Bubbles in A/C strainer

I'm trying to find the leak in this A/C strainer assembly. I'm calling it a leak because the entire assembly is below the water line and after cleaning it out and opening the seacock it fills with water and pushes water up another foot to the pump. The air bubble only show up when the pump starts pulling water. On the output side I'm getting around 4.0 to 4.5 gpm so everything is working ok.

So my theory goes like this. Once the pump starts, it finds pulling air in easier than pulling water and it so it pulls some air in from a poor connection.

Given that theory it seem like some water leak would show up when the seacock is opened up.

I've been fighting this leak for months. I've disassemble everything several times and replaced everything I could several times but I'm not having any luck.

Any ideas?

The Brockerts

Just out of curiosity, do you have a mermaid condensator or other venturi device draining your drain pan?"if so, which side of the strainer is it on? (It should be on discharge side.)

The only place it could suck air is between the thru hull and the pump. After that it is under positive pressure from the pump.

Is it happening at the dock or underway? If the intake gets air or churned water, such as while underway or if too close to the water line, it can suck air from the outside.

Is the gasket on the strainer basket in good shape?

How quick do the bubbles reappear? If it's repeatable at the dock then I'd be tempted to try something like smearing grease or vaseline on the threads when you reattach the basket and see if that stops the bubbles (for a while at least). That and/or use some silicone 'rescue' tape wound tightly around the threading part of the basket to help act as an added seal. If either of those calm the bubbles then I'd look at replacing the whole strainer assembly (cheaper than your time fighting with troubleshooting it any further).

But if it's only reproduceable under way then I'd concur with the idea that it's cavitation from where the seacock goes through the hull. Either due to the seacock's profile on the outside, or due to the amount of water being disturbed ahead of it while in motion.

All of this appears at the dock. I've not tested it underway. The bubbles appear within a minutes or two from when the pump starts. I've used 2 completely different strainers and got the same problem. Then tried to interchange the parts, got the same thing. The only thing that has not be replaced is the seacock and hoses. I've capped the hose out of the seacock and then opened the seacock. Left it open for day and no water leak.

I'll try the tape and/or vaseline. After that I'll go after the hoses. All seem rather strange.

The Brockerts

Maybe wipe some vasoline around the suspect areas and see if it goes away. Then you may be able to fix it if you can identify where it is coming from.

I mean, I suppose there's some potential for cavitation within the strainer itself. Have you asked the strainer vendor about it? There's not typically a vent or relief valve on the top of a strainer, so there's the potential of a pocket of air remaining caught up there. The flow dragging past it could cause enough turbulence for it to bubble, but you'd think it would eventually get sucked clear by the amount and speed of the flow.

And you, sir, get extra points for OCD concerns about things on the boat.

All good ideas. I think I re-work the installation tomorrow and install it upside down. That way I'll be able to make sure all air is out of the assembly from the start.

As for OCD, I'm in corpus christi, Texas. Low temp in 80's high temp in the mid 90's and humidity running around 90%. A/C is as important as water, so yes I provide care, feeding and OCD to my 4 A/C systems....LOL

The Brockerts

Heh, my Dad was stationed in Corpus for WW2 and had many things to say about the heat and humidity. And this was from someone that grew up in mid-Atlantic Baltimore, which gets it's own share of heat/humidity.

That and the bugs, 'big enough to steal things from you' as I recall him saying...

If you disconnected the hose running from the strainer to the pump at the pump and lower or keep it below water level, how freely does water flow?

What about if you disconnected the hose from the thru hull to the strainer at the strainer?

Maybe there is a bad clog, e.g. hard growth, for example in the the thru hull, seacock, or inlet to the strainer that is increasing the suction the pump is applying to the strainer, making it more likely to pull air past seals, e.g. at the strainer cap, hose, or fittings?

Are there any signs of leaks when the unit is off, but seacock is open?

Ultimately, unless there is more going on than I understand, the leak s coming from somewhere between the thru hull and the pump. Everything else is under positive pressure and would push water out, not suck air in. If this were above the water line, water could drain out letting air in, but that isn't the case here.

I question the wisdom of putting vaseline on any 'rubber' parts. I use GLY (waterproof silicone grease.) on my o'rings and hoses.

I do agree that the problem is between the hull valve through the strainer.
If you haven't done so, replace the hose from the hull valve to the strainer and the hose between the strainer and the pump. More than likely the problem is the strainer inlet or the strainer seal or perhaps a minor crack in the strainer body.
If after replacing the hoses and the problem persists, I suggest you replace the filter body. If I am wrong, at least you will, have a spare filter body and filter.

To be honest, I am not impressed with that 'plastic' filter body but, you do what you have to do.

What size feed to the pump and what size pump?


After checking for restrictions as in post #9....no hydraulic engineer here... but if mismatched, maybe cavitation at the pump impeller.

Thanks everyone for idea's. This problem has been solved. Lesson's learned. Took all of this systems apart. To many mismatches of hose and fitting. Everything is now 3/4 inch except for the last hose out that is 5/8. Found some adapters that were wrong size and hose's just squeezed on with hose clamps. Once everything was 3/4 water flow was smooth.

Lesson learned. You need to watch over everything on your boat. I was out on medical leave while this new A/C system went in and did participate in the installation. My normal process is to do it myself or at the very least work side by side with the contractor I've hired.

Thanks again.

The Brockerts

So, leaks or just cavitation due to size mismatches and resulting turbulence?

Glad you seem to have eliminated the problem!

wkearney99 said:

So, leaks or just cavitation due to size mismatches and resulting turbulence?

Glad you seem to have eliminated the problem!

Click to expand...

Hard to say what the exact problem was because I changed out several parts. I had 3/8, 1/2, 5/8 and 3/4 inch fittings and a reducer in the flow path. Probably a combination of fitting and cavitation.

The Brockerts