Question: Here's a picture of the bilge pump installation on my boat. The printing on the top of the pump says it’s rated to pump 1,000 gallons per hour. I’m certainly no expert, but I do have a basic understanding of physics and looking at the outlet line, I see a 90-degree bend and what appears to be some sort of check valve installed in the line.

The 90-degree bend at the outlet isn't the only factor that will affect this pump's ability to perform to its rating.

The 90-degree bend at the outlet isn't the only factor that will affect this pump's ability to perform to its rating.

I’m not sure how the pump is rated and am concerned that 1,000 gallons per hour may be a bit optimistic with this installation. What do you think?

Answer: Your instincts are quite correct. That pump, as installed, will in reality probably end up with a net capacity of 40 to 50 percent of what you see embossed on it. Most pumps are rated for capacity at the outlet of the pump, without consideration of how much lift is involved. This is an installation variable dependent on the depth of the bilge and the height of the overboard discharge point. The type of hose used will also impact the actual performance. The corrugated plastic “bilge pump hose” sometimes used will have a huge impact on performance due to the surface roughness inside the hose. All bends, especially 90-degree bends, will reduce flow rate by about 10 percent. The check valve you see could also easily reduce the flow rate by 10 to 20 percent, depending upon the internal design.

The wiring to the pump can have an impact as well. If the pump is designed to run at 12 volts and there's excessive voltage drop in the wiring to the pump, then its motor will not turn as fast as intended, affecting pump flow rate.

As a reminder, though, you should not be considering your bilge pumps as damage-control pumps. If you hit a submerged object and put a six-inch hole in your boat below the waterline, believe me, your bilge pump will never save the day! These pumps are installed to remove incidental water accumulation in the bilge due to small amounts of rain water or slow leaks from shaft seals and such, not a raw-water intake hose rupture or a hole in the hull. Properly functioning seacocks and other damage-control strategies need to be employed in those circumstances.

Written by: Ed Sherman
Ed Sherman is a regular contributor to, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.