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Old 29-08-2015, 08:22   #1
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Best Location For Raw Water Strainer?

Above, at, or below waterline? Seems like I have seen all of the above installations. Which is best and why?
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Old 29-08-2015, 08:33   #2
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Re: Best Location For Raw Water Strainer?

It depends on what system the raw water strainer is feeding. Some pumps just after the raw water strainer do not create a vacuum and must be placed below the waterline as well as the raw water strainer located before it or it will not prime. Other systems can draw great vacuum and the raw water strainer can be located above the waterline.


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Old 29-08-2015, 08:51   #3
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Re: Best Location For Raw Water Strainer?

Ideally above the waterline so that you can clean the strainer without having to close the thru-hull. But that depends on the ability of the raw water pump to draw.
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Old 29-08-2015, 09:25   #4
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Re: Best Location For Raw Water Strainer?

Mine is 9 inches tall. The top 3 or 4 inches is above the WL so I can unscrew the cover, pull out the screen, and clean it without closing the thruhull. Its also low enough that the engine raw water pump doesnt do much lifting.

OTOH, its good to open and close the thruhull valves periodically to keep them from freezing up.
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Old 29-08-2015, 09:41   #5
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Re: Best Location For Raw Water Strainer?

I vote below or as far down as possible. Self priming. You should be able to close that seacock anyway... and usually the strainer is right there.


However on caveat; if it's one of those plastic housing strainers "Above!"
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Old 30-08-2015, 05:43   #6
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Re: Best Location For Raw Water Strainer?

The strainer is a bronze Perko, the water pump on the engine can lift 2M. Can dismount strainer from the bulkhead with a single screw to drop it below the water line to prime it easily so think I will hang it above the waterline.

Agreed about using seacocks to prevent from seizing, in this case seacock is brand new.
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Old 30-08-2015, 06:15   #7
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Re: Best Location For Raw Water Strainer?

I am surprised that not one of the many experts on this forum that could do a lot better job of explaining this than I have responded but I will give it a go. Neither the pump nor the strainer should be at or above the waterline to ensure that there is some head pressure to push the water into the pump. As well it is always critical to make sure that there is an upward incline from the thruhull to the strainer and if possible onward to the pump. Not so critical and mostly not possible with engines and generators but with aircon, toilet, washdown, reefer it is essential for them to maintain their prime and serious damage to the pumps and compressors, etc can occur otherwise.
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Old 30-08-2015, 09:37   #8
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Re: Best Location For Raw Water Strainer?

I have two strainers on my boat and they are both below the waterline. I could not put these above the waterline if I wanted, there just is no space available. We have deep draft and the floorboards are about 8" below the waterline. Yes, I have to close a seacock when cleaning the strainer, but that's just the way it is.

Are you considering relocating a strainer?

Cheers!

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Old 31-08-2015, 06:37   #9
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Re: Best Location For Raw Water Strainer?

Quote:
Originally Posted by undercutter View Post
I am surprised that not one of the many experts on this forum that could do a lot better job of explaining this than I have responded but I will give it a go. Neither the pump nor the strainer should be at or above the waterline to ensure that there is some head pressure to push the water into the pump. As well it is always critical to make sure that there is an upward incline from the thruhull to the strainer and if possible onward to the pump. Not so critical and mostly not possible with engines and generators but with aircon, toilet, washdown, reefer it is essential for them to maintain their prime and serious damage to the pumps and compressors, etc can occur otherwise.
Onestep37 explained this well - you can have the best of both worlds by choosing a strainer design in which the top of it can be installed above waterline, while still maintaining full flow and head pressure to the pump.

Most pumps installed on auxiliary equipment are either self-priming impeller or run-dry centrifugal types. Neither of these will be damaged by losing prime and most compressors will shut themselves down very quickly if cooling is lost.

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Old 31-08-2015, 09:13   #10
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Re: Best Location For Raw Water Strainer?

Mark, I wish I could take credit for that, but it came that way from the factory. It still amazes me how many little things were well thought out by the original designer/builder of these boats.

For instance, every one of the seven bronze seacocks can be accessed in less than ten seconds.
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Old 31-08-2015, 09:20   #11
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Re: Best Location For Raw Water Strainer?

How about the little 2GM20 s? I would expect they could lift a foot or two wo a problem. Once the cover is back on, and seacock open, there should only be as much head as the tubing and fittings contribute if the pump is below the WL is most if not all 2GM20 installations.
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Old 31-08-2015, 09:39   #12
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Re: Best Location For Raw Water Strainer?

Below the waterline.
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Old 31-08-2015, 09:59   #13
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Re: Best Location For Raw Water Strainer?

Quote:
Originally Posted by Delancey View Post
The strainer is a bronze Perko, the water pump on the engine can lift 2M. Can dismount strainer from the bulkhead with a single screw to drop it below the water line to prime it easily so think I will hang it above the waterline.

Agreed about using seacocks to prevent from seizing, in this case seacock is brand new.
I'm not sure it will lift at all when the pump and strainer are full of air only. In that situation you will likely have to fill the strainer with water manually with the seacock closed until you start right?
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Old 31-08-2015, 12:04   #14
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Re: Best Location For Raw Water Strainer?

Most strainers are air-tight (a side function of them being water-tight), and most impellers can pull this small amount of air away without damage on initial start up.

The biggest issue would be a small air leak that allowed the strainer to empty every time the engine shut down. This would wear out impellers much quicker.

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Old 31-08-2015, 13:07   #15
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Re: Best Location For Raw Water Strainer?

Mine are below the water line and right next to the through hull feeding them. What's so hard about throwing the seacock lever?
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