Mysterious Bilge Pumping Explained
Thanks for publishing my letter on bilge pump cycle counter installation (February 15, 1998; other reader comments appeared in June 1998). In your closing statement you asked if the boat was flooding, which might account for the high count recordings when sailing in choppy seas. We have just returned from a two-week cruise from Baltimore to Norfolk and back with the following update to report.
Our Island Packet shower drains into the bilge. Every morning before we set out I would manually pump the bilge and turn off the automatic sense switch. At the end of the day’s sail I would flip the switch to automatic with no pump starts. No pump cycling was recorded during the day. By the next morning, we had one or two cycles recorded as shower usage being the contributor.
My conclusion is that sloshing bilge water caused the pump to start and stop many times while under sail in choppy seas.
Leaving the pump off while sailing is probably not a good practice, but the many false cycles will shorten the life of the pump. While sailing I could not hear the pump cycles so if it were left on and disaster struck I still would not know until the pump failed and telltales appeared, like water over the cabin sole. I think I will leave the pump off while sailing and on at all other times.
Thoughts On Pipe
Bill Seifert’s comments on pipe in the September 1998 issue left out some very important concepts:
1) All pipe is to be made of a long hole surrounded by metal, centered around the hole.
2) All pipe is be hollow throughout the entire length.
3) All pipe is to be the very best quality, preferably tubular or pipular.
4) All acid-proof pipe is to be made of acid-proof metal.
5) OD of all pipe must exceed the I D; otherwise the hole will be on the outside of the pipe.
6) All pipe is to be supplied with nothing in the hole so that water, steam, or other stuff can be put inside at a later date.
7) All pipe is to be supplied without rust as this can be more readily put on at the job site.
8) All pipe is to be cleaned free of coverings such as mud, tar, barnacles or any form of manure before putting up; otherwise it makes lumps under the paint.
9) All pipe over 500' in length must have the words “Long Pipe” clearly printed on each end so that the engineer will know that it is a long pipe. Pipe over two miles in length also must have these words printed on the middle so that the engineer will not have to walk the full length of the pipe to determine if it is a long pipe or not.
10) All pipe over six inches in diameter is to have the words “Large Pipe” painted on it so the engineer will not use it for small pipe.
11) All pipe fittings are to be made of the same stuff as the pipe.
12) All pipe closers are to be open on one end.
13) No fittings are to be put on pipe unless specified. If you do, straight pipe becomes crooked pipe.
In the June issue, you reviewed electrical wire crimpers and recommended the AMP Super Champ. I cannot locate this unit in the West Marine or BOAT/U.S. catalogs and the phone number you gave for AMP Super Champ is “not in service at this time.”
Do you have an update on where I can get this tool?
Sorry about the phone number; it worked when we wrote the story but was later disconnected. A working phone number for Contact East is 800/225-5370. The AMP Super Champ lists for $10.10 in their catalog (part #125-836, model 244).
There is also a web site: www.contacteast.com, which has some photos and is fairly easy to use and order from with a secure server.
Shurflo Pump and NPSH
I read on page 10 of the August 1 issue of PS about the suction lift “problems” with Nick Nicholson’s Shurflo water pump. I had the same “problem” with my Shurflo model 2088-423-344 not being able to self-prime once the level in my tank was less than half full, unless I opened one of the system taps. Here is what I found and how I solved it.
Most pumps require that a certain amount of the Total NPSH still be present in the water at the inlet so as to avoid cavitation. This minimum amount of NPSH is called the Required NPSH and is determined by the pump’s physical design, the fluid being pumped, the speed of the pump, and most importantly, the pump discharge pressure. A less confusing way to think about this is that the atmospheric pressure is pushing the water through the inlet piping, hose, valves, fittings and overcoming the vertical distance from the surface of the water in the tank to the centerline of the pump inlet. If the total of the suction side plumbing pressure losses and vertical distance are great enough, no flow will be established and the pump will spin dry and overheat. In the real world, pumps are not 100% efficient, water is not at 68°F, and inlet pumping pipes, hoses, valves and fittings do result in friction-induced pressure losses which diminish the 34' maximum theoretical “lift” capacity. In the West Marine catalog, my pump is advertised as being able to self prime up to 8'. Under what operating conditions this is true is not stated. I would guess that Shurflo has made some allowance for inlet plumbing losses. For example, if the pump’s Required NPSH is equal to 20' of water and the allowance for inlet plumbing friction losses is another 6' of water, then the remainder of 8' is available for the vertical change in distance (lift).
I put a vacuum gauge at the inlet fitting on my pump and measured a very low pressure (about equal to 10' of water or 4.3 psi). An Available NPSH at the inlet to a pump, which is less than the manufacturer’s Required NPSH, will cause cavitation in the pump inlet. This is a phenomenon where micro-boiling occurs due to the very low pressure in the water. As they flow through the pump, these bubbles collapse or implode on the pressure side of the pump and can erode away pump material, causing the pump to loose efficiency and increase its Required NPSH.
I have clear hose on my pump suction and was able to observe what was occurring. After running the pump I noticed air leaking into the pump suction hose, breaking the “vacuum” in it and allowing the water to drain back into the tank. Under normal conditions, this would not be a problem because the pump should be able to “lift” the water to its inlet from the tank. However, this was not happening, which caused me to suspect that the inlet plumbing was creating a high friction loss and that my pump’s Required NPSH had increased due to cavitation-induced erosion. This was confirmed when I noticed, as you did, that opening a water faucet, which temporarily reduces the pressure the pump must discharge against and simultaneously decreases the Required NPSH, allowed the pump to overcome the suction “lift” and re-establish suction flow. Once water is present in the suction side, the pump has an easier time maintaining the flow because water is thicker than air.
Water pumps do not pump air as well as water because the dimensional clearances inside the pump allow air discharged to the pressure side to flow back to the suction side more readily than water. I tried to seal the suction side plumbing to prevent the vacuum from being broken by the air leaks and managed an improvement in idle time between vacuum breaks.
To totally solve the problem, I purchased 3/4" hose adapters and re-plumbed the whole suction side. This reduced the suction side friction losses. I also put in a Raritan 3/4" check valve at the outlet pipe connection on my water tank. This kept water at the inlet of my pump, even if some small amount does leak away due to poor sealing in the plumbing. During a recent trip,when my 100-gallon tank was totally emptied twice, there was no reoccurrence of the problem.
In the July 1 issue, Cliff More says that, “…Copperkote worked well on the rest of the bottom, except the J strut and shaft, which were thickly coated with barnacles and soft growth.” We had a similar problem until we primed all the metal parts with Woolsey #640/#649 two-part epoxy primer before applying the bottom paint. I think the problem is that the bottom paint is slightly conductive electrically. If you put a piece of copper in seawater it will resist fouling pretty well as long as it can give off copper ions. However, if you connect a zinc to it and protect it from corrosion, it will foul.
In some tests I have done, Woolsey #640/#649 is a very good electrical insulator, plus being a good mechanical primer. Woolsey no longer makes #640/#649. They have “improved” it. I hope so. My last can is almost empty.