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Bottom thruhull vs bottom strainer.

2769 Views 27 Replies 10 Participants Last post by VIEXILE, Jan 18, 2022

deniseO30

Discussion Starter · Jan 3, 2022

This seems to be a common must or must not have decision to make,

the logic seems right with an open thruhull they are easy to clean from above with a tube brush softwood dowel or high pressure water. any debris will be stopped with the inline strainer.

With a clamshell or box type strainer on the bottom, when they clog, you pretty much have to dive on the boat or have the boat lifted to get the stuff out of strainer is that supposed to stop itgetting behind of 🤣lol

This came to light yesterday my engine intake is a simple 1/2" thruhull. No clamshell strainer.

The AC intake has a box type strainer on the bottom with very small holes in it about 60% clogged, that in addition to the inline strainer pretty much answer why I was having trouble with water intake when running the AC

Decisions decisions decisions.
Bottom side strainer or open thruhulls?

What I'll probably do is, replace the 1/2" thruhulls with 3/4" & low profile bronze ball valves, and a plastic three-quarter npt by 5/8 Barb on the top of the valve feeding into the AC intake and engine intake hoses, Reason for plastic above is to make it easy to remove and easy to replace with a plastic fitting & extension hose
for ramming a tube brush from above.

If it's found to clog with mud which is common when parked on Chesapeake Bay for long periods of time a strainer can be overlaid on the thruhulls

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I know you will give me your thoughts! it thank you!

.

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marcjsmith

· #2 · Jan 4, 2022

engine intake i would want open with a raw water strainer like the groco Groco Raw Water Strainer with #340 SS Basket 3/4" Short just for the simple fact that you know the clamshell strainer will get clogged and create an overheat on the either the coldest day of year, or on the roughest day. in which case going overboard would not be an ideal circumstance.

my ac has a clamshell type strainer as well as the groco and i cleaned the clamshell really well last year, but when I replace the thruhhull, I'll likely go with an open with no clamshell cover.

Jeff_H

· #3 · Jan 4, 2022

Denise,

I have wrestled with this since I bought my boat, It has always had an perforated clamshell on the engine intake. i will start by saying that as long as I have been around boats, the general approach is to put clamshells on intakes so that large debris does not get sucked into the intake hoses The logic has been that the clamshell is the course filter and the inline strainer is the fine filter. I basically agree with that concept.

But like you, I have had a real problem with barnacles clogging the clamshell. I have a diver clean Synergy's bottom on a regular basis during the warmer months. The diver makes a diligent effort to clean out the barnacles when the bottom is being cleaned. But that very cleaning process leaves enough debris to clog the intake hose at the thru-hull. I have made a rig that allows me to easily remove the intake hose from the raw water pump, and hook up a garden hose to the intake hose and flush the strainer and raw water line back through the strainer, seacock, and clamshell. I do that every time that the bottom has been cleaned and the combination of the cleaning and back-flushing it seems to work fine.

But it would be nice if i could minimally eliminate the back-flushing. I have never seen a fitting like this (below), but I really like it since it would allow you to apply anti-fouling paint inside the course strainer, and would allow the diver to clean in there more easily. Its not clear whether that particular version would lend itself to be opened by a diver, but in theory, it would be pretty easy to build or adapt that so a diver could operate it from below the boat. (perhaps with a simple pivoting gate that would lock it closed.)

deniseO30 said:
View attachment 141775

Of course that does not eliminate the diver issue, or the problem clearing it without a diver. In an ideal world there ight be a fitting like this that could be operated from the interior of the boat, or which had a internal arm that could be turned within the housing to clear the enclosed interior of the housing. I can image how I would engineer that, but cannot image anyone actually buidling such a thing.

Jeff

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paulk

· #4 · Jan 4, 2022

We had only the inline strainer on our J/36 for 24 years. We cleaned out a few barnacles from the intake each fall when we hauled out. Nothing else to report. The new boat had a clamshell. When we hauled we found out that it was not only almost totally clogged up with barnacles but that it had been mounted backwards. We are going to leave it off and see what happens.

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deniseO30

Discussion Starter · #5 · Jan 4, 2022

It does seem like a no-brainer to leave them open without an exterior strainer that does clog often & badly.

So, I'm going to go with just enlarging to 3/4" thruhulls. I will say the 1/2" are really small on the inside. And it won't take much to reduce the flow which I already discovered on the engine intake, but the sticky mud was easily removed

paulk

· #6 · Jan 5, 2022

Jeff_H said:
In an ideal world there ight be a fitting like this that could be operated from the interior of the boat, or which had a internal arm that could be turned within the housing to clear the enclosed interior of the housing. I can image how I would engineer that, but cannot image anyone actually buidling such a thing.

Jeff

Wouldn't a simple solution be to have the intake tube go straight up into the boat before having a hose clamped to it above the waterline? You would want this anyway so that you could stick a brush down it to clean it out, as Maine Sail has suggested in his posts on the topic. You could have another tube slide down inside the straight intake tube ("sleeved") with a mesh or screen covering on the end. If you set the mesh end so it was even with the outside of the hull, it would have less drag than a clamshell. Flow would be the same as with a clamshell or screen. If you thought anything might be clogging the screen you could undo the hose clamp, pull up the inner sleeve and clean it off. Best of both worlds. Seacock might be a problem, but you could remove the sleeve if you wanted to close the seacock.

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SanderO

· #7 · Jan 5, 2022

paulk said:
Wouldn't a simple solution be to have the intake tube go straight up into the boat before having a hose clamped to it above the waterline? You would want this anyway so that you could stick a brush down it to clean it out, as Maine Sail has suggested in his posts on the topic. You could have another tube slide down inside the straight intake tube ("sleeved") with a mesh or screen covering on the end. If you set the mesh end so it was even with the outside of the hull, it would have less drag than a clamshell. Flow would be the same as with a clamshell or screen. If you thought anything might be clogging the screen you could undo the hose clamp, pull up the inner sleeve and clean it off. Best of both worlds. Seacock might be a problem, but you could remove the sleeve if you wanted to close the seacock.

This idea is intriguing. Why not use a large diameter thru hull with no clam shell nor screen which has a pipe attached which extends above the waterline? Then you have an elbow which leads to an inline strainer and then to the engine etc/whatever. To clean, unscreww elbow, insert a cylindrical stiff bristled long handled brush to clean the tube. Reattach the elbo.

marcjsmith

· #8 · Jan 5, 2022

SanderO said:
This idea is intriguing. Why not use a large diameter thru hull with no clam shell nor screen which has a pipe attached which extends above the waterline? Then you have an elbow which leads to an inline strainer and then to the engine etc/whatever. To clean, unscreww elbow, insert a cylindrical stiff bristled long handled brush to clean the tube. Reattach the elbo.

could this cause a problem with the engine though. what if you loose your suction. then the engine raw water pump has to work that much harder to get the water to the engine? Ie much longer dry run for the water pump...

SanderO

· #9 · Jan 5, 2022

marcjsmith said:
could this cause a problem with the engine though. what if you loose your suction. then the engine raw water pump has to work that much harder to get the water to the engine? Ie much longer dry run for the water pump...

I haven't a clue.

deniseO30

Discussion Starter · #10 · Jan 5, 2022

When you're below the water line you really don't need suction although the engine impeller pump will self prime.

And as mentioned in my first post it's going to be set up so the hose connection can be taken off and a
a long hose connected so a long brush or a dowel can ram it clean

To quote ..myself
"What I'll probably do is, replace the 1/2" thruhulls with 3/4" & low profile bronze ball valves, and a plastic three-quarter npt by 5/8 Barb on the top of the valve feeding into the AC intake and engine intake hoses, Reason for plastic above is to make it easy to remove and easy to replace with a plastic fitting & extension hose
for ramming a tube brush from above"

paulk

· #11 · Jan 5, 2022

SanderO said:
This idea is intriguing. Why not use a large diameter thru hull with no clam shell nor screen which has a pipe attached which extends above the waterline? Then you have an elbow which leads to an inline strainer and then to the engine etc/whatever. To clean, unscreww elbow, insert a cylindrical stiff bristled long handled brush to clean the tube. Reattach the elbo.

What you describe here is what MaineSail has said in his posts on the topic of engine intake strainers - and what he has, obviously. It is simple, and easy enough to clean out that it makes having a screen on the outer end somewhat unnecessary.

deniseO30

Discussion Starter · #12 · Jan 5, 2022

Reading MS's information is what prompted me to go without bottom strainers.
Got to love Amazon without shipping costs it is cheaper talk about variations in price but I don't like buying from unknown online sellers no room for flanged valves (height and width)

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deniseO30

Discussion Starter · #13 · Jan 7, 2022

Looks like everybody uses 5200 on through hulls!

deniseO30

Discussion Starter · #14 · Jan 7, 2022

Gas Machine Engineering Nickel Metal

Nice!

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Steve Bateman

· #15 · Jan 7, 2022

Keeping life simple a 2" through hull , stop cock then a strainer. Very easy to keep clean ! From there to a sea chest Job done.

deniseO30

Discussion Starter · #16 · Jan 8, 2022

Unfortunately I may have to send the valves back and get the flange valves because there is not enough height under the sole thought I could use the flange adapter.
Most people don't realize know or care the mushroom threads are straight pipe threads NPS and the threads on the valves are NPT.
So either way you got to buy the whole works or do it the wrong way but I'll probably get the flange valve which is only about 3" high, I only have 4.5" to work with

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deniseO30

Discussion Starter · #17 · Jan 8, 2022

Like this

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VIEXILE

· #18 · Jan 10, 2022

Ah, but when you install the scoop strainer, which direction does it point? So water is pushed in by the forward motion of the vessel, or with the streamlined section forward? I install them "backwards" on sailboats and "forwards" on power boats (like the Dyer 29 I'm finishing off). "Backwards" because I don't want a water column pushed into my engine sailing hard - and I don't have to be concerned about closing the raw water thru hull to avoid that. "Forwards" on the Dyer with the Cummins to "scoop" water and push it into the system. Unfortunately, we're loaded with sargassum weed here at 18N, and sure, you can close your valve and clean out your inboard sea strainer (daily) if you don't have a hull strainer. But I came to the conclusion to keep the hull strainer (properly oriented). I think it may depend on your cruising grounds to an extent and the type of flotsam you're dealing with. Barnacles are another thing altogether, and I've seen the perforated strainers absolutely stuffed with them. At least the grooved strainers you can get a screwdriver up in there.

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marcjsmith

· #19 · Jan 11, 2022

I would have opted for marelon 93 series. get rid of the corrosion problem, get rid of the thread changes

Forespar Marelon Thru Hull Fittings | MAURIPRO Sailing

Shop Forespar Marelon Thru Hull Fittings at MAURIPRO Sailing Store. Low prices and complete technical information on all. Free shipping on orders over $99.

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ive done two thru-hulls with the marelon set up. pretty simple and smooth install.

deniseO30

Discussion Starter · #20 · Jan 11, 2022

marcjsmith said:
I would have opted for marelon 93 series. get rid of the corrosion problem, get rid of the thread changes

Forespar Marelon Thru Hull Fittings | MAURIPRO Sailing

Shop Forespar Marelon Thru Hull Fittings at MAURIPRO Sailing Store. Low prices and complete technical information on all. Free shipping on orders over $99.

www.mauriprosailing.com

ive done two thru-hulls with the marelon set up. pretty simple and smooth install.

I have considered plastic NOT! but height limitations pretty much steered me to the only one that's has a short measurement that's made by Buck Algonquin 2-11/16" high flange to top no others are that short 🔨📐📏

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