Re: Bilge pump as impeller?
Okay, so I?m going to put my head into the hornets nest with this one but?
MCM Manual #18 has a test procedure to determine if enough water flow is coming from the pickup pump in the outdrive. I assume the ?worst? case supply need of 4.5qt per 15 seconds. This comes out to 18qt per minute or 4.5GPM. That is the free flow rate, one can expect a reduction in flow from the resistances encountered in the hoses, risers, manifolds, etc..
Regardless, at 1000RPM the pickup pump is supplying 4.5GPM of water flow. Let?s assume that the water pump is magic and delivers flow squared per RPM and ignore all pumping losses. So Flow=C * RPM^2. Where C is a constant describing the pump. Here C=0.000 004 5 So then at 2000RPM Flow=0.000 004 5 * 2000^2=18GPM and at 3000RPM Flow = 40.5GPM
As far as engine RPM is concerned, I think we have to keep in mind that the boat can not be at a planning speed, how would you keep the pump in the water? And most any through hull fittings will be out of the water when the boat is planning. That being the case I think that 2000 RPM is more than sufficient given the hull speed formula of Knots=1.34* (LWL)^1/2. So a 20ft boat would have a maximum displacement mode speed of 5.99 knots or roughly 7MPH. This is not very fast, but it is motion, and you would have the power to keep the attitude of the boat under control in case of heavy seas until help could arrive. Also you would have the alternator system running in case you needed power for a bilge pump?
So, a pump that can flow 18GPM at 0? Head SHOULD work, but to be safe lets increase it by 50% for backup and say 27GPM or 1620GPH is needed. Looking at various submersible saltwater aquarium pumps there seems to be a few that fit the needed requirements. And these pumps are meant to run 24X7 for months on end.
The Model 6000 Quiet One Pump could do the job @ 1506GPH and 140Watts. The Danner Pro HY-Drive 2100GPM @ 190Watts pump could do it, and it operates at up to 22.5? of head. The Danner Mag-Drive Supreme 18 @ 1800GPM and 145 Watts would work too.
You?ll also need to power the pump, and most pumps are 120V, so an inverter is required as well. But a 250W Continuous Duty inverter is not that expensive, and can serve a number of uses. Also, the 120V power would be dangerous, but this is an emergency situation. Just don't touch the power cord unless the 12V source is unplugged.
The rest of it, plumbing, wiring, etc? shouldn?t be that hard if you set up for it on dry land so you?re prepared. Perhaps I am retarded for thinking this, but I can not see any reason this won?t theoretically work. It is for emergency situations only. I've even contemplated this before this thread... Is there any manufacturer that makes a purpose built electric or ancillary pickup pump? And if you were to carry a spare impeller pump, how would you install it in the middle of the ocean?
Boating makes engine breakdowns much more serious, so I am looking for redundancy wherever I can. And the impeller seems to be a very common failure. Although a 20HP kicker O/B would be the ultimate insurance
One more thing... Why does the pump need to match the water output to the engine speed? I think that is just an inherent result of being driven off of the engine and so it is slaved to the engine speed. There is the thermostat in the engine to take care of thermal mangment of the block so it shouldn't over-cool.
