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Old 12-21-2012
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Nettles clogging strainer

I have a sea chest that feeds several loads. Supplying it is a 1 ¼” ID through hull and a strainer. Because there are so many loads, there is a fairly significant volume of flow. When the sea nettles are bad (on the Chesapeake), they often get sucked into the clamshell intake. They then clog the strainer and have even gone through the strainer, into the seachest, and have destroyed the impeller on the genset. I’m now very careful when the genset is running to listen for a change in speed that will indicate I have nettles in the cooling. I’d like to eliminate or reduce the problem and was wondering what others might have done to address it.
Thanks in advance.
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Re: Nettles clogging strainer

Huh... learn something every day.. So 'sea nettles' are small jellyfish? How small generally?

We get large infestations of moon jellys but they are usually too large to get taken into intakes. Can't imaging what you might do to prevent pulling them in.. would another type of strainer handle them better?
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Re: Nettles clogging strainer

Nettles can be anywhere from Silver Dollar 5-6 inches diameter to 18-24 inches. nasty little pains in the ass. I am in the middle to upper Chesapeake and they move north over the summer as the salinity line moves north with less rain water into the Bay. Usually we canstill swin at a few of our regular spoits without them. They do present a problem in intakes of even the engine occasionally when they are think.

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Re: Nettles clogging strainer

Are you in a slip most of the time when you have this going on? Could you find a way to agitate the water near your intake. Do they try to stay out of moving water or would this exta water movment bring more food and increase your problem? I do not know if this helps. Lou
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Re: Nettles clogging strainer

The nettles do range in size from silver dollar size to 8 or 10" diameter. They also have long (6" to 4') tentacles that trail behind.
The boat is in a slip near the head of a river that keeps the water mostly fresh and all but eliminates the nettle problem. However, when I am out cruising in July to September, it is oftne a problem - some years worse than others. The solution I'm considering is to remove the genset intake from the seachest and to T it into the engine intake that is currently dedicated. My thought is that I never run the engine and the genset both at the same time. This will reduce the flow volume into the sea chest. Before I make this move, I'm simply looking for some other creative solutions.
Other thoughts might be a dp switch across the strainer, different strainer, backflushing, etc.
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Re: Nettles clogging strainer

Maybe a company that makes the systems or strainer might have insight? Do others in your area have ways to deal with nettels? I thought I would see a few more ideas. It might be like raking leaves in the fall. If you live near them, you have what they bring. best wishes , Lou
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Re: Nettles clogging strainer

Your 'sea-chest' is probably SLOWING the water flow (rate) in comparison to the its inlet/outlet. The comparatively higher flow rate in the piping and 'clam shell' strainer is high enough to 'chop up' the nettles, the low flow rate in the 'chest' is allowing or promoting 'packing together' (agglomeration) of the 'gel-like' particles in the low flow rate areas. This 'packing' is quite common with 'gels' and gel-like contaminants where the flow rates speed-up then slow-down, etc.

To 'filter' or 'strain' such 'gels' you need a LARGE surface area 'strainer' so that the gels are removed at first opportunity in the circuit; the LARGE surface area will slow the fluid velocity and prevent 'chop-up' ... but youre going to need an 'easily cleanable' strainer to do this, and for good measure you should install an accurate 'differential pressure switch wired to an alarm' to tell you WHEN to clean out the inlet strainer'.
Other possibility is a 'duplex' strainer one side 'on' while the other side is offline and 'backwashing' the gunk out the 'off side' ... requires another 'hole' in the boat bottom.

Not to worry however, as the infamous zebra mussels are now in the lower Susquehanna basin and the problems that they will cause in the brackish Upper Chesapeake will completely make sucked-up nettles seem like a 'privilege'.
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Last edited by RichH; 12-21-2012 at 04:05 PM.
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Re: Nettles clogging strainer

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Originally Posted by RichH View Post

Not to worry however, as the infamous zebra mussels are now in the lower Susquehanna basin and the problems that they will cause in the brackish Upper Chesapeake will completely make sucked-up nettles seem like a 'privilege'.
We get zebra mussels on the West coast too.. we have a nearby powerplant that uses seawater for cooling their condensors on the generator steam turbines. The mussels love the warm environment in the condensor and it's a huge issue. They heavily chlorinate the cooling water at the intake, but then have to neutralize it with SO2 before it's discharged back into the sea.

Maybe that's your solution, Harbin Get yourself a wee chemical treatment plant on board...
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Re: Nettles clogging strainer

i love lake sailing!
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Re: Nettles clogging strainer

RichH - thanks for your analysis. I guess it is important to know just exactly what the mechanism is for them to get through the strainer and then to destroy the impeller of the pump.
You have some good suggestions - one's I've thought about but not spent much time considering. I'll look into other strainers (although there is limited room below decks).
Of course, your thoughts also support my idea of moving the genset intake to a different seacock. This would also serve the purpose of reducing the flow. I have envisioned the flow into the genset being so great that it draws any nettles in that get to within a foot or two of the intake. I'm not sure if they ever attempt to resist being sucked in (sob sob). If I moved the genset intake to the engine, the flow would be reduced to both it and to the sea chest.
Anyway, some modification will be made this winter.
Thanks again to all.
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