Bilge Pumps, why do we see so many real  yachts with toy pumps! do this bit right if nothing else!! 


Pumps must be fastened down, they should be in the lowest part of the bilge space possible and it should be accessible. 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. You must not place the pump in the bilge unrestrained. If it falls over it will suck air and burn out.

If pumps are allowed to run dry for long periods they will over heat. Float switches must also be fixed down, slightly higher than the bilge pump in some cases to prevent surging of the pump. Normally plastic hose is used to carry the pumped water away, it is good practice to use a short length of silicone hose at the end to make removing the pump for cleaning or replacement easy as the plastic hose will harden in time and be difficult to pull off the pump.

Corrugated hose reduces pump output by as much as 30%, so always connect the pump to the discharge fitting with hose that has a smooth interior surface. If the discharge is submerged when the pump runs, sea water siphons back through the pump into the bilge when the pump shuts off.

The discharge fitting must never go below the waterline. If the fitting is through the transom, be sure it is high enough not to submerge when the stern squats. And if it is through the side of the hull on a yacht, which is not good practice in general, and especially on performance yachts. It must be high enough to remain above the water at the deepest angle of heel.

The through-hull fitting can further reduce pump capacity, to minimize this restriction, use a fitting with the largest possible opening to match the pipe. It is good practice to extend the discharge hose upwards in a loop to the highest position under the deck before it returns down to near the water line to the discharge fitting. No extra strain is put to the pump once the water is flowing and the siphon effect will clear the pipe when the pump is switched off.

It is essential to get the pump wiring out of the bilge as quickly as possible. Run the wires up and secure them so that they do not lay in the bilge water. The pump and float switch connections must be as high up as the wire supplied on the pump will allow. Crimp-on connectors will assure a good mechanical and electrical connection between the supply wires and the pump leads. Enclose these connections in adhesive heat shrink tubing to make them water tight. Better still use a small plastic enclosure and fill it with Vaseline or battery grease.

 A bilge pump big enough to deal with a real emergency will do a poor job of keeping the bilge dry because the water in the discharge hose drains back into the bilge when the pump shuts off. And a big pump requires a big hose.

The ideal bilge pump arrangement is a small automatic bilge pump mounted in the sump to dispense with rain and shaft-gland leakage, combined with a high capacity pump (3,700 gph) mounted higher to deal with more serious ingress.

Capacity of Pumps, be aware that small cheap bilge pumps do not perform as indicated in gallons per hour; 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. They're okay for use for de-watering small areas where water might accumulate, but never as a primary pump.

The most common sizes are the 1500 and 2000 pumps, with big leaps up to 3700 and 5000. Many of these pumps have been tested 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.

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 2000 and 3700, two of each, using the 2000 as the primary pump and the 3700 as the backup, the smaller pump has a lower power demand which is more desirable for normal dewatering. The 3700 serves as both a back up AND an emergency pump. The 3700 has a 19 amp draw, which can deplete batteries fast.

Stepping the discharge hose from the small pump down minimizes the backflow from the hose when the pump cycles, maintaining a dryer bilge. The large pump can be wired to a float switch if preferred, a manual switch makes more sense. An added advantage of this bilge pump configuration is that the high-capacity pump sits high and dry, extending its life indefinitely. If you want a dry bilge, the only way to get one is with a diaphragm pump.


Float switches can easily be protected from surge by simply locating it within 3" of a bulkhead with the flapper facing AFT. Always  AFT. If surging water catches the flapper from the front, it tears the flapper off its hinges.

Contrary to common belief, the pumps themselves ( good ones) rarely fail; it’s the electrical system from which they operate that is usually the cause of the failure. Because of this, one way to improve reliability is increasing the number of pumps to decrease the odds of complete loss of pumping ability.