Weird AC experience

[rob71]

My boat has 2 self contained AC units. Each has its own thru hulk, strainer and pump.
After a day trip, anchored for several hours then returning home at hull speed.
Turned on the ACs and neither would pump water. Troubleshooting revealed air lock or lost prime. Cracking open the hoses at the discharge fixed the problem. Never experienced this before. The only theory I have is that while we were anchored, a strong tide was running the entire time. Do you think it could have sucked the water out from the pickups ?

 

[Captain Ralph]

My boat has 2 self contained AC units. Each has its own thru hulk, strainer and pump.
After a day trip, anchored for several hours then returning home at hull speed.
Turned on the ACs and neither would pump water. Troubleshooting revealed air lock or lost prime. Cracking open the hoses at the discharge fixed the problem. Never experienced this before. The only theory I have is that while we were anchored, a strong tide was running the entire time. Do you think it could have sucked the water out from the pickups ?

Somewhere, somehow you picked up some air under your hull. A little wake to a slight chop will do it. Nothing is going to suck the water out. This foam or air collected before or in the pump chamber.
Next, AC raw water pumps are not self priming or self clearing. If there is air in that impeller chamber, nothing is going to happen. It takes just a few air splashes to mess it up.
Look over your system forgetting it's never happened before (ship happens).
Start with; Is the pump inlet below the water line?
Is there any rise in the hose from the thru hull to the water pump inlet? <-- Important..
Is the water pump discharge on the top of the pump?
Is there a big sag of the discharge hose just after the pump?
Pictures of the install will help.

 

[Pascal]

No, current would not suck Water out … we’re the ACs on while anchored or underway? What kind of thru hull do you have ? For air con I really like having South Bay strainers (wedge with a mesh) as it always keep pressure in the intake hose. Also eliminate strainer issues

Regardless, they should have reprimed once stopped. As Ralph suggested you must have a high point in the hose between the strainer and the pump inlet. That’s a big no no with air con pumps. You want the hose to go up to the pump or air will get trapped

 

[Bob Quinn]

I also have had that happen. Strainer was clean, just needed to re-prime. Don't know why.

 

[krush]

Aliens...or Putin.

 

[whoover]

This would happen to me quite often, not all the time. Going into a 180 degree engine room to bleed the pump was always fun! Running the gen and keeping the A/C on solved that problem.

Walt Hoover

 

[q240z]

My former boat with PO hose routing always lost prime on the AC pump despite the fact that the pump and AC units were in factory locations. If I went anything faster than just off idle, the pump would lose its prime. I eventually figured out that the PO had routed the hoses in ways that guaranteed the system wouldn't self-drain. I yanked all the PO hoses and installed new ones so all of them went slightly uphill from the raw water inlet, through the pump and condensers, then up to a high point, from which the hoses went downhill to the OEM thruhull outlet.

Once I did that, I could take that 427 Ford-powered Chris Craft Commander 42 up to WOT (23kts) and never had to reprime the AC pump.