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Old 05-08-2011, 06:24   #16
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Re: External Raw Water Strainer ?

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Originally Posted by osirissail View Post
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.
You can disagree all you want.... but it seems to be true on many engines...

I got water past my sea water pump running a trickle through my hose in the yard this year.

Ihave read the same caution on scoop striners on gennies for years and heard it a thousand times working around the marinas I've been employed at....

If the raw water stainer was perfect maybe...but often there is some "get by" by water and even the slightest pressure seems to force it through the system.
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Old 05-08-2011, 06:26   #17
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Re: External Raw Water Strainer ?

The way I'm going is a sea chest. It's used mainly in larger steel yahcts, but if you have a place it's the best thing.
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Old 05-08-2011, 06:30   #18
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Re: External Raw Water Strainer ?

Here's a repost from Kohler gensets

This is what Kohler says:
"In theory it sounds like a good idea, but in reality it has a catastrophic result. Let me explain, when the generator is not running and the vessel is underway, the pressure that is induced to the sea water pump varies; however, a sea trial has shown with as little as 14 inch water column or .5psi the sea water pump impellor will deflect. When this happens the sea water will trickle/flow (depending on pressure) through the heat exchanger, exhaust mixer, and ultimately stop at the engine silencer. At that point, water backs up through the engine exhaust resulting in flooding the engine and vessel. In order for the water to discharged out of the muffler, the engine must be running. In other words, it must be running. Exhaust pressure is what forces the water through the super silencer III.

Acceptable sea water through hulls are illustrated in Figure 3. Speed scoops and sea chests are not acceptable."
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Old 05-08-2011, 06:36   #19
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Re: External Raw Water Strainer ?

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You can disagree all you want.... but it seems to be true on many engines...

I got water past my sea water pump running a trickle through my hose in the yard this year.

Ihave read the same caution on scoop striners on gennies for years and heard it a thousand times working around the marinas I've been employed at....

If the raw water stainer was perfect maybe...but often there is some "get by" by water and even the slightest pressure seems to force it through the system.
The flow-by is easy to check for.
I put a small plastic ball in my strainer. It is active when engine is pumping.
But if I see it move when sailing, I know I am getting water past the vanes.
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Old 05-08-2011, 06:45   #20
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Re: External Raw Water Strainer ?

My motto is "never say never" as invariably there are exceptions and extraordinary events/circumstances surrounding any particular thing. Still I have never in many decades in the business seen scoops causing raw water to bypass a vaned raw water pump except as I mentioned when the impeller is missing vanes or in need of replacement. That doesn't mean it doesn't or can't happen as your experiences are different and and brings up very good information. Especially since few cruisers inspect or replace their impellers until the engine starts overheating.
- - Personally I don't like scoops or external strainers for the reasons Maine Sail's photos show. Sea life is so prolific it doesn't take long before the things are clogged. And as somebody else said without a external strainer you can clear a clog from inside the boat. Once I got a perfectly measured 1.5 inch diameter fish stuck in the raw water inlet which defied detection from both the outside and the inside (strainer). Only when the hose was removed did the fish reveal its presence.
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Old 05-08-2011, 06:57   #21
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Re: External Raw Water Strainer ?

Here's the same from the Yanmar help page...

Scoop: dont fit an exterior scoop unless you have an overheating problem caused by the position of the thru-hull fitting on the hull. When installing a scoop on a slower vessel (below 11 knots) install a slotted scoop facing aft. This helps reduce the possibility of a leaf, stray plastic bag, etc., blocking the intake and wont force water into the engine while sailing.
With a multiple engine installation and forward facing scoops (planning hull craft) the raw water cock on a non-operating engine must be turned off while motoring or the engine doctor will make lots of money again!
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Old 05-08-2011, 07:05   #22
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Re: External Raw Water Strainer ?

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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.
Blue Stocking, this sounds like a solution to my problem with removing the hose to clear the lower section. Thanks. So far, the weed seems to get caught at the right angle bend to go into the strainer. Once it came out nicely in a big clump. The other time, one strand at a time. Perhaps putting the T up at the strainer with the upper arm of the see-through arm capped and the T-arm going into the strainer would be work. That would give a straight shot to the through hull and only a short section from the strainer to the T.
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Old 05-08-2011, 07:27   #23
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Re: External Raw Water Strainer ?

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Blue Stocking, this sounds like a solution to my problem with removing the hose to clear the lower section. Thanks. So far, the weed seems to get caught at the right angle bend to go into the strainer. Once it came out nicely in a big clump. The other time, one strand at a time. Perhaps putting the T up at the strainer with the upper arm of the see-through arm capped and the T-arm going into the strainer would be work. That would give a straight shot to the through hull and only a short section from the strainer to the T.
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Old 13-07-2016, 03:30   #24
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Re: External Raw Water Strainer ?

A sketch of a seachest.
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