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Old 04-08-2011, 07:12   #1
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External Raw Water Strainer ?

I expect that the answer to this is probably geography and sea life dependent, but I'd be interested in perspectives from others. We recently had overheating issues two days out of three sailing in the waters around Nantucket and Martha's Vineyard. Both times we had sucked eel grass into the raw water intake. We have a Perko raw water strainer mounted below the waterline. Both times, the grass was stuck on the intake side of the strainer. Once, I was able to clear easily and proceed. The other time turned into a major PITA to get cleared. My prior boat had an external strainer and didn't have a problem in nine years.

The boat has been hauled to repair damage to its rudder from hitting an uncharted rock in a mooring field (another topic). I asked the yard about installing a strainer while on the hard. The yard guy, who I respect, strongly urged that I not add one because of the problem of barnacle growth clogging the apertures and being a much bigger problem than periodically clearing the internal strainer. I certainly see significant barnacle growth on our MaxProp -- and have the scars to show that I've had to scrape them off. What is the experience of others pro and con?

Thanks.
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Old 04-08-2011, 07:27   #2
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Re: External Raw Water Strainer?

Exellent advice from the yard guy. The strainer for the thru haul has a right angle to the thru haul and most times needs to be removed for cleaning.

One more pesky thing to monitor.
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Old 04-08-2011, 07:45   #3
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Re: External Raw Water Strainer ?

Always have had a strainer on the through hull. No problems. Just give it a good scrub when cleaning the bottom.
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Old 04-08-2011, 07:56   #4
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Re: External Raw Water Strainer ?

If you decide to go with an outside strainer, I recommend you use a Groco perferated strainer like this: Groco Perforated Hull Strainer
The perferations are small enough to keep out most grass and it can be opened to clean it. It is also very easy to replace the perferated strainer sheet if it is ever damaged. I have outside strainers on my boat and I like them. You have to be carefull not to plug the perferations with bottom paint when you paint the bottom.
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Old 04-08-2011, 08:06   #5
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Sailing out of Falmouth, MA I ran into the same problem. Installed a outside strainer two years ago and have been good to go. Yes it needs to be cleaned but I would rather clean it twice a season than have my boat overheat.
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Old 04-08-2011, 16:51   #6
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Re: External Raw Water Strainer ?

Thanks to all for your perspectives. The water is warm enough that diving and cleaning the strainer externally wouldn't be an issue. However, the caution about maintaining access to the through hull to clean any critters out of there is important. No rush to decide.
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Old 04-08-2011, 19:11   #7
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Re: External Raw Water Strainer ?

Here is another version sold by West Marine. Personally I prefer the Groco but sometimes the Conbraco installs better. The exterior part of the Conbraco separates from the bronze through hull and can be screwed into the hull over an existing through hull.
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Old 04-08-2011, 21:17   #8
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Re: External Raw Water Strainer ?

Over the years I have really grown to distrust and dislike external intake strainers of either the scoop type or the non directional round type, though I find the round type more reliable at blocking eel grass. The real danger in these devices comes when you do 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.

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 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.

To ream out an intake from inside some good pre-planning can make this a dry and easy task. It can be done in under three minutes if done right. All you need is an intake hose that can be held up higher than the static waterline and a snake. Close seacock, remove hose from strainer, hold hose above water-line, open seacock and ream out with your snake, done!

The harsh reality of external strainers is that they can become a safety hazard, if they have not already. I find when intakes are plumbed properly there is little to no need for one, and there are better & safer straining options. 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...

Some reasons why I don't like them....











Beyond these photos many boat yards, some builders, and DIY's install the scoops style strainers backwards on sailboats. Many a sailor have actually caused their engines to become hydrolocked and dealt with water in the cylinders because of the improper installation of a scoop style strainer. If a scoop strainer faces forward you risk physically scooping sea water up and over the siphon break then filling the exhaust and eventually the cylinders of your engine with sea water. This is not healthy for the engine or your own piece of mind.

This strainer, like many out there, is another potentially expensive mishap waiting to happen. All it takes is some rough weather and a good surf down a wave to fill your engine with salt water. Facing them backwards is better but can also add unnecessary vacuum on the raw water pump. Sailboats should ideally have round, non-directional strainers, not scoops, or my favorite, none at all.


If you feel you absolutely must use one please make sure it can be opened. This one is made by Groco and Hamilton Marine stocks them.


External strainers are absolutely not clog proof or growth proof. If they can't be opened it may be next to impossible to clear without removal of the external strainer screen. The worst of the external strainers are where the strainer is actually integral to the thru-hull fitting, and can't be opened, or even removed, without physically breaking the sealant bod and backing the whole unit out of the seacock......
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Old 04-08-2011, 21:32   #9
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Re: External Raw Water Strainer ?

The Groco series APHS addresses the problem of grass getting in by using small diameter holes on an angled plate. This also seems to reduce growth inside the thru-hull. They also open making it easy to clean inside the thru-hull when growth does happen.

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Old 05-08-2011, 04:53   #10
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Re: External Raw Water Strainer ?

Many thanks for the very illustrative pictures, Mainesail. I've been in the water up your way to clear my prop. I wouldn't do it for recreation. I wasn't aware of the opening strainers you and HopCar describe. Those sound like the way to go if I decide to add one.

I agree that it's much better to clear clogs from inside the boat. Unfortunately, my Perko strainer has a right angle turn and about an 8 inch hose run to its through hull. Since the hose is very stiff, it's almost impossible to get it off either the strainer or the seacock. A mechanic I talked to said he carries a SCUBA tank to blow air through the system to remove the weed. He suggested using our deck wash to back flush though I don't think it has the power required. I'm thinking I may upgrade my snaking options for the time being and see how things go.
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Old 05-08-2011, 05:03   #11
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Re: External Raw Water Strainer ?

Tartansail,

I changed my elbow for a T at the seacock, with the see-thru portion vertical. Short nipple and cap.can be removed, replaced with hose to above w/line, and rodded quite easily. No external strainer. Sargassum weed is a real bummer around here. Large diameter filter above waterline in boat easily serviced.
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Old 05-08-2011, 05:08   #12
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Re: External Raw Water Strainer ?

Wow this has raised some issues I would never have thought of

If I can ride on the coat tails of this thread and ask a related question, is an external strainer required if an inline strainer like shown in the attached picture is used?



If it makes any difference, my cruising grounds are in the tropics (NE Australia)
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Old 05-08-2011, 05:22   #13
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Re: External Raw Water Strainer ?

Quote:
Originally Posted by Reefmagnet View Post
Wow this has raised some issues I would never have thought of

If I can ride on the coat tails of this thread and ask a related question, is an external strainer required if an inline strainer like shown in the attached picture is used?



If it makes any difference, my cruising grounds are in the tropics (NE Australia)
That the inside filter I use. Unscrew cap, wipe basket,replace cap.
Takes longer to describe it, than do it.
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Old 05-08-2011, 05:57   #14
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Re: External Raw Water Strainer ?

Mainesail - I agree with most of your points except one. "Beyond these photos many boat yards, some builders, and DIY's install the scoops style strainers backwards on sailboats. Many a sailor have actually caused their engines to become hydrolocked and dealt with water in the cylinders because of the improper installation of a scoop style strainer. If a scoop strainer faces forward you risk physically scooping sea water up and over the siphon break then filling the exhaust and eventually the cylinders of your engine with sea water. This is not healthy for the engine or your own piece of mind."

Unless there is something new on the market, raw water for engine cooling goes through your internal strainer and then to a positive displacement (vaned) raw water pump on the engine. If the engine is not running then raw water cannot get past the raw water pump. And what is a siphon break doing in the raw water intake system ahead of the raw water pump? Normally it is located just before the raw water injection into the exhaust stream. See diagram below.
- - I can only see a problem if there are multiple vanes missing on the raw water pump impeller in which case there are a lot of other problems.
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Old 05-08-2011, 06:01   #15
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Re: External Raw Water Strainer ?

Main Sail....

Have a 6-8 knot trawler...would love to take the scoop off for just the reasons you have articulated. I know genny manufactures often say no scoop.

I don't think it would be a problem for my 135 Lehman at the speeds I go...any thoughts?
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