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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 10-07-2008
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Raw Intake Screen

Hi All,
I was out on a Dana 24 last weekend motor sailing from San Diego to Dana Point. The owner was kind enough to invite me along since he had read my questions on the Yahoo Dana group site and could tell I would really like to see one (the sailing community is the best). Anyway, at one point about 3 hours into to the trip the sound of the exhaust changed to more of a "pop-pop", or stuccatto sound. About 20 minutes later the high temp alarm came on an we shut down the engine. Long story short, the "logs and frogs" raw water strainer had some kelp and a small fish in it which, once removed, restored the water flow from the exhaust to normal. The "pop-pop" sound change was likely due to the reduced water level in the muffler.

The owner decided to take his Dana to a boat yard who recommended that a screen be installed over the raw water intake through-hull. It sounded to me like the screen would act as a scoop, by configuration, and I was wondering if, when the engine was shut down and under sail, if the scoop design would force water past the impeller, into the exhaust and somehow get back into the engine. The engine is a Yanmar 2GM20F I believe. Should he close the seacock once the engine is shut off?
Thank you.
Stu
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Old 10-07-2008
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I'm having trouble understanding how a screen over the intake thruhull, which is usually on the bottom, would be a scoop.

Then again, I don't get why you would want such a thing if you have a strainer. If something gets stuck in the strainer you can clear it from inside your boat--if something got stuck over an outside screen, you'd have to dive to clear it.
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Old 10-08-2008
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Quote:
Originally Posted by Stu01 View Post
The owner decided to take his Dana to a boat yard who recommended that a screen be installed over the raw water intake through-hull. It sounded to me like the screen would act as a scoop, by configuration, and I was wondering if, when the engine was shut down and under sail, if the scoop design would force water past the impeller, into the exhaust and somehow get back into the engine. The engine is a Yanmar 2GM20F I believe. Should he close the seacock once the engine is shut off?
The only screens on thru-hulls that I've ever seen are more of a "dome" than a "scoop" - the idea being that, so long as the boat was moving, current along the hull would push whatever it was off whilst ensuring that the intake coudn't ever get blocked (except by a plastic bag or similar, which is a problem at any time).
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Not a scoop... don't sweat it.
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They probably mean something like this Groco part:



You can also obtain the screen part separately, Groco makes a hinged version, and there are dome-shaped ones (separate).

I would imagine even the scoop-shaped ones, if the screen is facing forward, probably can't generate enough water-pressure to overcome the stopped raw water impeller.

Jim
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Old 10-08-2008
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This screen is a must if you want to avoid sucking in kelp and plastic bags. We have many plastic bags in the water. Sometime its just couple of feet underwater and looks like jelly fish. Anyway, my friend's engine sucks in a plastic bags once and it caused over heating alarm to come on. I've a screen (not scoup type) for my engine raw water intake.
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Thanks for the replys.
It seems weird that Pacific Seacraft (once on the West coast) wouldn't have the through-hull screened, especially with all the kelp out here.
Stu
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