Air lock in raw water

[IslandTimeInVancouver]

I have a recurring problem with my raw water system. Anytime I sail with the motor off I develop an air lock in the raw water system.

The problem is irregular so it took a while to diagnose. It does not happen every time I sail. If the sail is short and there is little wind I don't have a problem. When I sail close hauled in 12+ knots for a bit I always get the problem.

If I start the engine after a good sail I get no water coming out the exhaust. Down below there will be a bubble of air visible in my sea strainer. Interestingly, if I close my sea cocks (yes, my raw water system has two input) clear the sea strainer and replace it, and then I open the sea cocks there is so much back pressure in the raw water system that no water comes up to the sea strainer.

My current solution for dealing with the air lock problem after a good sail is to close the sea cocks and then open the zinc protecting my heat exchanger. After I drain the water out and reattach the zinc and open the seacocks the engine starts just fine and water comes through the system no problem.

I have looked at the following in the raw water system and all appears in order:

1. Through hulls. Ordinarily, water flows well through both of my through hulls. I have dived and there is nothing blocking them on the outside. My through hulls are close to the centre line and i doubt they ever get exposed to air when I heel over.
2. The hoses. I have isolated all hoses in my raw water system and flushed water through each one. There does not seem to be any restrictions in any of the hoses. I did not find any holes in the hoses. The hoses are not brand new and some are a bit sagged.
3. Sea strainer. The strainer is clear of debris. It makes a good seal and I do not see water dripping out.
4. Impeller. I replaced my impeller to be safe. It appears to be working. (except when I get an air lock). There are no water leaks there.
5. Heat exchanger. I removed my heat exchanger and had it serviced. I have installed a new zinc.
6. Anti-siphon valve. There is a vented loop in my system. After raw water leaves the heat exchanger it goes up to a vented loop. The loop is under my sink, The vented loop's air hole is at the level of my sink drain. I have tested the vented loop. Although it is probably original it appears to be in perfect order. When I took it out I was able to blow air through the water in and out openings. I can blow air through the air hole into the tube but I cannot blow air from the either of the water entry/exit points through the air hole. This all suggests to me that the vented loop is in good order.


I have not looked into the muffler. It seems to me it could not be putting backwards pressure on the raw water system. After raw water leaves the vented loop it goes down where the water meets up with the exhaust manifold. Everything seems to be in good order there. It seems to me that pressure could not back up from the muffler up the raw water tubes. If it did the water would also be forced up the exhaust manifold and into the engine which would be very bad and I think I would know it was happening.

Any thoughts on what could be causing air to get into my system? I suspect the vented loop is somehow malfunctioning. That seems to be the only spot in the water system that could allow air in.

I don't see how air could get in through the through-hulls. The boat does not heel that much. Is it possible big bubbles of air are being whisked under my boat and being sucked into the through hulls?

Why do I have two through hulls for the raw water system? It seems unusual. The previous owner bought the boat that way. Is it normal?

I can live with this problem. I just need to make sure I don't have an airlock every time I start the engine. It's easy enough to clear. I might try sailing with my through hulls closed and see if that prevents the airlock.

-David Hopkins
Vancouver, BC
1990 Catalina 30 #5845
Universal M3-20 diesel engine

 

[Benny17441]

This seems to me like one of those problems that can not be solved by deduction alone. Something is giving the indication that it may be working properly but it is not. I think you are in the right track with the Loop Vent. You are not drawing water into the engine and the problem evidences itself all the way up to the heat exchanger that leaves very little in between but the vent loop. You have tested the vent valve but since the problem seems to be of irregular occurrence the good results of the test cannot be accepted as conclusive. I think it is time to start a process of elimination. I would replace the vent hoses and valve. I would also go ahead an replace all saggy hoses. Replacing old hoses may eliminate other types of problems latter on. I would check the impeller to insure it has not been damaged by running dry. Check the output at the pump to make sure it is adequate. If everything checks out and the problem persists then I would inspect the exhaust mixing elbow. Out of curiosity at what point do the 2 water intake lines meet and how do they meet? Thru hulls are necessary but having two to serve the same purpose is irregular.

 

[jssailem]

Do you sail with the thru hulls open or closed? It is possible that the open thru hulls are drawing the air down the vent and into your system. As water passes the opening there is a vacuum created sucking water out of the thru hull.
I'd try closing the thru hulls and see if that is the issue.

 

[JohnVTX]

I would first close one of the raw water intakes.

 

[kloudie1]

We fought a problem that presented exactly like that on Catalina 34 a couple of years ago.. We found two problems.. one was that the non-oem impeller was about a 32nd of an inch narrower than the original.. It did not seal inside the pump housing as well as the original .. Additionally, we found a saggy pump discharge hose that leaked air but not water at the heat exchanger connection .. sounds fishy but it would not leak under pressure but it would allow air into the hose.. the water would leak past the (not wide enough, new "run dry") impeller, pulling air into the pump.. It took a while to do that so if the engine was started before 30-40 minutes had elapsed, everything was fine.. much longer and the pump would not prime.. We replaced the impeller and the hose and hose clamps.. cleaned the male ends of the hose connections on the heatex and the pump.. lubed the inside of the hose lightly with swimming pool silicone grease.. worked as new ..

 

[JamesG161]

I need some more information, to see if it is an "Air lock" that is preventing pumping.
1) Is your sea water pump above or below you actual boat water level?
2) Is the pump a typical flex vane rubber impeller?
3) Was the impeller intact when you changed it?
4) From sea cock to pump inlet are there other uses, like gear oil cooler?

I can blow air through the air hole into the tube but I cannot blow air from the either of the water entry/exit points through the air hole.

If you don't have water in your engine oil or water under you sink, it is working. Typical air vent part can be opened and cleaned, inspect the oil ring and good to go.

I open the sea cocks there is so much back pressure in the raw water system that no water comes up to the sea strainer.

Is this with the strainer top open or closed?
Jim...

 

[JamesG161]

one was that the non-oem impeller was about a 32nd of an inch narrower than the original

I am betting on @kloudie1 answer of under thickness sized impeller or worn cover plate.
Jim...

PS: kloudie hit POST before I read his post.

 

[IslandTimeInVancouver]

Good suggestions kloudie1. The new impeller is an original Sherwood (not OEM) so it should be the proper fit.

Jim- Pump is below water level. Typical flex vane impeller. Impeller was 100% intact when I changed it (a miracle because my temperature gauge was off the charts and I boiled away my coolant.) Seacock 1 is by the sink drain. It traverses the engine area going towards the bow and meets up at a T with water from the second seacock. The water then rises to the sea strainer. From there it goes straight to the impeller. No gear oil cooler. I mentioned the water not coming up to the sea strainer when the strainer is closed. If I open the strainer the water does come up.

Thanks all. I'll keep tinkering.

 

[JamesG161]

Seacock 1 is by the sink drain. It traverses the engine area going towards the bow and meets up at a T with water from the second seacock.

This has puzzled all of us so far.

If there is NO other service using the sea water, open the one closest to the engine and close the other.

Here is a guess (without understanding the sea water use on the MANIFOLD of sea water)

It is possible to force air into that manifold and trap it with these conditions...
1) The manifold has a vent or opening somewhere on the line.
2) The manifold has a high point loop, above your water line, downstream of your engine take off.

Air forced under your hull can enter a thru hull and be compressed by your boats forward motion, but to flow air into the manifold, it has to have an outlet ( item 1 above).

Now to trap air, there must be a seal to hold the air (trap it) and not refill buy just the the hydrostatic pressure of your boat. (Item 2 above).

Flex vane impellers are not great air pumps.

If my guess is correct, then the easy fix is to prevent Item 1, which will not let air in the line thru the sea cocks.

Jim...

 

[jssailem]

I like the approach James is using.

It does not happen every time I sail. If the sail is short and there is little wind I don't have a problem. When I sail close hauled in 12+ knots for a bit I always get the problem. ​

So slow speed no problem.
Sailing at Hull speed problem occurs.
? Are the thru hulls left open while you sail?
If yes try sailing at speed with they hulls closed to see if the problem occurs.

 

[HMT2]

I am with Jssalem. I would try closing your thru hulls when you turn off your engine, then sail, the reopen your thru hulls when done sailing at speed and ready to start your engine.

 

[JamesG161]

Sailing at Hull speed problem occurs.

BTW to sail at greater than displacement hull speeds, you HAVE TO force air under your hull.

My bet is you have a air leak in the sea water Manifold downstream of your un-located high point loop.

As all posters have suggested for a test...

Close one or more of the seacocks and close haul again.

That would show the "compressed air" source is or is not the thru hulls.
Jim...

PS: If yes, I would make sure I couldn't start my engine until they were open again or solve the problem.

 

[jssailem]

My thought is that the boat moving quickly through the water with the thru hulls open is exhibiting a Venturi effect at the openings.

The Bernoulli Principle and its corollary, the Venturi effect, are essential to aerodynamic as well as hydrodynamic design concepts. Airfoil and hydrofoil designs to lift and steer air and water vessels (airplanes, ships and submarines) are derived from applications of the Bernouoli Principle and the Venturi effect, as are the instruments that measure rate of movement through the air or water (velocity indicators).​

The boat hull compresses the water. The movement along the hull creates a low pressure. The two openings draw the water in the lines out of the lines to the sea strainer. The vacuum continues to draw water and eventually air from the system ( past any impeller, from the heat exchanger, perhaps all the way to the vent loop). When the boat is stopped and the engine is started the air in the system maybe compressed but as was stated the impeller pump moves water better than air. When the owner removes the zinc on the heat exchanger the system vents, the the air and water is allowed to move and rise to the zinc port (I think this is like bleeding the fuel lines).
Theory. Close the two thru hulls and sail. See if the problem occurs. If not try it with one only open. It may be that the combination of two thru hulls may be the reason for the amount of vacuum created.
This is an interesting problem.

 

[Benny17441]

If your pump is located below the waterline it should fill and prime automatically when the seacock is opened. Now the question is whether it is pumping with enough pressure to blow out any air that might have gotten into the system. There are various reasons why a pump would not operate properly but it all boils down to the impeller vanes failing to create a tight seal against the pump surface. You can check the pump output flow by removing the output hose and replacing with a hose directed into a bucket. You can safely run the engine for a minute or two which should give you time to check out the flow. It should be a steady flow strong enough to shoot water out 2-3 inches, with no air bubbles present. In Kloudie's case the impeller was not up to specs but sometimes a fouled pump surface and even a worn impeller cap can cause problems. Go ahead and do the maintenance and replace old and saggy hoses, replace clamps and clean connections and insure there is adequate pump flow and you should be OK. The venturi effect is a fact but it is always present and most boats do not suffer from the problem; air compresses a lot easier than water and the water coming in from the intake seacocks should be able to push any air out of the pump and allow it to prime. I think Kloudie may have pointed out your problem.

 

[JavaJones]

Hi All. Just found this thread from 2016. I am having the exact same problem. Have just noticed it recently as we have put a new huge headsail on and are sailing faster for longer. Coincidentally a few weeks ago had the raw water pump reconditioned as it was leaking. I don't know if that would be related. I'm still doing tests, but I'm thinking the Venturi effect is the culprit. When we replaced the through hills we we advised to put the intake grate on with the grate facing backwards as we were a sailboat, ie unlikely to spend much time above 12 kts. I'm wondering if you , IslandTimeInVancouver, have managed to get to the bottom of your problem.
Our salt water cooled fridge also runs from the same through hull but I have tried isolating that and still the problem occurs. In any case it hasn't been a problem in the past. The water strainer is at water level. Any advice much appreciated.