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Old 10-11-2013, 11:17   #1
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Adding External Strainer to Open Through Hull

We are having many little fishes swim up our head intake through hull. It is a pain to clear them out when they expire at the internal strainer. The through hull is of the open variety. Is anyone aware of a way to add a strainer on the outside of the hull around the through hull to keep the little critters out?
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Old 10-11-2013, 12:06   #2
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Re: Adding External Strainer to Open Through Hull

Neko,

You can pull the boat and install a new thru hull fitting with a screen. For a head it probably isn't a huge issue, but to keep the same water flow you also need to increase the size of the hole to compensate for the blockage from the screen. This is abut an hour job on the hard.

Technically it is possible to do it in the water, but it takes a really good diver who trust a lot to do it. I would pay for a lift long before being willing to take this route.
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Old 10-11-2013, 12:29   #3
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Re: Adding External Strainer to Open Through Hull

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Neko,

You can pull the boat and install a new thru hull fitting with a screen. For a head it probably isn't a huge issue, but to keep the same water flow you also need to increase the size of the hole to compensate for the blockage from the screen. This is abut an hour job on the hard.

Technically it is possible to do it in the water, but it takes a really good diver who trust a lot to do it. I would pay for a lift long before being willing to take this route.


You don't need a whole new through hull fitting, with the hassle of removing the existing fitting and installing new. That's a lot of expense for a strainer. There are many external strainers that are cheap, effective, and easy to install for any diver; no haul out required. Just make sure to bottom paint it first, and reinstall it at the next haul out when it's dry. If you have a cored hull make sure the screws don't penetrate into the core!


Perko 0086DP4PLB Round Bronze Strainer - 4" : Amazon.com : Kitchen & Dining
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Old 10-11-2013, 12:40   #4
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Re: Adding External Strainer to Open Through Hull

Just keep in mind if stuff grows inside the external strainer it's actually even more of a hassle to clear out. In comparison, an internal stainer is a lot less pain to check and clear. I got rid of the external strainer on my boat.
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Old 10-11-2013, 12:49   #5
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Re: Adding External Strainer to Open Through Hull


Groco RSC Series Strainer makes it easy to clean inside the thru-hull fitting.
Link: Groco Round Strainers (RSC series)
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Old 10-11-2013, 13:32   #6
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Re: Adding External Strainer to Open Through Hull

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Just keep in mind if stuff grows inside the external strainer it's actually even more of a hassle to clear out. In comparison, an internal stainer is a lot less pain to check and clear. I got rid of the external strainer on my boat.


Just make sure to paint both sides of the strainer with antifouling before installation on a through hull which has also been painted.
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Old 10-11-2013, 19:20   #7
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Re: Adding External Strainer to Open Through Hull

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Groco RSC Series Strainer makes it easy to clean inside the thru-hull fitting.
Link: Groco Round Strainers (RSC series)
It makes it easier, but you still need to get under the boat to open it up and clean it.

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Originally Posted by minaret View Post
Just make sure to paint both sides of the strainer with antifouling before installation on a through hull which has also been painted.
Antifouling will help. But I still think that having one internal strainer to clear is easier.
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Old 10-11-2013, 19:43   #8
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Re: Adding External Strainer to Open Through Hull

A lot of large power boats are eliminating internal strainers all together. They are using fine mesh external strainers. They are pretty much self cleaning and if stuff can get through the strainer it can get through what ever the strainer is protecting. Whether you have an external strainer or not, barnacles and oysters can grow inside the thru-hull and it's no fun trying to clean them out while the boat is in the water. One way or another you've got to clean them from the outside.
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Old 11-11-2013, 01:10   #9
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Re: Adding External Strainer to Open Through Hull

Hop,

I have seen ones like that, and they work fine for a while. Right up until someone paints them, or anything grows on them. Not to mention they require larger thru hull sizes. The flow restriction on some strainers can surpass 50%, requiring a much larger hole in the boat.

As for screwing into the bottom of your boat. Yes it is possible, but no reputable yard should do so. It is guaranteed to cause glass fracturing, and god help you if you hit core.
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Old 11-11-2013, 01:23   #10
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Re: Adding External Strainer to Open Through Hull

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Hop,

I have seen ones like that, and they work fine for a while. Right up until someone paints them, or anything grows on them. Not to mention they require larger thru hull sizes. The flow restriction on some strainers can surpass 50%, requiring a much larger hole in the boat.

As for screwing into the bottom of your boat. Yes it is possible, but no reputable yard should do so. It is guaranteed to cause glass fracturing, and god help you if you hit core.


How is an external strainer any different than the integral strainer you just suggested yourself? Same flow restriction, same cons. And flow restriction generally only matters in high capacity fittings like an engine intake, it won't affect their head intake one bit. Guaranteed to cause fracturing is silly, maybe if you don't know how to properly size a pilot hole for fiberglass. It's easy, as long as you don't have core in way of the screws (even many cored boats have solid glass around the through hulls), it's a go. Just about every reputable yard everywhere installs these, they are extremely common. Failing to make this extremely simple install well would be disreputable.
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Old 11-11-2013, 06:00   #11
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Re: Adding External Strainer to Open Through Hull

I hate to ask a stupid question, but why not use an in-line filter and not mess with the thru hull fitting at all
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Old 11-11-2013, 06:31   #12
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Re: Adding External Strainer to Open Through Hull

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I hate to ask a stupid question, but why not use an in-line filter and not mess with the thru hull fitting at all
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Old 11-11-2013, 06:39   #13
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Re: Adding External Strainer to Open Through Hull

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I hate to ask a stupid question, but why not use an in-line filter and not mess with the thru hull fitting at all
I just figure a 15-20 dollar item that is readily accessible.
To me it is a KISS kinda thing. I have had octopus in the head then installed the strainer and no problems.
On another note, anybody ever suck a jelly fish into their strainer,(A/C)? It makes the strainer basket basically waterproof
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Old 11-11-2013, 06:40   #14
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Re: Adding External Strainer to Open Through Hull

Why do you need a strainer (internal or external) on the head intake? Been doing fine all these years with no strainers at all on the head intakes.
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Old 11-11-2013, 06:57   #15
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Re: Adding External Strainer to Open Through Hull

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Why do you need a strainer (internal or external) on the head intake? Been doing fine all these years with no strainers at all on the head intakes.
We were good until we sucked an octopus into the head. It got up around the top of the bowl and would not come out without the use of a shop vac!!!! The wife required a strainer after that incident
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