cableties的部落格

The problems

In 1980s, most marines just used manually operated pumps. Since some boats are internal-ballast/keel design, there aren't any keel bolts to leak, and, if hull integrity is maintained, there should not be much water coming aboard, save for the dripping from the prop shaft. Of course, even a slow drip from a prop shaft can accumulate considerable water over time.

Here is an example: In a marine, the keel sump is quite deep, extending four feet below the cabin sole, and about five feet below the waterline. Actually this is an advantage. Any water that comes aboard is contained in this deep sump, and it doesn't slosh around the cabin--at least not until you've collected many, many gallons of it in the sump.

Early in the boat’s career, it had an electric bilge pump installed deep in her keep sump. Well, I should put quotes around the word "installed", because all that was really done was to lower a small submersible pump into the keel sump, where it was left loose to bang and slosh around with the bilge water.

The pump was a portable centrifugal water pump, with an enormous rating for output in terms of gallons-per-hour. I never could understand how that mini pump was able to push all the water up through the 0.75-inch-ID outlet hose fitting out of marine. It would clear the keel sump, but it would take a minute or two to pump a gallon or so of water out. Since the pump was loose in the keel sump, it could land upside down—as it usually did--which meant that it would leave six inches of water in the sump after its intake was exposed. So it would never really get the sump dry.

The mini submersible water pump has a few things going for it, however, namely:

1.affordable

2.easy to install and remove in place

3.easy maintanence

4.very low power drain

5.quiet operation

6.actually did pump water out from the sump

Surprisingly, It did the job for about ten years, but finally gave up in 1996. I suspect that its quiet operation is the main reason for its long life. The pump made so little noise that you could barely tell it was running, so it was very common to leave it on. You'd come aboard, clear the bilge of water with the pump, start the engine--making enough noise that the pump was totally inaudible now--and leave the pump running for hours, until you finally shut the engine off and heard the pump running dry. Ten years of that type of abuse ultimately killed the thing, not to mention ten years of being submerged in oily bilge water.

When the pump run out, the boat was up in the North Channel on charter. A new pump was purchased and taken up by the next charterer, and when "installed", it proved the equal of the original, with the exception of item no. 5. This pump would not actually pump water out of the sump. it ran and made a try at it, but the buyer of the pump had overlooked one critical specification: HEAD capacity. He'd purchased a pump that claimed a zillion gallons per hour of output, but only if the path was all downhill from the pump. In the marine, the submersible pump has to lift its output water almost as high as seven feet. There's the five or so feet to get out of the bilge sump itself, then a couple of feet more to come up to the cockpit deck level and join the cockpit scupper lines to exhaust the water overboard. Seven feet of head on the pump was way more than it could manage. All the pump could do was to maintain water in the first five feet of output line, but it could not quite pump the last couple of feet to get it out of the boat. To make matters worse, the pump exhausted into corrugated flexible hose, whose inside was filled with hundreds of little ridges from the corrugation, making resistance to flow higher.

Installing a Better Bilge Pump

I'd been wanting to install a real bilge pump on marine for a long time. Now it comes the chance! I'd never liked the mini bilge pump for one simple reason: it seemed wrong to put an electrical motor under water to operate. There was not a compelling reason to submerge the pump, and most things-electrical would prefer to operate high and dry, but not underwater. The new bilge pump would be one of those big, ugly, noisy, diaphragm pumps that could mount in the cockpit locker, well above the oily water of the bilge sump. When you think about it, every one of those attributes is an advantage:

Advantage of Diaphragm Type Bilge Pumps

1.Electrical Above Water. The electrical motor is not submerged. This just makes sense, as electricity and water don't mix well.

2.Powerful. This motor draws some amps! It takes a certain amount of work (as in Physics) to lift water. You can't get the work done without some power being applied. More power equals more work being done.

3.Noisy. When these pumps are running, you can hear them. This would tend to decrease the occasions when the pump unintentionally was left running all day.

4.Diaphragm-Bilge Pump. Water isn't the only thing in a bilge sump. There's little bits and pieces of stuff floating around down there, the kind of thing that does not go through an impellor type pump very well, but can be sucked through a diaphragm pumps with ease. Besides, the pump can run dry without damage.

5.High-and-dry mounting. A submerged pump sits in oily water most of its life. The repair of those pumps is nil. The new pump is mounted clean and dry, and if something breaks, a repair can be made. In additional, the pump is securely mounted in place, and it is not banging around the bilge. You've gonna like it!