Air entering raw water strainer.

[Scoid]

I have an older Groco SVS-1000. I have rebuilt it several times and so has my mechanic.

The problem is air keeps getting into the strainer. Usually, it runs at about half full of air and half full of water (I'm an optimist).

Now here's the weird part.........It only happens when the boat is moving.

I can run it at full power at the dock for an hour and not a single bubble. But move along at 7 knots and the air comes in. There are no items ahead of the intake that could cause cavitation. Hose replaced. Pump checked for leaks. Etc., etc.

About the only thing I can figure is that there is cavitation happening at the intake. Had a diver check it. Couldn't see anything.

Has anyone ever had this problem?
Did you fix it?
How?

Thanks!

 

[Larry M]

Add a scoop? It sounds like you have air bubbles running along the hull while underway. Where is the intake and how low is it below the water line? On planning vessels they are added frequently.

 

[Comodave]

Try slathering silicone grease around the O ring to make sure that the air isn’t coming in around the seal.

 

[Scoid]

Add a scoop? It sounds like you have air bubbles running along the hull while underway. Where is the intake and how low is it below the water line? On planning vessels they are added frequently.

It's a GB 32 with a 3'9" draft. The intake is probably 2' under the waterline. the intake is scooped. It's the brass one with 4 slots and the uptake pipe in the back. No planing. 7.5Kts cruise.

 

[Scoid]

Try slathering silicone grease around the O ring to make sure that the air isn’t coming in around the seal.

All slathered up. It's got more silicone than Ivanka Trump.

 

[STB]

Is the strainer above or below the waterline? Or is the waterline right at the "fill line"? Is the intake clear? does it rapidly fill to the waterline when the cap comes off? Or could the seacock be stuck or intake partially clogged?

 

[Arc]

Cavitation is not always air mixing with water . It is often pressure change that boils water making air. Also just because you do not see water leaking doesn’t mean you do not have an air leak. Under vacuum air can actually come through a hose and for sure come through a hose barb. We see this often without ever seeing a drop of water .

 

[Scoid]

Cavitation is not always air mixing with water . It is often pressure change that boils water making air. Also just because you do not see water leaking doesn’t mean you do not have an air leak. Under vacuum air can actually come through a hose and for sure come through a hose barb. We see this often without ever seeing a drop of water .

Agreed. Why then would this only happen when underway?

As I said in my first post, I have run at full cruise power at the dock for an hour with no bubbles. Replaced and lubed all seals several times. Replaced hose to pump. Double clamped both ends. Had pump checked by mechanic.

 

[Arc]

Agreed. Why then would this only happen when underway?

As I said in my first post, I have run at full cruise power at the dock for an hour with no bubbles. Replaced and lubed all seals several times. Replaced hose to pump. Double clamped both ends. Had pump checked by mechanic.

Probably because the water is not static like at the dock. Your pump is pulling maybe 10in/ of vacuum that means air has the opportunity to to enter being much thinner the water.

 

[Scoid]

Probably because the water is not static like at the dock. Your pump is pulling maybe 10in/ of vacuum that means air has the opportunity to to enter being much thinner the water.

Huh? I don't follow.

 

[jgwinks]

Is the scoop facing forward or aft? It's counterintuitive but they are supposed to face aft. Most of them get installed backwards. Facing aft they resist bubbles or anything else but water getting sucked in.

 

[Arc]

Huh? I don't follow.

For the moment let’s assume there is a permeable hose, fitting, gasket. Something that can allow air through but not water[ water much dense then air ] . At the dock, water is forcing itself up into the seacock by boat displacement alone. Op an run the engine all day no air.
Now the boat is underway. Water is traveling under to boat at speed. Like blowing across the top of a bottle it makes vacuum. The pump is also making vacuum trying to pull water. If it can find a weak link in the supply line it will suck in air and mix it with whatever water it can pull. I deal with this every day. Finding that air source is the problem. It could even be the drain screw on the bottom of a strainer or the stem of the seacock valve . They will not leak water but will definitely suck air.

 

[Scoid]

Is the strainer above or below the waterline? Or is the waterline right at the "fill line"? Is the intake clear? does it rapidly fill to the waterline when the cap comes off? Or could the seacock be stuck or intake partially clogged?

Hard to say exactly where the waterline is in relation to the strainer. It does fill very rapidly when the cap is loosened. So it's likely below the water line(?).

As I said, I had a diver check the intake. He didn't see, or feel, anything but it is not easy to see inside this type of scoop (described earlier).

The SVS uses a rotating black rubber cylinder that has a hole through it. Due to it's, age I suppose it might not be opening all the way when the handle is open. But I do keep it well lubed and it's no hard to turn. This cylinder is no longer available and hasn't been for some time.

My next step is to have a diver put a cup over the intake while I disassemble the SVS valve in the boat to see if there's anything in the pipe that goes thru the hull.

 

[Scoid]

For the moment let’s assume there is a permeable hose, fitting, gasket. Something that can allow air through but not water[ water much dense then air ] . At the dock, water is forcing itself up into the seacock by boat displacement alone. Op an run the engine all day no air.
Now the boat is underway. Water is traveling under to boat at speed. Like blowing across the top of a bottle it makes vacuum. The pump is also making vacuum trying to pull water. If it can find a weak link in the supply line it will suck in air and mix it with whatever water it can pull. I deal with this every day. Finding that air source is the problem. It could even be the drain screw on the bottom of a strainer or the stem of the seacock valve . They will not leak water but will definitely suck air.

I have tightened both drain plugs considerably. I have replaced all seals three times. I have replaced the hose and clamps and had my mechanic check the pump seals. Is there something else I can do or check for an air leak?

 

[Scoid]

Is the scoop facing forward or aft? It's counterintuitive but they are supposed to face aft. Most of them get installed backwards. Facing aft they resist bubbles or anything else but water getting sucked in.

It's facing forward. However, this boat is 30+ years old and never had this problem until 6 months ago.

 

[Arc]

The seacock valve stem is often overlooked. Did you remove the handle and repack the stem? If it is a spring loaded wedge valve it may need to be lapped .

 

[firstbase]

Went through this same thing with my 2 SVS1000 feeding Lehman 135s. Original to the boat, 1987. Water flow would diminish as RPM's went up. I could not run at WOT for more than a minute or two before temps started climbing. Backed off on the RPM's and everything was fine except the strainers stayed at 1/2 full. Loosened the wing and they would fill right up but go back down in a few minutes. I tried all of the above solutions but finally got tired of it and replaced both. Very glad I did. When I got them out on a table in the sunlight they were in very poor shape. Much worse than they appeared to be when installed in the engine room.

FYI, I don't "run" at WOT. Just check it out for a couple of minutes every year or so.

 

[O C Diver]

Had to look up a Groco SVS - 1000.
IMO, any design that can trap a substantial percentage of the strainer's capacity with air, is a poor design or at best outdated. I need a strainer that will self fill with water, not self empty.

Now that you see the potential problem first hand, change the strainer before it causes a more expensive problem.

Ted

 

[C lectric]

What are the seals for the strainer made of? If cork they can leak and seal poorly even when new.
I modded my even older Perko strainer years ago to use O rings. Never had a problem since and it was done close to 25 yrs ago. Perko eventually offered nitrile rubber seals but I had already gone my route.


I agree about the air entry that if there is ANY gap or poor fit of the parts
that air can be sucked in.
--This includes valves at the stem. If you have gate valves they are prone to this. The stems can often be resealed with a new piece of sealing twine. THe nut just below the handle needs to be adjusted periodically to maintain a tight seal. Gate valve though should not be used for this at all. My opinion.
--hoses to old or damaged.
--loose clamps.
--Scratches in the hose barb fittings.
--through hulls that use a gasket between the actual through hull and an elbow to redirect the water flow horizontally.. This happened to me on a previous boat.

Hope this helps.

 

[STB]

It really does seem like it has to be a suction leak. The good news is that it it is most likely at a point between the intake thru-hull and the raw water pump, because everything after the raw water pump is positive pressure and the intake thru-hull is the beginning of the relevant part of the system. And, it has to be inside of the boat, because water is outside of the boat (assuming it isn't some type of cavitation or air bubble movement that is unlikely at trawler speeds).

I tend to be an Occum's Razor type of person. I'm betting there is something wrong with the strainer other than the seals that you replaced. My next bet is a problem with one of the two hoses. And, my last bet is a problem with one of the fittings between from thru-hull up up to the raw water pump.

I also tend to be a person that doesn't like throwing parts at things if there is a way to prove the failure first, if for no other reason than to be able to prove I fixed it. Is this a twin screw? Can you swap strainers and see what happens?

I wouldn't let it run too long underway with that air in it. If the strainer isn't the highest part of the loop, there could be a lot more air north of it that you can't see.

If it filling up with water rapidly to the top when the cap is off, it isn't unduly constricted on the intake side. And, since it filled to the top, or at least above that waterline, it isn't draining back down filling with air from the discharge to the waterline (which is really, really unlikely in any case, but a theoretical possibility).

Personally, I agree with those who are guessing that, when not underway, it is sealed up well enough, but once underway, something about the dynamics of the boat, e.g. Bernoulli Principle, cavitation, efficiency of pump, etc, is decreasing the relative pressure of the cooling loop to the air around it, causing the cooling loop to suck in air.

The thing that I'm not understanding is why it always seems to fill up to air to the same point, if that is the case (vs me misunderstanding, the cruises just being about the same length, etc).

 

[Scoid]

The seacock valve stem is often overlooked. Did you remove the handle and repack the stem? If it is a spring loaded wedge valve it may need to be lapped .

No. But I have lubed it through the drain by putting a zerk there. I lubed it until grease came out the t-handle threads. The boats in the water. I might have a diver cup the hole while I open up the valve but if the rubber cylinder is shot I might be in trouble.

 

[Scoid]

It really does seem like it has to be a suction leak. The good news is that it it is most likely at a point between the intake thru-hull and the raw water pump, because everything after the raw water pump is positive pressure and the intake thru-hull is the beginning of the relevant part of the system. And, it has to be inside of the boat, because water is outside of the boat (assuming it isn't some type of cavitation or air bubble movement that is unlikely at trawler speeds).

I tend to be an Occum's Razor type of person. I'm betting there is something wrong with the strainer other than the seals that you replaced. My next bet is a problem with one of the two hoses. And, my last bet is a problem with one of the fittings between from thru-hull up up to the raw water pump.

I also tend to be a person that doesn't like throwing parts at things if there is a way to prove the failure first, if for no other reason than to be able to prove I fixed it. Is this a twin screw? Can you swap strainers and see what happens?

I wouldn't let it run too long underway with that air in it. If the strainer isn't the highest part of the loop, there could be a lot more air north of it that you can't see.

If it filling up with water rapidly to the top when the cap is off, it isn't unduly constricted on the intake side. And, since it filled to the top, or at least above that waterline, it isn't draining back down filling with air from the discharge to the waterline (which is really, really unlikely in any case, but a theoretical possibility).

Personally, I agree with those who are guessing that, when not underway, it is sealed up well enough, but once underway, something about the dynamics of the boat, e.g. Bernoulli Principle, cavitation, efficiency of pump, etc, is decreasing the relative pressure of the cooling loop to the air around it, causing the cooling loop to suck in air.

The thing that I'm not understanding is why it always seems to fill up to air to the same point, if that is the case (vs me misunderstanding, the cruises just being about the same length, etc).

Single screw. Only one hose with an SVS. Not always the same point. Usually somewhere around half. I can see a small bubble getting sucked in to the engine quite often.

 

[Scoid]

What are the seals for the strainer made of? If cork they can leak and seal poorly even when new.
I modded my even older Perko strainer years ago to use O rings. Never had a problem since and it was done close to 25 yrs ago. Perko eventually offered nitrile rubber seals but I had already gone my route.


I agree about the air entry that if there is ANY gap or poor fit of the parts
that air can be sucked in.
--This includes valves at the stem. If you have gate valves they are prone to this. The stems can often be resealed with a new piece of sealing twine. THe nut just below the handle needs to be adjusted periodically to maintain a tight seal. Gate valve though should not be used for this at all. My opinion.
--hoses to old or damaged.
--loose clamps.
--Scratches in the hose barb fittings.
--through hulls that use a gasket between the actual through hull and an elbow to redirect the water flow horizontally.. This happened to me on a previous boat.

Hope this helps.

Rubber. No elbow on an SVS. None of the other items apply.

 

[Scoid]

Went through this same thing with my 2 SVS1000 feeding Lehman 135s. Original to the boat, 1987. Water flow would diminish as RPM's went up. I could not run at WOT for more than a minute or two before temps started climbing. Backed off on the RPM's and everything was fine except the strainers stayed at 1/2 full. Loosened the wing and they would fill right up but go back down in a few minutes. I tried all of the above solutions but finally got tired of it and replaced both. Very glad I did. When I got them out on a table in the sunlight they were in very poor shape. Much worse than they appeared to be when installed in the engine room.

FYI, I don't "run" at WOT. Just check it out for a couple of minutes every year or so.

Hi Hal,

1988 GB32 Lehman 135. Thanks. Did you ever try removing the rubber cylinder and sanding it, lubing it, and reinstalling it. I’m now thinking after 34 years that rubber cylinder has probably succumbed to old age. We’re yours in bad shape?

 

[STB]

Well, if you see bubbles getting sucked in the direction of the engine quite often, that suggests the air is coming from the somewhere from the thru-hull up to, but not including the discharge of the strainer. So, your search space just got smaller.

You probably can't tell, but if you were able to see bubbles moving up from the drain plug or from the intake, that could limit the search space more.

So, suspects...

Strainer cap, including threads on the cap; threads on top of strainer for cap; strainer top gasket; strainer drain plug o-ring; strainer drain plug; strainer threads for drain plug; strainer bowl or bronze work; intake-barb on strainer; strainer threads for intake barb; strainer intake hose; seacock and related fittings; thru-hull and related fittings; any of the interfaces or hose clamps along the way.

If you've got enough extra hose, you can try cutting off a little bit on each end and reclamping using new clamps and, when you do, taking the opportunity to inspect the barb on each side and any other fittings along the way, and to reseat the barb on the intake side and any other fittings using new paste/tape.

 

[Greg S]

Find the leak and replace that item.
Buy a roll of plastic wrap. Tightly wrap the entire outside of one of the potential air leaks and go for a cruise. If that stops the air from coming in, you found the culprit. If not, then tightly wrap the next item. Repeat.

The item must be securely wrapped. The plastic wrap is flimsy enough that it will pull into any suction point and seal it.

 

[Scoid]

Well, if you see bubbles getting sucked in the direction of the engine quite often, that suggests the air is coming from the somewhere from the thru-hull up to, but not including the discharge of the strainer. So, your search space just got smaller.

You probably can't tell, but if you were able to see bubbles moving up from the drain plug or from the intake, that could limit the search space more.

So, suspects...

Strainer cap, including threads on the cap; threads on top of strainer for cap; strainer top gasket; strainer drain plug o-ring; strainer drain plug; strainer threads for drain plug; strainer bowl or bronze work; intake-barb on strainer; strainer threads for intake barb; strainer intake hose; seacock and related fittings; thru-hull and related fittings; any of the interfaces or hose clamps along the way.

If you've got enough extra hose, you can try cutting off a little bit on each end and reclamping using new clamps and, when you do, taking the opportunity to inspect the barb on each side and any other fittings along the way, and to reseat the barb on the intake side and any other fittings using new paste/tape.

Brand spanking new hose, new double hose clamps each end. No threads on cap. SVS uses a wing nut. All seals new. Replaced 3 times. Always lubed with silicone grease. Drain plugs tight. Etc., etc. Everything you mention has been checked and double checked. The one possibility you mentioned is the rubber cylinder in the valve. I will have to check that at haul out because if it’s cracked or disintegrates I’ll be taking on water otherwise. The SVS is a seacock and valve combination unit. I’ve had the strainer part out of the boat and the valve did not leak water when the strainer was separated. Still, I wouldn’t pull the cylinder out unless I was sure I could get it back in. I’ve checked or replaced everything else you mentioned. I’ve discussed the problem with Groco several times. I guess a haul out is in order. I appreciate your help and that of everyone who replied.

 

[Scoid]

Find the leak and replace that item.
Buy a roll of plastic wrap. Tightly wrap the entire outside of one of the potential air leaks and go for a cruise. If that stops the air from coming in, you found the culprit. If not, then tightly wrap the next item. Repeat.

The item must be securely wrapped. The plastic wrap is flimsy enough that it will pull into any suction point and seal it.

Thought about that but if you’ve ever seen an SVS you know it’s not possible. Thanks though!

 

[STB]

Brand spanking new hose, new double hose clamps each end. No threads on cap. SVS uses a wing nut. All seals new. Replaced 3 times. Always lubed with silicone grease. Drain plugs tight. Etc., etc. Everything you mention has been checked and double checked. The one possibility you mentioned is the rubber cylinder in the valve. I will have to check that at haul out because if it’s cracked or disintegrates I’ll be taking on water otherwise. The SVS is a seacock and valve combination unit. I’ve had the strainer part out of the boat and the valve did not leak water when the strainer was separated. Still, I wouldn’t pull the cylinder out unless I was sure I could get it back in. I’ve checked or replaced everything else you mentioned. I’ve discussed the problem with Groco several times. I guess a haul out is in order. I appreciate your help and that of everyone who replied.

Shoot. Yeah. I was misremembering which model. I just looked at the parts diagram. If you've replaced all the seals and squared away the one hose....I hate to say it, but I think it is time to replace it.

I'd recommend doing it sooner than later. If I work with them on timing (not just time of day, but finding a quiet day for them), the yard over here will often do a "short haul" for quick maintenance like this. They have the parts ready and just take care of it in the sling and put me right back in the water. I'm in and out in a couple of hours or so. They usually give me a break on the haul for not needing to block it, but not always.

I've done things like this in the water, but I don't recommend that for the faint of heart or those without lucky charms. In particular, I had all of the parts on hand, disconnected/loosened everything up on the inside, and then snorkeled underneath and plugged the external strainer with toilet wax. I made the wax into a "very large hamburger" and placed it onto a small plastic bag and then squashed it against the strainer, clogging all of it and sealing over the whole thing, using the plastic bag to keep it off of me as I packed it, and leaving the bag in place to keep the wax together (it was the same home depot bag I got when I bought the toilet wax rings). I then swapped the inside boat parts, only, leaving the body of the thru-hull in place. Then, once done, I removed the bag and wax, which did take some cleaning, but wasn't too bad. I did it just holding my breadth making a bunch trips under, not with a tank. If anything went wrong, I had plugs in various sizes and foam cone plugs and actual emergency packing on hand.

Again, not recommended at all for a setup like yours. I've only done one similar in complexity in water once. On my last boat, I replaced a bunch of seacock valves (leaving the entirety of the thru-hull, including nut) in water. But, in those cases, since I didn't touch the thru-hull and the valves weren't seized onto the thru-hull, I had many, many more options for reliably plugging the leak if things went badly and didn't risk getting surprised by the hole in the hull vs thru-hull.

When I bought my current boat, I found a leaky thru-hull-to-hull interface. I was getting ready to swap it in water when I realized that the boat had a 0.75" thru hull and a hole drilled for a 1" thru-hull. The PO replaced the original with a smaller sized, filling the gap with some sealant. I gooped it to no end with 5200 on both sides and scheduled a short haul for replacement with emergency priority! Thankfully, the yard had mercy on me!

 

[mvweebles]

Had to look up a Groco SVS - 1000.
IMO, any design that can trap a substantial percentage of the strainer's capacity with air, is a poor design or at best outdated. I need a strainer that will self fill with water, not self empty.

Now that you see the potential problem first hand, change the strainer before it causes a more expensive problem.

Ted

THis caught my attention, so I too looked-up the SVS series.

https://www.groco.net/media/file/file/s/v/svs-series.pdf

It has the old-school tapered plug style of valve. I have no idea why it would leak-down, or where the water would go, but I agree with Ted's guidance here. Given the amount of futzing the OP has tried with no success, time to hit the reset button.

BTW - I am seeing many people using Groco Flanged NPT>NPS Adapters with a ball valve instead of a traditional all-in-one flanged seacock. Allows the ball valve to be easily replaced needed, which is admittedly rare.

https://www.groco.net/products/valves-seacocks/flanged-valves/ibvf-flange-adaptor

Peter