AC looses prime when underway

Seems to happen when i am in any bit of a sea. 3 foot sea and it stops working. Any suggestions?? Sending this message while underway...

Not uncommon. Those pumps lose their prime easily. Apparently a bit of air is getting into the intake.

On one boat I had, a planing hull, I put a "T" in the pump intake, with a pipe straight up well above the waterline and a cheap plastic ball valve at the top. Whenever it lost prime, I'd open the valve to release the air.

Certainly common enough. As said you can put in a priming valve to make it easier to reprime.

Mine has that problem, mostly 3-4'+ seas. My solution was to put a switch on the lower helm that when "ON" the pump runs continuously, not cycling with demand from the AC's. The fix works.

When the AC's cycled off, it would get a little air with each wave, then when it restarted it had a mouthful of air. If on continuous, it can handle a little air with each wave.

The switch also makes it easier to reprime, the AC's can be off but pump on.

I did two things. One was to add a forward facing scoop to the exterior of the hull. Two was to add a Yandina bubble extractor after the pump BubbleOut.

My own preference would be to move the intake farther back (but not so far back that the props cause a problem) and to use a forward facing scoop (if it isn't what is there now) to give it some assist and avoid any suction when the water is passing by fast.

Basically, I'd want to locate the intake away from air, protect it from suction, and give it some help getting water in to widen the tolerances.

Adding things like valves and air traps may help, but my own preferences is to avoid adding parts. There is an old saying in software, "Novice programmers add corrective code; experienced programmers remove defective code." Increasingly, I've been trying to apply the idea, a.k.a. "The KISS principle", to my boat.

Yes, a good once-over of the whole system should identify any weaknesses in the design. Obviously in an ideal world these would be engineered away.

Personally, I'm OK drilling one less hole in my boat, and using a priming valve. Funny thing, I'm dealing with this right now on one of my air conditioner intakes. It's a massive 1-1/2" seacock, reduced down for the A/C, and it's not in a great location. I'll be making improvements, but for the seacock I'm stuck with what I've got.

rgano said:

I did two things. One was to add a forward facing scoop to the exterior of the hull. Two was to add a Yandina bubble extractor after the pump BubbleOut.

Click to expand...

OK, I had to check it out:


OMG that's just an "air can," AKA high-vent, for a hydronic heating system. $14 at Lowes.

Even in the fresh, nearly oxygen-free water of a home heating system, these things fail within a few years. I can't imagine putting them in a salt water system. I don't know what the attachment on top is, so I'll defer any further judgement.