raw water strainer location

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Bruce T.
Posts: 169
Joined: Sun Feb 27, 2005 2:52 pm
Location: Almonte, Ontario, Canada

raw water strainer location

Post by Bruce T. »

I had a great idea for locating my raw water strainer.

Most people seem to mount their raw water strainer near the intake, usually mounted to the stringer. I decided to mount mine high, above the water pump. I figured that this would have many advantages. Since the strainer has a clear lid it would allow easy inspection and cleaning without having to get down on my knees. It would allow me to see if sufficient water is flowing to the water pump. It would also provide a small reservoir of water above the pump that would insure that the pump is always primed.

So now that it's installed I'm having second thoughts (as usual). I'm thinking that to draw the water from the pick-up, up through the strainer, which is about 24" above the pick-up, will require quite a bit of suction. This will require the strainer to be under a lot of negative pressure. I'm wondering if the rubber gasket seal on the top of the strainer will hold the vacuum. I've heard that engine cooling systems are more prone to leaks on the negative pressure side rather than the positive pressure side (before the pump rather than after) and that leaks on the negative pressure side are more problematic since they introduce air and reduce the water flow to the engine.

My strainer is a plastic body unit with the large, clear, 4" diameter screw on lid. The lid has a flat rubber gasket to seal it.

Any thoughts on this?

Bruce.

Scott C
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Joined: Sat Dec 03, 2005 6:52 am
Location: Bacchus Marsh, Victoria, Australia
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Post by Scott C »

My experience with raw water strainers is a bit more 'industrial' than what Bruce is discussing but I suppose that the same principles will apply.

I formerly crewed on barges that were powered by two Detroit 6-71 inline twinpacks (four engines, two transmissions). The raw water system was set up simarly to what Bruce has proposed in that the sea chests were located close to the intake with the pumps located above waterline (approx 20"). We certainly did suffer the vacuum issues of poorly sealed sea chests and the subsequent induction of air to the system. The raw water pumps were Jabsco impellor pumps which did not like being run dry, in fact the smell of burning rubber still haunts me :lol: but they would draw water from a mile away if primed properly.

Our problems were that we did a lot of work in rivers and beachheads where the chances of dragging crap into the chest was highest. Therefore, we were regularly cleaning them out and always carried spare gaskets so that any that were not up to scratch were replaced immediately. The location below deck plating was a real pain in the backside for cleaning and service so we got pretty good at making sure things were right before firing up the mains again.

Bruce how close to the strainer is your pump set up? Obviously the further away means higher suction will be required over and above the fact that you will be trying to draw 24" anyway. I would think that if the strainer body is good for the vacuum and your pump can draw that far that as long as you have a good (and I mean good) seal on the strainer lid you should be ok. Maybe the flat type seal isn't up to it, you might need a twin ring seal for the vacuum. Speculation on my behalf, but certainly something you want to be sure about. You might be able to go down the path of fabricating a strainer body to suit a good gasket that you can get hold of, again speculation.

Good luck,
Scotty

Stoker: Tug boat AT 2700 'Joe Mann'
I used to have a handle on life ................. but then it broke off.

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kens
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Location: Coastal Georgia

Post by kens »

I would think that you place it just at, or, barely above the waterline at rest. This way it is possible to service a leak (heaven forbid!) at rest and there is little vacuum of the pump. A small loop in the line can hold enough water to keep the impeller wet.

If you use a seacock, then it installs below the waterline to close off the water to all parts. This is the best setup since the water can be mechanically closed for safety and the strainer can be below the waterline to provide the best intake for the pump. Again, you can run a little bit of hose above the height of the pump to keep it wet. Also, the pressure side of the pump is not likely to dry out either, it will stay wet.

clearlakepelican
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Joined: Fri Jan 23, 2004 3:29 pm
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Post by clearlakepelican »

Just make sure it isn't in line with a trailer bunk or boat-lift bunk. Nothing like the sound of the "craaaack" of your hull as you hoist your boat up on the strainer housing.

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