Bilge pumps are like flares… you never need them until you REALLY need them, and when you do, you will wish you had 2 fitted!
The recent test of bilge pumps carried out by Profs. at Southampton Solent University for Practical Boat Owner has a few gems in it. I didn’t realise for instance that the recommended discharge point is at the stern just like the engine exhaust. The discharge pipe has to be as short as possible because the energy required to overcome friction can exceed the energy required to lift the water!.
Most of the pumps tested were 12v, manual switched, and under £50. The most impressive were the Attwood Tsunami and the Vetus EBP 80 . Both capable of lifting 40 litres (10 gals) of water 1 metre in 1 minute, and a good flow rate for power consumed.
Due to lift height / friction loss in outlet hoses, bilge pumps are only running at (estimate) 80% of their stated capacity. So a 1200 gph bilge pump, is in the real world only giving you 960 gph.
A wave that swamps the cockpit can easily dump 500 litres (132 gals) of water in the boat in one hit. I realise that that does not mean that the water goes into the bilge -it should drain out of your cockpit drains. But, just imagine for arguments sake that the water all goes below. The Attwood Tsunami rated at 1200gph (4,542 lph) running at full capacity would take 6.6 minutes to pump this out. If you say that friction losses cause by the length of pipe reduce efficiency to 80% then the time to pump 500 litres would be more like 8 minutes.
Of course if the water was from wave then at least you have the inter wave delay to pump water out – if the water was coming from a leak in the hull then there would be no respite to the water ingress and the pump may be fighting a losing battle, Nevertheless it would, in theory, buy you a few extra minutes to find the right sized bung!
I think I would rather blow the budget of £50 and fit a higher capacity pump such as the Johnson 2200 gph pump.