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Old 14-01-2014, 08:48   #1
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Sea Chest or stock strainer?

I need to replace the sea chest on my boat and am trying to decide between recreating the current one or using an oversized Groco strainer leading to a manifold to distribute sea water to the 88hp engine, 5kw genset, water maker, and wash down pump.

The current sea chest is cylindrical in shape with 5 outlets scattered around its circumference (one capped and unused), each equipped with ball valves.

Would it be a bad idea to replace this with a very large (like you see on motor yachts) Groco strainer leading to a short manifold made up of stock pipe fittings and valves? I would plan to have the outlet from the strainer be below the waterline, as large in diameter as possible and gradually step down to the appropriate hose sizes for each user of seawater. My fear is that the suction of the engine raw water pump might cause problems with the other users. For example, if both the engine and genset were running, would the 17gph watermaker have any problem getting enough water to function properly? Any other potential problems anyone can think of?
In favor of the stock Groco strainer is that it is a known quantity and all the parts of the manifold could be replaced or modified using stock parts. Even if I spent $400 on a large strainer, it would still be cheaper than having a stainless steel sea chest fabricated from scratch.

If anyone has experience with this sort of thing and can share their insight, I'd appreciate it a lot. Am I asking for problems by going to a large Groco strainer and manifold arrangement, or is it a viable alternative to replacing the existing sea chest?
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Old 15-01-2014, 17:03   #2
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Sea Chest or stock strainer?

First of all, even if you rebuild the sea chest, you should have the fill go through a big strainer. Much easier to monitor clogging and avoid problems. That is how I set up my sea chest.
Most people say that the engine should always have its own thru hull fitting.
That is a knee-jerk position, but it is conservative and safe. Even if you know the flow rate needed by your engine, the efficiency of the raw water pump is going to be affected by the height of the intake over the waterline, pump wear, etc. and this entails a sophisticated calculation.
It is reasonable to believe that if you engine raw water pump inlet is 1.5 " in diameter, that a thru hull that size would be adequate for the engine alone.
If the genset has a 1" inlet on its pump, the air conditioner has a 5/8" inlet, etc. then the required through hull for a sea chest serving all three can be calculated based on the formula . Area of a circle is pi times radius squared.
1.77 square inches area for the engine inlet, .78 for the genset and .31 for the air conditioning pump.
Total is 2.85 square inches of inlet for all three.
Or 1.9 " diameter inlet. So if you had a 2" thru hull for the sea chest/manifold, you could service all three.
As long as the manifold does not get clogged ( you could inspect it easily) why wouldn't it work? You would want it to be substantial. How about an 8" diameter aluminum pipe"
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Old 15-01-2014, 17:30   #3
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Re: Sea Chest or stock strainer?

Thanks, I did the same sort of math that you did and arrived at the same conclusion but just was wondering if I was missing anything. The numbers in my case are a 1.5" through hull leading to the sea chest. The sea chest has an integral strainer. If I go with a Groco strainer, it would have the same 1.5" inlet and outlet leading to a 1.5" manifold. The current engine inlet is 1" and the genset inlet is 3/4" and the watermaker inlet is 3/4". So, doing the pi R squared thing yields and area of about .44sqin for the two 3/4" outlets and .78sqin for the engine, which all add up to slightly less than the 1.76sqin total and that would only come into play if all 3 were operating at the same time, which isn't likely to happen often. I'm not worried about the engine and genset because the Groco strainer we'd use has plenty of capacity and they are self priming, but the feed pump for the watermaker is not self priming so if there was a suction applied to it, that could cause an air bubble which might cause it to shut down. But with the size hoses involved and the whole thing being below sea level it seems to me that the pressure downstream from the strainer shouldn't decrease appreciably unless the strainer got pretty dirty. But I still don't know......
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Old 16-01-2014, 08:40   #4
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Re: Sea Chest or stock strainer?

What about a standpipe? Same idea, but it trades width on the hull for height, and can be reamed out from above if needed, and plugged from below if the hull strainer cover is hinged or pinned.
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Old 16-01-2014, 08:51   #5
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Re: Sea Chest or stock strainer?

If you decide to go the manifold route, Groco makes a thing called a Raw Water Manifold that might help to keep the installation neat.
Link: GROCO MARINE PRODUCTS

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Old 16-01-2014, 13:16   #6
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Re: Sea Chest or stock strainer?

Thanks for the ideas all! I just checked and there isn't enough vertical room for a standpipe so it will have to be horizontal. The good news is that I've found another 3/4" thru hull that I can dedicate to the watermaker so this one will only have to handle the engine and genset, and usually not at the same time. I will also have 2 more valved outlets on the manifold, one as a spare and one for the deck washdown pump, but their water demands are inconsequential compared with the engine(s). Now that the watermaker is eliminated from the equation, I think it's safe to go with the large Groco strainer supplying both the engine and genset.
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