next. The following table provides a reasonable set of requirements:

Boat Length   No. Pumps    Total Capacity - GPH
 16 -- 20        2               2500
 21 -- 26        2            300 - 3500
 27 -- 35        3           3500 -- 4500

There are two factors that must be considered: the capacity of pumps and the number of pumps. The number of pumps is important since bilge pumps are not reliable because they are electrical devices submerged in water. 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. For sailboats, that is easy because the fore and aft trim doesn't change much, so the cen- ter bilge is usually the target area. For most powerboats, the water will accumulate in the mid-section at rest and aft while underway.

Capacity of Pumps

A word about those little 4" square boxes that companies that make them call bilge pumps, for example 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. 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.

Rule does make excellent pumps, however, such as the 1500 and 2000 pumps, with big leaps up to 3700 and 5000. These pumps have been tested and the one thing to be aware of is that they do not pump at those rates. Those numbers seem to be 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.

For sailboats, you really have to pay attention to how high the water is being pumped. For a 40 foot sailboat, pumping the water up 3 feet or more, consider two 3700's. 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?

Rule pumps are 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 options there include 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 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.

Float Switches

Those wonderful little devices 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, it would cost a lot. 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.

You must keep your bilge clean. Nothing, but nothing is going to survive a bilge with sludge and debris in it. And 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.


Pumps only work if the batteries can supply energy to them. Go for a heavy duty commercial or marine battery. Surette, American, Exide, any of the big battery makers. You can tell if its for real if its big, black, very heavy and costs twice as much. Good batteries are heavier because they have more lead, for one thing. You are better off with one size 8D battery than you are with two smaller, cheap ones. Capacity is DIRECTLY re- lated to size. Paring up two small ones is no match for one large one. An 8D (250 AH) costs about $250.00; two 90 AH auto batteries are going to cost around $100 each, so the cost isn't that much more. A pair of 4D (125AH) will work nearly as well.

Hope this is of interest and if you can check out Mr. Pascoe's web site. It's full of excellent information and some real eye openers.