A/C freshwater pump prime problem

[Jim Wharton]

I'm having problems getting my freshwater pump to stay primed aftersailing. When I'm at the dock the pump seems to stay primed, but if Itake it out for a sail almost every time I have problems getting thepump to prime. In the install manual it did note this could be aproblem. However, being an engineer I got to figure out why and fix it..We installed a 12K reverse cycle unit in a 1991 Hunter 30 (under frontVberth). The boat had a 1.5in seacock at the aft end of the boat (headdischarge not being used) which is about 2ft aft and to port 1.5ft theprop shaft log (as close to bottom of the boat as you can get). Wereduced the output to the seacock from 1.5in to .75in and installed astrainer. From the strainer we ran a short hose red striped to the pumpprovide in the kit. All the equipment to this point is below the waterline. From the pump output we ran hose to the A/C unit under frontVberth approx. 25ft. Then setup the water discharge approx. 7in abovethe water line.Once the pump is primed at dock the unit can be turned off and on andworks fine (even leaving the boat idle for a week (use boat onweekends). However, every time we take the boat out, we have to turn theunit on and off several times to get the pump to prime. Looking at thehoses at the pump there is air in the line (not a lot). I'm trying todetermine if the line is getting a suction (back flow) on the line, orair is being brought in due to the prop, or what?I'm open to any ideas you guys might have.Thanks Jim jim.wharton@fnc.fujitsu.com

 

[Wayne]

Some suggestions

I am in the process soon will be installing AC on my 340 and have been reading all I can as to what can go wrong. If your thru hull where the pump picks up is not near the centerline of the boat, air can get in when the boat heels. Possibly just the turbulence under the boat and some cavitation allows an air lock to develop. I would suggest closing the thru hull before leaving the dock and then open it just prior to starting the AC. I installed a dedicated thru hull for my water intake. I installed it adjacent to the keel. I hated to put another hole in the hull but it was the only way to go. I was going to make an arrangement to share the head water pickup thru-hull but decided not to as it would come out of the water when the boat heels and I would have to reprime the pump when back at the dock. There were warnings about this in material I read on the MarineAir web site. Good luck with your problem. My system came with a bleeder valve which you install on the outlet of the pump to help in priming the pump.

 

[Jim McCue]

prime

On MY 28.5 I initially had similar prime loss after heeling with low tank water level.On MY 28.5 the pump wat located on the side of the forward berth. I emptied the water tank, put a clear vynl tube on the tank side of the connect and made it ( the tubing) "loop" like a "sink trap" before it got to the supply side of the pump. This allowed me to manually and easily bend and twist any air bubble out of the loop.