Internal vs. "South Bay" strainers?

[ranger58sb]

I'm considering installing external plate ("South Bay") strainers, and then removing our internal sea strainers.

Our internal strainers work fine, but the installation -- with our V8 engines -- is cluttered. I think removing the internals and rerouting the intake hoses could offer some improvement.

Might also consider the same change for our generator intake... and maybe even for the AC intakes...

Pros and cons?

I would also add a service port on the inside for injecting anti-freeze to winterize...

-Chris

 

[twistedtree]

This is worth a read. https://marinehowto.com/external-strainers-omg/

 

[rslifkin]

I don't mind external strainers, but in my mind, they should be large and not too fine, such that they keep any large junk from getting sucked in, but don't gunk up or clog easily. I'd much rather be able to take care of most cleaning from inside where I don't have to get in the water.

 

[Circumnavigator]

This is worth a read. https://marinehowto.com/external-strainers-omg/

Great pix here -- thank you.


Jim

 

[ofer]

This is worth a read. https://marinehowto.com/external-strainers-omg/

the most convincing proof i have seen.

No argument.

 

[kapnd]

Local conditions will play a big part in deciding which to use.
The under hull strainers will do pretty well for most large-medium sized debris and sea grass, but will not exclude sand and silt from from your cooling system.
I utilize both types of strainer, and call it cheap insurance!
Rslifkiin makes a valid point about having to enter the water to service the external strainers.

 

[O C Diver]

I installed an external strainer on Slow Hand after having one on my charter boat. The above link shows you what not to buy in addition to poor maintenance and why you need to check your boat bottom after the village idiot painted it.

This is the strainer on Slow hand:


The objective is to have large enough holes and a large enough area for the suction of the water to not hold weed and organisms against thte screen. My screen has 5/32" holes and they occupy 60% of the surface area.

Personally, I wouldn't remove the internal strainer as I prefer progressive raw water filtration. My Groco strainer has smaller holes and the basket provides a place for small amounts of sand to collect as opposed to going through the raw water pump, scratching the bore.

Ted

 

[twistedtree]

I've never heard of using ONLY and external strainer. Is this actually done? I have only seen internal strainers with or without an external screen/grate.

 

[O C Diver]

I would also add a service port on the inside for injecting anti-freeze to winterize...

-Chris

If this is the main reason for removing the internal strainers, buy replacement lids and modify them to allow an antifreeze hose connection. By closing the seacock and replacing the lid, you should be able to accomplish winterizing.

Ted

 

[ranger58sb]

I installed an external strainer on Slow Hand after having one on my charter boat. The above link shows you what not to buy in addition to poor maintenance and why you need to check your boat bottom after the village idiot painted it.

This is the strainer on Slow hand:
View attachment 146525

The objective is to have large enough holes and a large enough area for the suction of the water to not hold weed and organisms against thte screen. My screen has 5/32" holes and they occupy 60% of the surface area.

Personally, I wouldn't remove the internal strainer as I prefer progressive raw water filtration. My Groco strainer has smaller holes and the basket provides a place for small amounts of sand to collect as opposed to going through the raw water pump, scratching the bore.

Ted

Yep, that's the kind I have in mind. Not the kind in Rod' article (which we already have now).

I've never heard of using ONLY and external strainer. Is this actually done? I have only seen internal strainers with or without an external screen/grate.

Thanks for that link. Interesting, although Rod only mentions the one-piece kind of external strainer, not the "South Bay" screen plate version.

And yes, apparently some new boats are (were, many of our previous boat brand, for example) delivered with only the external "South Bay" things. In the case of our previous brand boat, I think the internal basket strainers were optional. We had internal strainers, but other owners who have only "South Bay" things have said they work decently. (So posting here for additional ground truth, hopefully to avoid false positives...)

And they all mention that "don't paint" thing, too.

If this is the main reason for removing the internal strainers, buy replacement lids and modify them to allow an antifreeze hose connection. By closing the seacock and replacing the lid, you should be able to accomplish winterizing.

Not the main reason. We can do the winterization through the internal lids easily enough... but the whole mess of original strainer assemblies and fat raw water hoses is serious clutter. Partly also because the intake hose for the internal port strainer comes from a seacock on the the starboard side of the boat, and vice versa...

-Chris

 

[rslifkin]

Not the main reason. We can do the winterization through the internal lids easily enough... but the whole mess of original strainer assemblies and fat raw water hoses is serious clutter. Partly also because the intake hose for the internal port strainer comes from a seacock on the the starboard side of the boat, and vice versa...

-Chris

If you've got space at the seacock location, there's always the option of the (expensive) Groco BVS units that are a strainer mounted right to the top of the seacock. That would simplify the plumbing a bit and avoid needing space for a separate strainer.

 

[ranger58sb]

If you've got space at the seacock location, there's always the option of the (expensive) Groco BVS units that are a strainer mounted right to the top of the seacock. That would simplify the plumbing a bit and avoid needing space for a separate strainer.

Thanks, hadn't known about those, worth a look.

I'll have to look closer at our current strainers and hoses, but I think they're in the neighborhood of 15" tall with 2 or 2-1/2" or maybe even 3" hoses...

I started this idea thinking about the concept... but I'm short on details.

-Chris

 

[SteveK]

This is worth a read. https://marinehowto.com/external-strainers-omg/

Has anyone made a large hole in the external strainer, is that worth considering if you also have an internal basket strainer?

As for the hose lenght above static water line, if the lenght is not enough, make a dedicated hose and remove/attach to the thru hull directly.

 

[ranger58sb]

I'll have to look closer at our current strainers and hoses, but I think they're in the neighborhood of 15" tall with 2 or 2-1/2" or maybe even 3" hoses...

I started this idea thinking about the concept... but I'm short on details.

I had a quick look; Our strainer housings are ~20" tall, and the hoses are ~3.5" outside diameter.

While I was there, I snapped a quick pic of the "clutter" I'm trying to simplfy.

-Chris

 

[twistedtree]

The picture is helpful. I actually think that looks well done, and would not change it. If the problem is that you want to be able to get in that area to work, or to make it look better, I would build some removable floor panels to put in the space. I did something along those lines in a Grand Banks 47 EU in the laz. Most of the space was curved hull and stringers making it difficult to sit or kneel, and difficult to store stuff. It took some scribing to create floor panels, and various blocking and cleats to support and hold the panels in place, but the end result was flat floors throughout, with a couple of steps so it followed the curve of the hull. It worked really well from a storage, service and access, and appearance perspective.

 

[ranger58sb]

The picture is helpful. I actually think that looks well done, and would not change it. If the problem is that you want to be able to get in that area to work, or to make it look better, I would build some removable floor panels to put in the space.

Good idea, thanks. I'm not overly concerned with looks, just want to get around easier... and I can see where floor panels could help that.

I need to replace our leaking water heater, which is forward of our starboard engine. I haven't created a model WH to see about getting it into place yet, but I'm guessing I might have to disconnect and maybe even remove some of that strainer/hose stuff to do it...

Whether I do the thing with the strainers or not.

This (or similar) is the strainer I'm considering as replacement:

https://www.groco.net/products/raw-water-strainers/hull-strainer/aphs-series

-Chris

 

[rslifkin]

Good idea, thanks. I'm not overly concerned with looks, just want to get around easier... and I can see where floor panels could help that.

I need to replace our leaking water heater, which is forward of our starboard engine. I haven't created a model WH to see about getting it into place yet, but I'm guessing I might have to disconnect and maybe even remove some of that strainer/hose stuff to do it...

Whether I do the thing with the strainers or not.

This (or similar) is the strainer I'm considering as replacement:

https://www.groco.net/products/raw-water-strainers/hull-strainer/aphs-series

-Chris

Those are good external strainers. Personally I'd go for the largest available holes (or the slotted version) and keep internal strainers, as that will minimize the risk of needing to dive to un-clog something (but still letting the external strainers reduce the amount of junk that gets sucked in).

 

[twistedtree]

Good idea, thanks. I'm not overly concerned with looks, just want to get around easier... and I can see where floor panels could help that.

I need to replace our leaking water heater, which is forward of our starboard engine. I haven't created a model WH to see about getting it into place yet, but I'm guessing I might have to disconnect and maybe even remove some of that strainer/hose stuff to do it...

Whether I do the thing with the strainers or not.

This (or similar) is the strainer I'm considering as replacement:

https://www.groco.net/products/raw-water-strainers/hull-strainer/aphs-series

-Chris

I have those Groco strainers on my boat and like them. I've also used these where you want neutral pressure as the boat moves.

https://www.groco.net/rsc-4000

With the wedge scoops, you need to be sure you have them correctly facing forward or aft depending on what they feed. If facing forward, boat movement will push water through the line and flood things like engines and generators that aren't running when you are underway. Engine and generator manufacturers generally prohibit such an installation for just this reason. But I have seen watermaker manufacturers who want the scoop facing forward. So you need to make a careful decision for each thruhull.

But by screens are all in addition to internal basket strainers, not in place of. I see the role of external screens as primarily to keep jelly fish, plastic bags, and large chunks of seaweed from getting sucked in and plugging the hoses, etc. So really a trash screen, not a filter.

 

[ranger58sb]

Those are good external strainers. Personally I'd go for the largest available holes (or the slotted version) and keep internal strainers, as that will minimize the risk of needing to dive to un-clog something (but still letting the external strainers reduce the amount of junk that gets sucked in).

I have those Groco strainers on my boat and like them.

I see the role of external screens as primarily to keep jelly fish, plastic bags, and large chunks of seaweed from getting sucked in and plugging the hoses, etc. So really a trash screen, not a filter.

Thanks for comments. Have to admit, I don't have much appetite for adding the external plate strainers -- assuming it'd be an expensive project -- unless it buys me better interior space and improved access around stuff up there in our ER...

I haven't yet looked at cost of parts, or labor...

-Chris

 

[HopCar]

I think some new boats are coming with just the external strainers. The hole type, not slotted. I suggest the Groco APHS as it has a replaceable plate and can be opened to clean inside.

 

[jlombardo]

External Strainers

I own a Mainship 31 Sedan Bridge and it came with external strainers only. After 5 years of getting tired of overheating and having to remove hoses off the raw water pump and intake pickup to flush the grass out of the trans/oil coolers, I installed internal strainers. The boat is still on land for winter, but I am looking forward to a carefree summer without the worry of overheating and having to rip the intake water system apart to clean out the grass. Now if I notice the temps starting to rise, I can stop the boat, close the ball valves and clean out the strainers - done! Honestly, I can't believe that they would produce a boat without raw water strainers.

I will add that our bay averages about 4 feet deep, so grass is definitely an issue.

 

[ranger58sb]

I think some new boats are coming with just the external strainers. The hole type, not slotted. I suggest the Groco APHS as it has a replaceable plate and can be opened to clean inside.

Thanks, Parks, that's what Ive had in mind.

I own a Mainship 31 Sedan Bridge and it came with external strainers only. After 5 years of getting tired of overheating and having to remove hoses off the raw water pump and intake pickup to flush the grass out of the trans/oil coolers, I installed internal strainers. The boat is still on land for winter, but I am looking forward to a carefree summer without the worry of overheating and having to rip the intake water system apart to clean out the grass. Now if I notice the temps starting to rise, I can stop the boat, close the ball valves and clean out the strainers - done! Honestly, I can't believe that they would produce a boat without raw water strainers.

I will add that our bay averages about 4 feet deep, so grass is definitely an issue.

Well, that doesn't sound great. Are your external strainers the ones with the removable perforated plate? Or the version with only the open slots?

I've read the perforated version was specifically invented for eel grass (whatever that is) in the Great South Bay (wherever that is).

-Chris

 

[jlombardo]

Mine are the plate type, but they are not housed within a frame. My issue may be that the grass is getting sucked in from around the outside. Maybe next year, I will build a frame of some sort that will prevent that and make it easier to get on and off for cleaning and paint. I clean/paint them every season.

 

[Steve DAntonio]

Coarse hull "south bay" strainers are fine, the strake variety, but not in place of an internal "fine" strainer. I'm less enthusiastic to the ones with many small holes, as they are easily clogged with growth or anti-fouling paint.

Some guidance on external and internal strainers below. Side note, hull strainers should be fastened with bronze rather than stainless fasteners, the former are prone to crevice corrosion and failure. If the vessel is bonded, use at least one through bolt for that purpose.

https://stevedmarineconsulting.com/photo-essay-hull-strainer/

https://stevedmarineconsulting.com/photo-essay-hull-strainers-revisited/

https://stevedmarineconsulting.com/photo-essay-easily-serviceable-hull-strainers/

https://stevedmarineconsulting.com/wp-content/uploads/2014/03/Strainers127_02.pdf

 

[Steve DAntonio]

Thanks, Parks, that's what Ive had in mind.





Well, that doesn't sound great. Are your external strainers the ones with the removable perforated plate? Or the version with only the open slots?

I've read the perforated version was specifically invented for eel grass (whatever that is) in the Great South Bay (wherever that is).

-Chris

Great South Bay, south shore of Long Island.

 

[ranger58sb]

Thanks, Steve, very useful review.

-Chris

 

[jlombardo]

Funny you say that as I spent two days with an 1/8" drill bit poking every hole on both strainers to remove debris and anti-fouling paint.

I do have a bonding system and include the strainers with one thru-hull screw to a grounding wire. Definitely recommended.

John

 

[ranger58sb]

I'm noticing most of the "cons" so far have focused more on issues with external slotted or clamshell strainers, not the perforated plate style... and with the external types that are essentially non-opening for cleaning and so forth. Many of the cons are also associated with how not to manage a system. For example, excessive paint might well clog the intakes... but then again, it shouldn't be too difficult to NOT apply excessive paint.

That said, I do appreciate all the input, and I'm paying attention.

My most current thought is to not replace our existing internal basket strainers with external perforated plate strainers. And if I don't replace the existing setup -- primarily to reduce interior engine room clutter and to make access up there easier -- there's no huge need to change to better external strainers. (Might revisit again later, though; buds on another forum who only have external perforated plate strainers have said they've never had any issues with clogging...)

Systems forward of those internal strainers include main engine electronic control boxes, the main engine emergency stop buttons, the automatic fire suppression system, the general systems monitor buss box, the VacuFlush vacuum generators, the main sump, and the water heater. So far -- and aside from our upcoming water heater replacement -- I've been able (or our engines guys have been able) to deal with the obstructions. IOW, it's not like I've had to be up in that area on a daily basis... so maybe I can live with the clutter for a bit longer... and at least move the whole idea to the bottom of our (still long) list of stuff toe address.

I also like the idea of a platform to help protect the lower level obstructions up there (float switches and pumps, etc.) -- so maybe I'll invest more time in that idea in the meantime.

And actually, our more impactful clutter issue is about spare props stored just in between the access ladder and the engines. Down, up, down... just to get into position to do any work. I have a solution to that, though, as described by another owner: move the props, and he's identified a likely destination. I have to hire that done, though; I can't actually lift one of our props. To be implemented, finally, after we return home.

-Chris

 

[jlombardo]

My recommendation would be, if you do replace them save the parts that you removed in case you are not happy with external only and want to go with both.

John

 

[Waterman]

My factory external strainers would get completely clogged with chopped up seaweed(fresh water)
My internal strainers were clean but due to reduced water flow my Hino’s would start to run hot. I tried back flushing but the weeds packed in so tight the only way to clear was to get under the boat ( on a hoist) and physically remove the seaweed. I bought the plate style strainers- they have a frame and would fit over the old intake strainers. I removed the outer dome area of the original strainers to improve water flow. No more issue since the plates were installed. I would not recommend eliminating the internal strainers. They serve other purposes.