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Tuesday, November 16, 2010

Forward Bilge Pump Testing

The most common defense against a sinking boat is a bilge pump. Should you get a hole in your boat, or experience a failed underwater thru-hull, the first thing to do is pump out more water than that which is leaking in.
Bilge pumps come in all sorts of different sizes rated by pumping capacity often described in GPH (Gallons Per Hour). Question is, how big is big enough? There is no real answer to that in a boat this size. It is like trying to answer the question "How big of a hole do you plan to put in your boat?" Well, the SMALLEST of course!

So, in our case, a pump was chosen by physical size. The largest that would fit in the space planned for the pump. As our primary electrical system will be 24 volts DC, our choices are limited (suppliers tend stock mostly 12 volt pumps, 24 volt versions available by special order).

Our choice: The Rule 16A, 24v, 3700 GPH Bilge Pump. It has a 1 1/2" hose discharge which matches the thru-hull. For the float switch, we are using the Aqualarm 20378 Bilge Pump Switch and Alarm, which provides automatic and manual operation from a remote panel, plus an alarm should the pump run (in automatic mode) for more than two minutes.

As a test, I emptied one of the fresh water tanks into the bilge. The water tanks have been full since the summer as a test for leaks (there are none). With winter coming and the occasional freezing temperatures, I thought it a good idea to empty the tanks.

The initial setup includes 1 1/2" hose, up through a vented loop, to the overboard discharge thru-hull. At the pump outlet, a "non-return valve" (also known as a check valve or one-way valve) is installed inline with the hose. This helps keeps the remaining water in the hose from rushing back into the bilge, and tripping the bilge switch, restarting the pump, again, in an endless cycle. (The reality is, even this valve will leak, so the bilge will back-fill anyway,  cycling the pump, just more slowly and less often. I am still working on how to best deal with this.)

The results: The 3700 GPH pump easily kept up with the draining of one of the tanks. In fact it outperformed the tank. Which revealed a problem: these centrifugal pumps, when they start sucking air, begin to lose their ability to pump against the head pressure in the discharge line. As a result, the water level in the bilge kept rising until it was well above the base of the pump (intake strainer). Then the pump would eventually "prime" and empty the bilge, then do it all over again, causing a "spitting" behavior. All this because the pump is "too powerful"!

This is not necessarily bad. As in the event of a real hole in your boat, the water ingress could very well be more than a tank draining. To help this situation, I will install a smaller capacity pump (maybe 500 GPH) along side the big one and adjust the float switches such that the small pump will be the "first line of defense". If it can't keep up, then the big one (with the float switch a couple inches higher) will "kick in".

More later...


5 comments:

Anonymous said...

The problem with installing the 'rarely' if ever used bigger pump is that even the most anal boat owner forgets or ignores servicing the bigger pump. There needs to be a convenient way to trigger the float and run water through the big pump regularly.

robert said...

Good point. In this installation, the remote panel has an override switch to manually cycle the pump.

She:Kon said...

Good write up on the pump installation. I've got a similar 12 volt pump (4000 gph) that I'll use as a "crash pump" basically for emergencies only and two smaller 500 GPH for general "dewatering" duties. There's also the pump in the shower sump that can be pressed into action in the case of an emergency.

The task once they're installed is to keep them all maintained properly as mentioned. I have found, on my old boat, putting the back flow valve down near the pump instead of up near the outlet. Seems to work better for me.

robert said...

S:K, thanks for that. Current thinking is a similar installation for the same reasons as you described.

Rhys said...

The shower sump pump is a great idea. I have a Rule 3700 in my "well" aft of the engine; all limber holes lead to here, but I will put a Johnson sump pump in the shower.

I also have a Henderson diaphragm pump as part of the Lavac head set-up. If you put in a Y-diverter, this becomes a manual bilge pump good for "local leaks" like overflows or (bad scenario) backup from the galley sink because someone forgot to shut off the thru-hulls in bad weather.

Lastly, I have a large-capacity Patay manual pump which I intend to fit to the pilothouse floor. This will probably be my "first" pump before the Rule to sop up stray drips. The advice on where to put the check valve (aka backflow, flapper, one-way or "joker" valve) is sound. No one can tell me why this type of valve has so many names.