I had a boss once whose favorite line was, "Show me the data." I learned a lot from him...
Ratings are all fine and good, but the real world can always change things. So I just timed how long it took my Amel original diaphragm pump to move 12 liters of water: 42 seconds. A flow rate of: 17 liters per minute (or 4.5GPM) I have read the "32 l/min" number several times, but don't know its original source.
Whenever I see someone write down a number and proclaim that "this is the minimum capacity for a bilge pump for this boat" I wonder how they came up with that number.
I think of bilge pumps in two categories: Dewatering and emergency.
Dewatering is just the normal day to day emptying of the bilge. On most boats that would include shaft drip, rain water leaks, etc. We don't have those, so for us it is just the routine emptying of gray water from the sump. The primary pump selection issue here is not about capacity, but rather picking a pump that will not choke on any lumpy bits from the galley sink. Given that requirement, we do not have a lot of options.
An emergency bilge pump is another matter. Any hole in the hull below the water line will overwhelm most bilge pumps. 8gpm is a good number for a 1/2 inch hole, and the flow rate rises with the square of the diameter... so a 1 inch hole would be 24gpm, and a 2 inch hole pushes 100gpm. That's a LOT.
A boat like an Amel with watertight bulkheads has an extra issue. Any hole outside the engine room can not drain to the bilge anywhere near as fast as water comes in from the ocean. So the size of the pump in the sump doesn't matter.
My own personnel takeaway is that a bilge pump that could actually keep up with a significant hole in the boat (on an Amel, maybe a broken engine raw water hose?) is larger than any recommendation I have seen, and borders on impractical. If you have a hole in the boat too big to plug with your thumb, and you can't stop it, you are sinking--eventually. (Watertight bulkheads aside, of course!)
Ratings on centrifugal bilge pumps are a pretty sad joke. They are all rated at Zero head, and that's just plain goofy. Most of them don't even supply a curve of output flow vs head, and even if they did most people would not know how to interpret it ("pressure head" is much more complex than just discharge height.) In a real world installation you'd be very lucky to get even 1/4 of the flow rate listed on the box.
All that is a very long winded way of saying... With all the variables and considerations I don't pretend to know what a "proper" capacity is. I do not even know what kind of logical criteria one would use to set one. I have seen many "authorities" and committees proclaim a number, but their logic is either (arguably) flawed or not specified.
On Harmonie we have:
- The standard Amel installed diaphragm bilge pump,
- A bilge level alarm,
- An identical spare electric pump ready to plug in as a replacement if needed,
- The Amel installed manual bilge pump,
- And a portable manual bilge pump for really serious emergencies.
I'm comfortable with that list.
Fort Lauderdale, FL
, wrote :
Thanks for the confirmation on the duckbills being nitrile in the Sealand pump. I suspect that this pump would be great for pumping the sump. What is you opinion about the capacity of this pump in regards to being the sole electric bilge pump aboard? I am used to installing centrifugal pumps in boats this size with ratings 10X higher. I think something as small as a 1/2” hole 4’ below the WL would flow almost 8 GPM….
The only rubber in the T-series pumps in contact with pumped water are the joker valves.
The sealand joker valves (they call them "duckbill valves") are made of nitrile rubber, which is good for oil contact.