External Water Strainer

[markbarendt]

One of the problems I had when I got my boat was that the external water strainer for the engine was nearly plugged off with marine growth. It also seems quite tough to clean without getting wet (and cold), essentially a swim becomes necessary to do a good job.

I found this https://marinehowto.com/external-strainers-omg/

Essentially, the article suggests skipping the external screen.

Having no screen allows the plugged valve to be shut, the hose removed, and then open the valve and use a dowel (or similar) to push the blockage out.

Once the junk has been pushed out, pull the dowel, shut the valve, reinstall the hose.

What are your thoughts about external strainers?

 

[psneeld]

Got rid of all mine, but have way oversized through hulls so suction is minimal. 1 1/2 inch intakes necked down to 1 one inch for actual supply.


Over 20,000 miles of Atlantic ICW with only one blockage/high temp...had 3 small fish swim in and clog the entry into the internal sea strainer basket...just took a minute to clear.


Single engine Lehman 120 hp, run at low rpms usually.

 

[Lou_tribal]

One of the problems I had when I got my boat was that the external water strainer for the engine was nearly plugged off with marine growth. It also seems quite tough to clean without getting wet (and cold), essentially a swim becomes necessary to do a good job.

I found this https://marinehowto.com/external-strainers-omg/

Essentially, the article suggests skipping the external screen.

Having no screen allows the plugged valve to be shut, the hose removed, and then open the valve and use a dowel (or similar) to push the blockage out.

Once the junk has been pushed out, pull the dowel, shut the valve, reinstall the hose.

What are your thoughts about external strainers?

I have external strainers and when going in heavy fouled area like some canal sections it can grab quite some grass. This is why the PO was clever enough to get a second intake installed, less chance of fouling both. Never really had an issue if one strainer is partially fouled when I stop suction stops and the fouling grass fells off.

L

 

[O C Diver]

When doing my refit I installed an over sized external strainer. The larger the surface area, the less likely it is to be completely blocked by debris or a plastic bag. In an effort to reduce marine fouling and reduced water flow, I switched to stainless steel perforated sheet metal with a slightly larger hole and a larger percentage of surface area opening. The below picture shows the larger percentage of open area by optimizing layout and the 5/32 holes will go several times longer before marine growth can close them. The larger holes are also much easier to clean with a brush. I don't expect the stainless to last as long as the original bronze plate. I'm on year 3 with the original stainless and will probably swap it out at the next haulout. I made several spares should electrolysis eat the original one. Material is available on Ebay as scrap pieces from fabrication, so very reasonably priced.



Ted

 

[jgwinks]

I've had external strainers on all my boats. One marina I was in had heavy invasive water milfoil growth in the fairways. The boat next to me had an internal strainer and as he backed out of his slip, the prop would chop up the weeds and the pieces would get sucked in and completely plug his strainer. Literally every time. My boat had an external only I never had a problem with weeds plugging up the intake. Fouling the prop, yes, but never the cooling system. There is a caveat though, we're in Michigan so the boats get hauled and cleaned every fall. The boats shown in the marinehowto article might not have been hauled or cleaned in years. Big difference.

 

[psneeld]

The real difference is inflow into the throughhull.


I have read where others have oversized intakes, eliminated external strainers and had zero issues.


I have been in heavy duckweed, bay grass and through think and well dispersed Sargasso weed. Never a sliver.


So, external strainers are necessary for some setups, but not all. Probably their greatest use in a case like mine is if stationary and almost touching bottom.

 

[Dougcole]

One of our favorite anchorages in the Bahamas has really strong current and a lot of grass blows through there on an incoming tide. My generator used to clog with grass two or three times a night there. I added a dome type strainer over the intake, the hinged kind, and have not had an issue since.



My mains seem to hardly ever pick up anything though. Not much on my AC either. I suspect it may be something with how the genset pick up is placed, perhaps the grass gets concentrated over it due to hull shape/keel shape.


I wouldn't add one if I wasn't having any issues though.

 

[rslifkin]

The only time I've clogged my external engine strainers with weeds (the typical slotted scoop type), I discovered that with twin engines and no prop tunnels, stopping the boat and giving a hard blast of reverse pushes a good bit of water along the hull and across the strainers. Alternately shutting down one engine, then the other while moving at low speed, then giving a blast of reverse on both was enough to dislodge the weeds without manual intervention.

If I have a reason to mess with my existing intakes, I'm planning to replace the strainers with an oversized set of these: https://www.groco.net/asc-1250

I'd go with the slotted type again rather than the mesh version of those same strainers, figuring the slots are less likely to clog than even large mesh. I have strainers inside, so I don't need super fine filtration on the external strainers.

 

[markbarendt]

Got rid of all mine, but have way oversized through hulls so suction is minimal. 1 1/2 inch intakes necked down to 1 one inch for actual supply.


Over 20,000 miles of Atlantic ICW with only one blockage/high temp...had 3 small fish swim in and clog the entry into the internal sea strainer basket...just took a minute to clear.


Single engine Lehman 120 hp, run at low rpms usually.

I'm upsizing too, 1-1/4.

I'm really thinking that it's worth the occasional minnow in the internal strainer to avoid a cold winter swim in the PNW.

I'm also thinking I may add a valve to be able to switch to the house seawater intake as a backup.

 

[Comodave]

The bigger size you go the slower the water flow so the less crap will be picked up. It won’t stop the marine growth though.

 

[markbarendt]

The bigger size you go the slower the water flow so the less crap will be picked up. It won’t stop the marine growth though.

Understood, the marine growth is what caused the problem I listed in the OP. The screen it had before I got it didn't stop that problem either.

Without the screen I can at least clean it from the inside, maybe even with a bottle brush of sorts.

 

[psneeld]

Understood, the marine growth is what caused the problem I listed in the OP. The screen it had before I got it didn't stop that problem either.

Without the screen I can at least clean it from the inside, maybe even with a bottle brush of sorts.

Exactly...and I have not had an issue with marine growth despite being in high growth areas.

But growth can vary wildly so who knows.