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Old 15-06-2020, 22:25   #1
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Raw Water External Strainer Debate

The biggest concern about a raw water intake, other than it not leak and so be a proper seacock installation, is that it also not block-up easily - and this is especially true for a shoal draft boat that is bound to be near eelgrass.

So I have oversized the intake diameter and the internal strainer, and am now considering putting on an external strainer of the round, removable variety for sailboats...

The debate https://www.pbase.com/mainecruising/external_strainers

One school of thought is that the more strainers the better in keeping things like eelgrass out so use both an internal and external strainer

The other school of thought says that since barnacles and mats of eelgrass can plug up a raw water external strainer quite quickly, requiring you to dive overboard to clean up the strainer, why not rely on the internal strainer only to keep the water clean, and even arrange the hosing so you undo one end and then can ream out the blockage at the intake from inside the boat, and skip the external strainer altogether. The benefit of an internal strainer is precisely that it is internal, so you don't have to get wet trying to clean out a blockage, so why undermine that by also employing an external strainer

I tend to disfavor the second school of thought, the no external strainer school, because

1- You have to normally dive on your boat and scrape the hull pretty regularly anyway in the tropics so keeping the strainer clean is not much harder, which may be true in some waters than others

3- I wonder if the intake isn't going to get clogged by barnacles regardless of whether you're using an external strainer or not

4- and there's no reason you can't ream out the blockage from the inside while using an external strainer, which tend to be domed and provide space for the blockage to fall out into the sea rather than blocking the blockage

5-The point of the strainer is to exclude things like plastic bags which pass by and will eventually be pulled way from the intake by the force of water, while if you relied only on an intenral strainer it would be sucked into the seacock at least - s0 an external strainer lets blockages self-clear more easily
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Old 16-06-2020, 03:16   #2
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Re: Raw Water External Strainer Debate

Iíve been sailing all my life and have never used or felt the need to use an external strainer

Additionally I never have problems with the internal sea chest strainer clogging with grass

Grass does get into the strainer, but not at a rate that exceeds routine inspection maintenance

Is your boat refrigeration water cooled ... do you have air conditioning ? Do you run a water maker

These systems draw water when the boat is in port ... stationary ...and are most likely to pick up weed , jellyfish and bags

This problem is not possible to solve

Adding a second sea strainer ...ac , water maker and refer only ...is one way to isolate and protect the engine cooling system

Many many boats do this

An engine , when propelling a boat forward never clogs a strainer

Another tip , for operating in weedy zones ...when retrieving your anchor ...as soon as the anchor breaks free of the bottom put the boat into reverse

The anchor and chain will now shed it weed load clear of the boat .. the sea chest will be less likely to pick up weed
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Old 16-06-2020, 05:27   #3
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Re: Raw Water External Strainer Debate

I personally hate my external strainers and plan to get rid of them, or at least change them to oversize ones that are less likely to clog. If you clog one, you have to get in the water to fix it. With no external strainer, you can rod out the thru hull and clean the strainer from inside the boat.
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Old 16-06-2020, 05:34   #4
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Re: Raw Water External Strainer Debate

I have 1 external strainer and HATE that thing. I see no point to it as there is an internal one also on the line. All the external strainer does it provide a place for barnacle to grow and make it impossible to clear any blockage for inside the boat with a screw driver. Next haul out that thing is going to be history.
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Old 16-06-2020, 05:43   #5
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Re: Raw Water External Strainer Debate

Not discussed here is the external strainer used as a "speed scoop". (the one that look like a scoop, not the round plate w/holes in it). Was on an old Alden schooner and it needed the speed scoop to stop the engine from overheating.

Every part of the raw water system was taken apart, cleaned, replaced impellers, etc. and it still overheated. Eventually it was determined that the overheating was due to sucking in air into the raw water intake causing it to hydrolock. Once the speed scoop was added, it fixed the problem and never overheated.
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Old 16-06-2020, 05:47   #6
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Re: Raw Water External Strainer Debate

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Originally Posted by Bill O View Post
Not discussed here is the external strainer used as a "speed scoop". (the one that look like a scoop, not the round plate w/holes in it). Was on an old Alden schooner and it needed the speed scoop to stop the engine from overheating.

Every part of the raw water system was taken apart, cleaned, replaced impellers, etc. and it still overheated. Eventually it was determined that the overheating was due to sucking in air into the raw water intake causing it to hydrolock. Once the speed scoop was added, it fixed the problem and never overheated.
I've got those on my engines (and oddly my A/C intakes). I've been trying to decide how to get rid of them. Maybe just take one of those scoop strainers and cut the strainer slots out to make an open scoop?
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Old 16-06-2020, 06:00   #7
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Re: Raw Water External Strainer Debate

I suppose the best thing to do is try without and see if it is necessary later; I'd just hate to have to find out it was necessary in a time of crisis when I need the engine running
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Old 16-06-2020, 06:05   #8
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Re: Raw Water External Strainer Debate

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Once the speed scoop was added, it fixed the problem and never overheated.
Well this is a separate issue - the type and installation of the strainer, rather than whether they are even necessary. As we all know scoops are discouraged for sailboats compared to motorboats because the force of water entering can get into the cylinders when the boat is under sail and the engine is not running, causing a hydrolock when you do turn on the engine

I don't understand how there would be an airlock without the scoop unless the intake was placed too low, and the scoop was therefore necessary to keep water in the intake when the boat heeled or something? The fix would be to change the location of the intake, unless for some reason that was not possible?
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Old 16-06-2020, 06:08   #9
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Re: Raw Water External Strainer Debate

My boat was built w/ the Groco STH scoop through hull. IMO, these are horrible, permitting interior marine growth that is virtually impossible to remove, even when on the hard, picking away through the slots.

My boat has been changed to using the Groco APHS strainers w/ standard mushroom through hulls. These are infinitely better. I have almost no interior growth, from one the hard service to the next, easily cleaned when applying new paint. My APHS strainers were properly bonded using a long bolt to a green bonding wire.

The APHS screen exterior is easy to clean while floating. My underwater metal is protected w/ Rustoleum Cold Galvanizing Compound, including this screen, with good results.
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Old 16-06-2020, 06:13   #10
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Re: Raw Water External Strainer Debate

I know those APHS strainers as Great South Bay strainers because they're used to fight off eelgrass in the Great South Bay NY where eelgrass is a problem, and people swear by them.

For a sailboat, the perforated round kind is recommended over a scoop type. If I were to use strainers they'd be the Groco RSC series (hinged, so they can be opened to clean)

https://www.groco.net/products/raw-w...ner/rsc-series

I'm just not sure if they're a benefit or a hindrance
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Old 16-06-2020, 06:50   #11
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Re: Raw Water External Strainer Debate

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Originally Posted by Cyrus Safdari View Post
Well this is a separate issue - the type and installation of the strainer, rather than whether they are even necessary. As we all know scoops are discouraged for sailboats compared to motorboats because the force of water entering can get into the cylinders when the boat is under sail and the engine is not running, causing a hydrolock when you do turn on the engine

I don't understand how there would be an airlock without the scoop unless the intake was placed too low, and the scoop was therefore necessary to keep water in the intake when the boat heeled or something? The fix would be to change the location of the intake, unless for some reason that was not possible?

We have sailed our boat for many years w/the water intake open (w/speed scoop) and never have pushed water into the motor. Again, maybe specific motors are prone to this, but we haven't experienced it.

The owner of the Alden was an engineer and meticulous. The airlock occurred while motoring and motor sailing alike, so don't think it was due to the boat being heeled. Can't imagine that an intake being too low would cause an airlock unless possibly the thru hull dia. was too small. Bottom line, the scoop fixed the issue.
Have heard of similar issues of air accumulation in the primary filters of water makers on certain boats. Maybe its poor placement of the intake or the hull design, don't know, but it happens.
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Old 16-06-2020, 06:53   #12
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Re: Raw Water External Strainer Debate

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We have sailed our boat for many years w/the water intake open (w/speed scoop) and never have pushed water into the motor. Again, maybe specific motors are prone to this, but we haven't experienced it.

The owner of the Alden was an engineer and meticulous. The airlock occurred while motoring and motor sailing alike, so don't think it was due to the boat being heeled. Can't imagine that an intake being too low would cause an airlock unless possibly the thru hull dia. was too small. Bottom line, the scoop fixed the issue.
Have heard of similar issues of air accumulation in the primary filters of water makers on certain boats. Maybe its poor placement of the intake or the hull design, don't know, but it happens.
Sorry I should have written "unless the intake was placed TOO HIGH" and so may have had to be lowered to ensure that it was always sucking water not air

ABYC code suggests that scoops not be used on sailboats for the reason stated, FWIW

The gasket on the internal strainer can be a source of airlock as can leaky hoses etc but if everything was replaced ...
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Old 16-06-2020, 07:18   #13
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Re: Raw Water External Strainer Debate

First never put a scoop on a sailboat engine, or on a twin engine power boat. They are often on air conditioners and Webasto at least recommends them, but as we don’t run the AC when underway, we don’t have scoops on the intakes.
A few years ago I installed the Groco RSC domed, hinged covers so they can be opened if needed to clean out.
My opinion is if you want strainers do as Maine Sail recommends and just glue them on with 5200, don’t use the screws, that way they can be removed by knocking them off from inside using a broom handle or similar, or get the hinged covers.

If your always in warm clear water then it’s not so relevant, you can remove them if needed by diving the boat using a screwdriver, if you have a Hooka or dive gear. But in cold or nasty water or maybe even where there are critters you don’t want to get into the water with, then you really don’t want an external strainer.
It’s not that the external strainer will get clogged so much as it’s that given enough time there will be barnacles growing under the external strainer, and they will require removal of the strainer to clear.

If you want an external strainer, go with the Groco domed / hinged RSC one that you can open if needed without removal.

I bring this up as some may not realize it, but external strainers are not attached to the thru hull in any way, they are held on by screws that screw into the hull.
There are I believe some thru hulls built with strainers as a part of them, avoid them, cause to get barnacles out from under them would require removal of the thru hull.

You should not be able to get an “air lock” on an impeller pump like used on our motors as they will pump air, but you can of course have an air leak on the suck side that will cut down on the amount of water pumped.
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Old 16-06-2020, 07:34   #14
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Re: Raw Water External Strainer Debate

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My boat was built w/ the Groco STH scoop through hull. IMO, these are horrible, permitting interior marine growth that is virtually impossible to remove, even when on the hard, picking away through the slots.

My boat has been changed to using the Groco APHS strainers w/ standard mushroom through hulls. These are infinitely better. I have almost no interior growth, from one the hard service to the next, easily cleaned when applying new paint. My APHS strainers were properly bonded using a long bolt to a green bonding wire.

The APHS screen exterior is easy to clean while floating. My underwater metal is protected w/ Rustoleum Cold Galvanizing Compound, including this screen, with good results.
Here are mine, before I switched from protecting my underwater metal using barrier coat / bottom paint / Prop Speed to Rustoleum. The Rustoleum is waaay better.

The screen on these APHS strainers could be pointed to the rear for sailboats to avoid hydrolocking the engine.

Mine have never required opening the door while floating for cleaning.


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Old 16-06-2020, 07:38   #15
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Re: Raw Water External Strainer Debate

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I bring this up as some may not realize it, but external strainers are not attached to the thru hull in any way, they are held on by screws that screw into the hull.
There are I believe some thru hulls built with strainers as a part of them, avoid them, cause to get barnacles out from under them would require removal of the thru hull.

You should not be able to get an “air lock” on an impeller pump like used on our motors as they will pump air, but you can of course have an air leak on the suck side that will cut down on the amount of water pumped.

The only reason we have a scoop on ours was after seeing how well it worked on the Alden. I'll guarantee that there were no air leaks on the suction side on the Alden and the RW side was systematically gone through. The final change was the addition of the speed scoop which fixed it.
Will agree that the thru hull should be a plain mushroom type and the strainer added after, adhered to the hull.
Never had any fouling issue w/our scoop even after years between bottom jobs. Was contemplating removing the scoop as I read it could reduce the hull speed by ~0.25kts. Don't know it its true though, but easy enough to find out.
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