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Old 18-03-2024, 18:33   #1
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Seawater strainer filling with air

I'm puzzled. I recently installed a Seaflo sea strainer ( picture below, and very similar to the Vetus FTR straners) . I like the clear lid to see what's going on and make visible the state of the strainer basket.

The problem is that it keeps running dry! If I start with a full strainer of water, it runs dry in just a few hours at cruising rpm on my Yanmar 4jh2e 50- hp engine.

Now, to be clear, this strainer is actually slightly BELOW the waterline. I know the manual says to install above the waterline, but this is the same vertical hose from the ball valve, so the original builder seemed happy with it.

So, at any time, and at any rpm, when I crack that lid open I can refill the strainer. And I have used Molycoat silicone lubricant/sealant on the wide, flat, rubber seal and threads for the lid. I believe that lid is airtight! ( with the ball valve closed and the strainer full I can open the impeller pump( below the strainer level) and that water just stays up there in the strainer).

So...what's going on? Where is the air coming from? What do I do? Thanks for any advice!
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Old 19-03-2024, 08:18   #2
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Re: Seawater strainer filling with air

Somebody told me that the air gets into the strainer from Bubbles that travel under the hull when underway. I must say the I not tried running the engine at cruise rpm in idle with the boat stationary for several hours to test that theory!
Even if the air enters from under the boat, why doesn't the raw water pump just "ingest" and pass the air to draw in more seawater? The raw water pump seems to be OK
The impeller is fine, and after I check the impeller and close up the empty pump- full of air- it draws water ( from a full strainer) without difficulty.

The problem behind this, of course, is that after several hours of running and the strainer filling with air, there is no water supply to the engine and it starts to overheat.

So, again, any ideas of what to do? Does everyone have to babysit their sea strainers like this? Thanks for any ideas...
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Old 19-03-2024, 10:04   #3
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Re: Seawater strainer filling with air

This is an interesting problem and I don’t know the solution. That said, it is not normal for raw water strainers to draw air. We use a Groko strainer that has a clear cap and body so the level is visible and you can monitory debris in the strainer. If we are long term motoring I check the engine a couple times per day and there is never air in the strainer. Whatever air that may enter the strainer due to sporty sea conditions gets pulled through the pump.
Flexible impeller raw water pumps are robust but water acts as a lubricant and coolant. Intermittent air will eventually damage the impeller. Again IMHO a healthy flexible impeller raw water pump will draw air through the system (unlike many pumps).
If it was me, I’d start by tightening the hose clamps and throughly look at each component. Take a hard look at the raw water pump for seal leakage. With the engine off ,run your hand around the base of the pump to feel for water leakage. Any seal leakage, overhaul or replace the pump.
Take out the pump impeller for inspection, put a strait edge on the pump face place to look for damage. From my past experience and gut feeling I’d focus on the raw water pump.
I change impellers yearly and overhaul or replace the raw water pump every 1000 hours. Yanmar probably has a suggested service interval which would be a good place to start. At minimum replace the impeller and lubricate it with impeller lube prior to startup.
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Old 19-03-2024, 12:26   #4
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Re: Seawater strainer filling with air

I feel like the most likely cause is the filter lid. I understand you are confident that is not leaking, but I would revisit that. Maybe overtightening is distorting the gasket?

Is the filter rated for the flow rate of your raw water pump? That is, is it undersized such that it ends up with more internal vacuum than intended?

I cannot see how a raw water pump issues would cause the filter to fill with air.

Are you always getting a good flow of water out the exhaust, at least until the filter becomes air bound?

I assume you did not have a similar issue with your previous filter. The seacock is far enough below the waterline that you are not introducing air while rolling? I had a poorly placed seacock for an air conditioner system that would drain when the boat healed.

Good luck! I know these kinds of problems can be a challenge to solve.
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Old 20-03-2024, 13:00   #5
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Re: Seawater strainer filling with air

Fellows- Thank you for your detailed replies- they have led to much discussion on board over the last 24 hours.

In summary, given your input and our experience, my thinking is that we have an air leak in the strainer area combined with some inefficiency in the raw water pump which is preventing the pump from passing the air once the strainer is empty.

On the leakage area, we're going to try some additional non-permanent seals to the strainer lid, like electrical tape, wrapping in saran wrap, or rescue tape. The hose clamps are already tight. I may also wrap the vertical hose leading from the ball valve to the strainer in rescue tape. That is a 25 year-old hose and one of the few I didn't replace when we bought this boat two years ago.

On the raw water pump air-pumping efficiency, I'll pull the impeller and lube, check the cover plate, consider flipping the cover plate over. I have already considered taking the pump home for rebuild when our cruise finishes in June.

So the theory, again:
Air leaks in only when the lid is closed and under significant negative pressure from sucking water at cruise rpm
All is fine until the air displaces all the water in the strainer and then the pump inability to suck only air causes loss of water flow and engine overheat.

Other notes:
I don't like it, but you can't observe the raw water exhaust on this boat because the exhaust is underwater.
The previous Sea strainer also caused occasional overheating, but we always assumed it was the weed I would find at the inlet to the strainer. That strainer had an opaque cover suit was impossible to see inside.

Well, enough for now. It may take 7-10 days to get to all these items- company coming for a week- but I will report back! Thank you again
..
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Old 20-03-2024, 13:10   #6
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Re: Seawater strainer filling with air

Thanks for the updates. Your plans for continued troubleshooting sound reasonable to me.
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Old 20-03-2024, 14:37   #7
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Re: Seawater strainer filling with air

Quote:
Originally Posted by Cap Bill Des View Post

In summary, given your input and our experience, my thinking is that we have an air leak in the strainer area combined with some inefficiency in the raw water pump

On the leakage area, we're going to try some additional non-permanent seals to the strainer lid, like electrical tape, wrapping in saran wrap, or rescue tape.
So the theory, again:

Air leaks in only when the lid is closed and under significant negative pressure from sucking water at cruise rpm
..
If there were inefficiency in the raw water pump it wouldn't be trying to draw so hard.
Before you spend a lot of time and effort with all kinds of tape and such try this simple thing.
Get a container of "water faucet grease" at the local hardware store.
Liberally coat the gasket and threads of the strainer lid.
See if that works.
If not, fill a squirter-bottle with water and food coloring, when the strainer starts filling with air squirt the colored water all over the strainer, see where the colored water is drawn into the strainer.
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Old 20-03-2024, 15:36   #8
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Re: Seawater strainer filling with air

Do you have a water intake thruhull with attached/built-in strainer?
Is your 25 yr old suction hose collapsing or somehow sucking air?
Some types of old suction hose can delaminate internally.

When you remove the strainer cap with engine running at cruising rpm,does it unscrew easily,or is there vacuum holding it down.
With cover off,place hand over filter.Is there vacuum?
Cheers/Len
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Old 12-04-2024, 12:04   #9
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Re: Seawater strainer filling with air

This is Bill, the original thread poster, with an update.

On the strainer itself, we carefully examined the flat rubber washer that seals the bowl and lid, and found some non-uniformity in the sealing footprint. This led to two actions:
- The molded plastic strainer basket had two spots with a 1 mm raised edge that appeared to not be part of the design. Concern that these were interfering with lid-to-bowl seal. These were dremeled off to be flush with rest of basket.
- We cut and added, with silicone grease coating, an addition sealing washer - from gasket mayerial we carry- on top of the rubber washer.

For about 6 engine hours, spread over several days, these actions seemed successful. The strainer stayed full.

Yesterday we sailed about 40 very fast miles in just over 5 hours- 20 knots of wind just behind the beam, 4-6 ft seas on the aft quarter, some heeling but not excessive. Yes, averging almost 8 knots in a 42 ft monohull.

As we sailed into harbor, flat water, I looked at the strainer before starting the engine. It was not empty, but surely no longer full. I thought of the boisterous sIl and did air get under the boat and up into the strainer, displacing strainer water back down the hose into the ocean.

Started the engine and it didn't sound right- strainer was empty and no water flowing!

We quickly anchored under sail. Cracked open the strainer lid and of course it fills with water as usual.

This morning checked the water pump. Impeller vanes all fine. Noticed that the cover plate had burned-on evidence of earlier dry-and-failed impellers. Cleaned cover, lubed inside of cover and o-ring, and put back on. Pump now full of air of course.

Started engine, immediate water flow in strainer.

My summary guesses, from my earlier theories in my post above:
I think we may have made progress on sealing the strainer
I think the boisterous sail put air up into the strainer
I think/hope that the burned residue on the inside of the cover decreased it ability to pump air and self-prime.

Comments? It will be several days before we move the boat again, but I'll come back with an update!
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Old 12-04-2024, 12:07   #10
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Re: Seawater strainer filling with air

Oh, I forgot to mention, we did put in a brand new 9" piece of 1.5" ID "Wet exhaust" (spiral wire) from the ball valve to the strainer.
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Old 12-04-2024, 12:35   #11
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Re: Seawater strainer filling with air

Good news-progress!
Your raw water pump should have no problem sucking seawater,instantly,from the thru hull to the pump,even if that complete hose & filter circuit was bone dry & sucking it from several feet below pump. It is a self priming pump.
This assumes that you have zero air leaks in the suction circuit & that the pump outlet is not blocked/severely restricted.
Sounds like you have sealed the filter cap.
Interesting about the pump cover.I know the cover needs to be perfectly flat & the impeller needs to just scuff the cover in order to suck properly.Water prevents impeller from melting against the cover. Some people sand their cover by rubbing it over a sheet of 220 grit lying on a flat surface.
I can't understand losing much water from the filter during your sail.Perhaps a little,due to gravity-but your impeller to cover clearance may have allowed some air to be sucked into pump via pump shaft seal?? Just a shot in the dark In any event,your pump ,being self priming,should instantly draw water,even if system has gone dry.
Cheers/Len
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Old 13-04-2024, 03:56   #12
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Re: Seawater strainer filling with air

Could the cooling water drain-down be the result of a siphon [Bernoulli principle] effect?
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Old 13-04-2024, 08:31   #13
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Re: Seawater strainer filling with air

I don't have any new suggestions for your immediate issue, but I do have 2 that can help with early warning of low raw flow.

1. What is the condition of your anti-syphon valve? It could well need cleaning as salt often plugs them up. In my case I removed the valve and plumbed in a hose to an outlet in the hull where I could see it. When flow is good, a small piddle results letting me know the raw water flow is OK.

2. I also added an Aqualarm to the cooling system. This alerts should the flow reduce and lets you know something is wrong well before the engine overheats. Moreover it is "watching" all the time even when you are not paying attention to the engine temp or the piddle.

https://aqualarm.net/index.php?main_...products_id=54
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Old 13-04-2024, 10:28   #14
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Re: Seawater strainer filling with air

First, this is 100% normal and expected with an intake plumbed like yours.

When you are sailing, even at modest speeds in flat water, air is entrained under the hull and flows back toward the stern. When these air bubbles reach the vertical thru-hull they do exactly like air normally does, and rise up into the strainer. Since there is no way out for it, air continues to accumulate until, in the limit, the entire system is full of air. Sailing fast in rough water this will happen quickly, slow sailing in flat water, very slowly, if at all.

No need to invoke Berrnolli or syphons, which are actually two completely different phenomena and unconnected with this. The fact that air rises in water is all the physics you need to understand.

The good news, as has been pointed out above, your engine's raw water pump will blow this out in seconds without an issue or harm. Most positive displacement pumps will not have a problem here.

If you have a large volume of air, and a very small pump--such as the size raw water pump that might be on a small generator--clearing the air can take long enough that the pump suffers reduced life span from running dry. In the case of intake strainers, bigger is not always better! Match the rated flow of the stainer to the flow rate needs of the user.

This can also be an issue if you have centrifugal pumps that are not plumbed correctly, and they can get air locked from this effect. In this case "correctly" means the flow rises continuously from the trru-hull to, and then past the pump.
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Old 13-04-2024, 10:46   #15
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Re: Seawater strainer filling with air

It is good practice to immediately check your oil pressure gauge & your water flow out the exhaust Thru hull each time you start engine.
Alarms,as noted,are good also. /Len
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