External Strainers OMG

“RC My Engine is Overheating?”

Over the years I have really grown to distrust and dislike external intake strainers of either the scoop type or the non directional (sailboat style) round type. The real danger in these devices comes when you have a blockage and can’t get to the crud to clean it. They often require a dive over the side with a coat hanger but that does not always work.
Inside Strainer:
To ream out an intake from inside, some pre-planning can make this a dry and easy task. It can be done in under three minutes if set up correctly.
1- You need is an intake hose that can be held up higher than the static waterline.
2- You need a snake or a BRT (blockage reaming tool). Mine consists of an oak dowel cut to the correct length.
3 – If a blockage occurs before the internal strainer, close the seacock, remove the inlet hose from strainer, hold the hose high and straight above water-line, open the seacock and ream it out with BRT…
The harsh reality of external strainers is that they can become can be a safety hazard, if they have not already. I find when intakes are plumbed properly there is little to no need for any sort of external strainer. You can far more safely ream out your intake from INSIDE the vessel if you don’t have an external strainer and this can be a huge safety margin when you don’t have to physically get into the water. In just one season here in Maine I was in the water three times with a coat hanger…(cussing)
If you’re still concerned about clogs in teh intake, and not noticing them, you can insert a flow detector alarm on the intake hose or just pay close attention to your exhaust note. When it starts to sound a little hollow you’ll know you have some flow issues.
Below you’ll see some of the reasons why I don’t like external strainers, some operator error, and some strainer error…..

A 1″ Perko Strainer With 1 Piece Sweeping Elbows

This is what a internal strainer looks like. Also note the 1 piece bronze sweeping elbows. They are far less prone to plugging up before the debris hits the internal strainer basket. I prefer to see a bare minimum of 1″ for an engine intake.
Internal Strainer Tips:

1 – Make your intake hose, from the seacock to the strainer long enough so that when removed from the strainer inlet it can be held above the static waterline with one hand and reamed out with the other hand. If it plugs between the strainer and seacock remove hose from strainer inlet, hold it high and straight, and use a pre-made “blockage-stick” to push the blockage back into the ocean. If done right this takes less than two-three minutes.

2- Use a minimum of a 1″ intake seacock, intake hose and 1″ strainer, I much prefer the bronze Perko or Groco strainers to the all plastic models.
3 – Any elbows in the intake path to the strainer should be sweeping 90’s (they are gently curved 90’s) not hard turn 90’s.
4 – Any of these bronze 90’s should be one-piece not a male adapter and an elbow. Using sweep-ells helps get any crud passed to the strainer basket and not stuck in-between.

Wrong Elbow for a Strainer

This is absolutely the wrong type of elbow to use with an internal basket type strainer. They are prone to plugging at the elbow instead of allowing debris to pass through and into the strainer basket.

A Well Designed Internal Strainer System

This owner hit a nasty patch of eel grass. All the eel grass wound up in the basket, exactly as it should, not clogging the intake seacock, hose or elbow.
Cleaning this required no diving, no shrinkage (“I was in the pool” George Costanza) & no danger. Simply removing the strainers clean out cap and lifting out the SS basket was all that was required.
This is a 1″ Groco ARG-1000-S bronze strainer with 1″ Groco bronze sweeping elbows feeding it.

The Dreaded 1 Piece External Strainer

In areas with high barnacle or muscle growth these creatures know no bounds and often take up residence in the no-mans-land hiding behind the intake screen. After spending nearly three hours one day, in really, really cold water, even with a wet suit, trying to get eel grass out of one, I vowed to never, ever have one again. Any time you dive in your risk goes up. A diver spent two hours trying to get the barnacles out of this one and it still netted insufficient water flow.
This particular strainer is one piece meaning the thru-hull fitting and strainer are one integral piece. The grate could not be removed for a proper cleaning so the boat had to be hauled aat a fairly large expense. because the woenr could not see up into the area, or get bottom paint into the thru-hull he had no idea the barnacles had nearly blocked off his entire intake.

Cause = Eeel Grass & Muscle Family Taking up Residence

Owner dove on this one four times in one season, was still over heating and burned up two impellers.
A reamable 1″ internal bronze Perko strainer was installed and he’s not been in the water since.

Boat Moored in a Tidal River

This owner did not even need to run the motor for the scoop strainer to clog up. This was on a sailboat and the direction/orientation was incorrect. If installing a scoop in a sailboat it should face backwards. The correct strainer for a sailboat is round and non-directional.
Cause river current, improper strainer orientation.

Cause = Muscles & Marine Growth

Again here is another one piece strainer installed improperly on a sailboat with the scoop facing forward. According to the owner this scoop was spotless when he launched the vessel because “I spent nearly two hours with a coat hanger and picks cleaning it out before she launched.” The owner even used an artists paint brush to get bottom paint “up in there”.
Result of the blockage, engine over heating..

Cause – Eeel Grass

The eel grass plug on this one was so bad the 74 year old owner lost all engine water flow. He had no way to clear the blockage from inside the boat and his wife was adamant he was not getting in the water.. Boat was towed into the marina and hauled $$$$$.

Cause = Barnacle Colony

The photo does not depict this very well but the entire thru-hull was plugged solid with barnacles because the owner could not get to it to paint it..

Cause = Muscle Squatters

A family of muscles decided to become squatters in this owners intake strainer..

Cause = Barnacle Colony

Here we don’t have an external strainer and it shows what a barnacle blockage looks like behind and intake strainer. If you don’t protect the thru-hull with bottom paint the barnacles with grow. With external strainers this requires removal, if they can be, and a proper application of bottom paint.
So what’s the problem with this vessel? This was a 1/2″ intake with an ID of less than 3/8″. Owner stated he did not have a paint brush small enough to “get up in there”… What the boat builder was thinking using a less than 3/8” ID intake is beyond me….

Cause = Improper Orientation of Scoop Strainer

This owner was the lucky recipient of an engine hydrolock event while under sail. When he went to start the engine a cylinder was filled with water, and water is in-compressible.
“But RC it worked for 6 years with no issues.”
Another way to look at that:
“You got lucky for 6 years.”
Yes, a professional boatyard did this installation… Apparently no one informed them that when sailing with a “scoop strainer” water can be forced up and over the siphon break, into the water lift muffler and can eventually back fill a cylinder. Raw water pumps are not shut off valves and they do and can leak water by. Facing a scoop strainer forward is an incorrect installation for a sailboat. Heck directional scoop strainers should not even be used on sailboats..

Cause = Boatyard

This owner pays the boat yard to bottom paint. He pays too much!!
“My engine is over heating and spits steam out the stern.”
Gee I can’t understand why….??? (bang head with mallet)

Cause = Lack of Maintenace

This external strainer is removable and the owner fully admitted to never removing it, cleaning and painting it. Thru-Hull totally clogged.

Another 1 Piece Strainer

1 – Lack of maintenance caused by the strainer internals being inaccessible due to the 1 piece design.
2 – Another improper scoop orientation on a sailboat. A hydrolock event is only a matter of time

Owner Had Already Done a “Dive Cleaning”…

Engine still over heating…


Muscles, poor maintenance and an incorrect scoop orientation for a sailboat.

Engine Over Heating…….. WTF???

Yep you guessed it “The boat yard does the bottom painting”……. While this is the correct strainer it is totally occluded by bottom paint.

Cause = Muscles & Marine Growth

Correct type of strainer for a sailboat, but totally plugged due to inaccessible marine growth.

A Well Designed External Strainer Opens…!

If you still insist on an external strainer there is only one good method. This Groco non-directional sailboat strainer opens and can be cleaned and maintained.
Face the hinge towards the bow so that when, not if, you drop the pin or ring when “diving” to clean it, the strainer door will not hang open and catch weeds when sailing or motoring and will instead tend to close.

This Was Done by the Owner Himself.


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