I was having a jolly old time mucking around in the bilge - cleaning out, getting covered in gunk, remaking wire connections etc, and my geek brain thought it might be fun to check the actual real-life pumping rate.
The Rule brand pump has a name-plate rating of 1400 US gallons or 5600 litres per hour. The actual discharge out of the through-hull filled a 10 litre bucket in 20 seconds. That's about 1800 litres per hour, or just under one third of the pump rated flow. It would seem that the maker's ratings relate to something like an open discharge. In other words they are useless rubbish.
The discharge height on my boat means the pump works against about 1 meter of static head, plus the resistance in about 2.5 metres of 1 1/8" line. I also have a check valve to prevent short-cycling which no doubt has some effect too, although it shouldn't be much since it opens up full-bore.
The flow rate would vary a bit with battery voltage - my test was at a healthy 12.6V. I'm sure you'd get a fair bit more with the alternator supplying 14+ volts, but a whole lot less with a flattish battery and dead engine.
So there you go, don't bask in the numbers on the bilge pump. Try the bucket test, or divide the rated flow by about 3 if your configuration is like mine. But as the old saying goes, it'll probably never keep up with a frightened man and a bucket.
Cheers, Graeme
As a civil engineer who deals with discharges through pipes (and channels, rivers and other systems), I like your thinking. It is not often you find someone who understands headloss (the energy needed to drive the water through the system and the resistance of various components) through a pump and pipe system.
Your conclusion that the pump is rated for a resistance-less situation (i.e. something you would never experience in reality) looks good.
Second year fluid mechanics at Uni: Bernoulii, Reynolds, Froude et al inform my watery thoughts.
It was one of those "doh, of course" moments, but I bet I'm far from the only one who derived worthless assurance from that nice big number on the pump.
But as the old saying goes, it'll probably never keep up with a frightened man and a bucket.
OK. I'll jump in on this one. It is a case of expectations versus reality.
The expectation is that an electric bilge pump (no matter of the rating) will stop the vessel from sinking.
The reality is that it won't.
When I was working as a port engineer in Singapore I watched a 5000 ton freighter sink in the roads anchorage. Very entertaining.
High capacity bilge pumps going full bore as the crew threw their suitcases off the stern and swam away.
Our small vessel bilge pumps, no matter what the rating, will discharge the slop in the bilge, be it a window leak, a through deck leak, sterntube leak or split beer.
That's it.
Do not expect anything else. The only thing that will stop her going down (as James Cook and a recent post on here about a lightning strike showed) is to close the leak promptly. Then use the bilge pump (and a bucket in my case ) to clean up.
A small pump running for a while or a large pump running for a short time will do that.
Yes, the capacities of these pumps is over-rated, but that is no surprise. Think about the mileage for motor vehicles. These are set under the most suitable conditions available , just so with our bilge pumps. No surprise there.
gary
It is interesting that for boats in NSW survey the 12v bilge pump does not count. The surveyer looks at it but ignores it. 240v and 415v pumps count and hand pumps.
Ramona went with out a 12v pump for about 8 years. Just pumped out the water with the hand pumps. It's a regular check whether they are still working as well. Now she has a small Rule pump [500gph]. This is small enough to pump out the sump and allow water to drain through the limber holes so the water can just be pumped in one go. Fitting 1500gph pumps as well for an emergency situation is OK but for just clearing the bilge the small pump is the way to go.
On my fishing vessel I used two 415v pumps as emergency bilge pumps. 2 inch outlets and they shifted a lot of water. The bilge for normal operation was cleared by a 500 gph Rule switched by a Waterwitch switch.
Ricochet has had the same installation with the Waterwitch switch running off it's own battery and solar panel. It's interesting that the switch cost 3 times as much as the pump!
My view is that a bilge pump is there to pump out small amounts of water that accumulate over long periods of time. That means drips from rainwater and joints in the hull, so that you can leave your boat for several weeks knowing that a drip from the drive shaft will not sink it. It does not mean emergency leaks or big holes in the hull.
I don't have a bilge pump. The bilge is shallow, only about three inches (75mm) at its deepest. I fixed the windows, which was where most of the water came in. I have a dripless seal on the drive shaft, which works very nicely. A little water does get in, probably rain down the keel-stepped mast, and is easy to sponge out every three or four weeks when I visit the boat - I rarely sponge out more than about a litre. Apart from having one less thing to go wrong, not having a bilge pump lets you know how dry your boat actually is.
I watched a test of three or four different pumps last night and only one which was a rule brand could pump up to a 6 foot head. The rest made about 4 foot and that was it.
