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Old 05-03-2008, 13:24   #1
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Explain a water chest..

I couldn't find anything on a quick Google, so...

What is a "water chest" and how does it work?

Here's my speculation. I just want to see how close I am.

It is a gravity fed water tank: that is it is below water line and fills through a (probably large) thru-hull, or maybe more than one? So, no pump is used to fill it. Might be that they are sometimes built as an integeral part of the hull? Or maybe are some built where the water goes through strainers first? Probably some sort of one-way air lock so air could bleed out would be needed as well. Seems to me you wouldn't want to complicate it by using pumps to fill it and having to sense whether it was full, etc.

Then various taps off it for engines, A/C, raw water wash down, etc. Taps would be down low on the tank.

Anyway, as usual, my mind is just pondering. I'm kinda wondering if it'd make any sense to try to convert mine to work that way. Not very seriously, just pondering.

So, after my guesses above:
Theories?
How are they built?
Variations?
Features?
Pros/Cons?
And the marketing question: What color should they be? (slightly obscure reference for some of you...)
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Old 05-03-2008, 15:20   #2
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The idea is you build a small water tight compartment below the water line. You make one large sized through hull so the chest fills. You can then add as many through hulls to the chest and have a way to close off all of them at one time. Making it all water tight yet able to be cleaned forces some design issues.

The pros is it limits the number of holes in the boat and the cons are you still need gray water drains. It may also force you to make longer raw water runs since they all come from one central place. You may also use some central strainer as well to reduce costs. The main through hull does need to be large so you have enough supply for all the various uses.

Personally I wouldn't want one on any of the past boats I have owned since the runs would be so long. Long raw water runs tend to grow stuff in them and clog easily. Not all boats are set up the same so I would weigh all those issues before you start off down that road.

It might work best on some sort of twin plus engine generator setup on a power boat though. Anything that planes can have a hard time having enough places to poke holes in the bottom that don't come out of the water on plane.
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Old 05-03-2008, 15:58   #3
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Quote:
Originally Posted by Pblais View Post
The idea is you build a small water tight compartment below the water line. You make one large sized through hull so the chest fills. You can then add as many through hulls to the chest and have a way to close off all of them at one time. Making it all water tight yet able to be cleaned forces some design issues.

The pros is it limits the number of holes in the boat and the cons are you still need gray water drains. It may also force you to make longer raw water runs since they all come from one central place. You may also use some central strainer as well to reduce costs. The main through hull does need to be large so you have enough supply for all the various uses.

Personally I wouldn't want one on any of the past boats I have owned since the runs would be so long. Long raw water runs tend to grow stuff in them and clog easily. Not all boats are set up the same so I would weigh all those issues before you start off down that road.

It might work best on some sort of twin plus engine generator setup on a power boat though. Anything that planes can have a hard time having enough places to poke holes in the bottom that don't come out of the water on plane.
Thanks. That's pretty much what I thought it was.

But as I read what you said, I realize that in a planing hull, the effective waterline while up on plane might mean the box wouldn't fill. There are strainers with scoops like you use on a jetski, but with the resultant air coming in as well, doesn't sound like it would work well to me.

So, any thoughts? Anybody heard of a waterchest being used on a planing hull? The only place I've heard of them have been on full displacements, like the Nordhavn, Molokai Strait etc.

I was never very serious about doing it on mine, but thought there might be advantages I hadn't thought of.

What did make me think of it is:
  • I don't like the single pump for my 2 A/C units (+1 thru-hull)
  • I've considered adding a 3rd A/C (+1 more)
  • If I actually did do something really crazy like do a heat exchanger cooled PC (now we're up to 3 more thruhulls)
With all the above and especially if there were other benifits as well then maybe it might make sense. But even if I went that crazy, if it's a problem on plane that would put an end to it.

But as always, the info may come in handy on my next boat...

-dan
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Old 05-03-2008, 16:52   #4
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You don't have to have just one input to a water chest. I believe I read an article somewhere where Dave Gerr suggested one inlet on each side of the keel. This would greatly diminish (if not eliminate) the possibility of plugging your engine raw water intakes with say a plastic bag and could save an engine.

A water chest does not have to deliver water to all areas of the boat. It could just be used for engines, generator, A/C units and perhaps other uses in the vicinity of the water chest.

The biggest problem I see with a water chest on small boats is finding a place for it.
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Old 05-03-2008, 18:25   #5
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I have one. Not a great idea on smaller boats or planing hulls...but it works great for me. Mine is a stainless steel cylinder attached via a 2" thorugh hull and then I have 6 taps coming off it. A big screen basket sits inside to filter debris and a clear plexi top allows viewing and removal for cleaning. It is very convenient and eliminates multiple filters and does not clog as easily as small filters. Good idea...but obviously not for everyone or every boat. BTW...they are also known as manifolds.
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Old 05-03-2008, 18:29   #6
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In addition to sea chests, I have also seen sea chests for exiting water...multiple a/c
is one example.

"The fewer holes in my boat....the happier I am"
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Old 05-03-2008, 19:09   #7
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I will have a sea chest on my boat. It will look a bit like this one:
Website Updates Main Page

Only mine will be round stainless steel. The principle is the same.
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Old 05-03-2008, 20:22   #8
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Well, based on the boat you have, there should be no problems using a sea chest. Planing hulls leave about 2/3 of their waterline length in the water, so water pickup is only a concern on the smallest planing hulls that bounce around a lot. In fact, that's a benefit of a sea chest--more reliable water flow in heavy weather (or when a planing hull goes airborn). Your intended use, air conditioners, don't draw much water compared with main engines. You're not talking about a very big hole, and you can easily cover it with a bronze scoop. I think you have a solid glass hull, making new through-hulls pretty simple. So if a sea chest works for you, go for it!.

But remember that if you install a new all in one a/c, it is not meant to go in your engine room (not ignition protected), so you'll either have long piping runs or you can solve your problems with a new through hull. Then there's the issue of patching the hull over an old through hull. You have to grind out a big cone--a real pain in the butt. If you do a full cost/benefit analysis, I suspect you'll come down on the side of putting in another through hull. After all, you're not likely to go more than 100 miles from land. Check you through hulls regularly, carry a set of wooden plugs and a couple of good radios. Heck, for an a/c sized hole, your bilge pumps may keep up.

Brett
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Old 05-03-2008, 22:18   #9
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Quote:
Originally Posted by Stranded Mariner View Post
I will have a sea chest on my boat. It will look a bit like this one:
Website Updates Main Page

Only mine will be round stainless steel. The principle is the same.
Your link was to an index page. I didn't see the sea chest...
Quote:
Originally Posted by LtBrett View Post
Well, based on the boat you have, there should be no problems using a sea chest. Planing hulls leave about 2/3 of their waterline length in the water, so water pickup is only a concern on the smallest planing hulls that bounce around a lot. In fact, that's a benefit of a sea chest--more reliable water flow in heavy weather (or when a planing hull goes airborn). Your intended use, air conditioners, don't draw much water compared with main engines. You're not talking about a very big hole, and you can easily cover it with a bronze scoop. I think you have a solid glass hull, making new through-hulls pretty simple. So if a sea chest works for you, go for it!.

But remember that if you install a new all in one a/c, it is not meant to go in your engine room (not ignition protected), so you'll either have long piping runs or you can solve your problems with a new through hull. Then there's the issue of patching the hull over an old through hull. You have to grind out a big cone--a real pain in the butt. If you do a full cost/benefit analysis, I suspect you'll come down on the side of putting in another through hull. After all, you're not likely to go more than 100 miles from land. Check you through hulls regularly, carry a set of wooden plugs and a couple of good radios. Heck, for an a/c sized hole, your bilge pumps may keep up.

Brett
Yep, no A/C units in the engine room, just 2 engines and a genset. If I add another A/C it will go in the equipment room (under the helm, directly in front of the engine room) to join the main A/C, water heater, and holding tank. Holding tank may move back to the engine room, although it could stay where it is, it'd just be a little crowded in there. The other A/C is under the v-berth, now that's a long run for the water, and the return comes all the way back to equipment room. The A/C system has 1 pump and a Y that splits the flow to the two A/C units. Something somewhere makes more of the water flow go through the larger unit than through the small one. Could be a smaller coupling, a restriction in the unit itself, or just all that extra run of hose (and probably fittings) running to the bow and back. Might be on purpose because of the unit size, or could be a problem. Don't know, and not really that concerned about it. But I'd rather have a separate pump for each.

As to 100 miles, I have a 600 mile range at hull speed, about 400 up on plane. I plan on max of 300/plane 450/hull for safety. So 100 miles offshore is a fairly good bet.

But back to my useless quest for knowledge: Will the sea chest stay full when the boat is up on plane where the bottom of the sea chest will actually be barely lower then the water level? I just am not quite getting the physics of how it works if there is no pump pressurizing it. I totally get it for a full displacement boat, just not a planing hull.

One thing you said strikes me. I was figuring I'd do a sea chest for everything (and figuring that there was really no compelling reason for me to do it at all). But doing a small one for just the A/C units might actually be a good idea. Could be the existing thru-hull can handle the flow for all of them. It handles a 16k and a 12k now, maybe it could handle another 12k? If that's the case, I could do it all myself with no haul-out. Worst case would be haul-out and enlarge the hole for a larger thru-hull. So the question remains, how do it work on a planing hull?

-dan
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Old 06-03-2008, 00:51   #10
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Something went wrong with the link. Here's the picture:
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Old 06-03-2008, 08:52   #11
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Something went wrong with the link. Here's the picture:
Great! That helps a lot. The only other ones I have seen were for firefighting boats and large boats. I envisioned something larger than this, but thought something smaller should work.

Thanks.

-dan
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Old 06-03-2008, 09:20   #12
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Sea chests are kept full in three ways:
1. if below the water line, even by 1 inch, static head pressure will fill them.
2. Once you have a solid water colum (a full sea chest and supply lines going out), a vacuum will keep the sea chest full. Think about filling a straw, pinching off the end, and lifting it out of your glass.
3. When on plane, water is rammed into the sea chest. You coupld probably put a sea chest 10' high and it will stay perfectly full while on plane.

To figure out if your current through hull is sufficient, you need a little more info. Your a/c manufacturer can tell you the gpm requirement for each condenser. Knowing this, it is a simple calculation (or empiracle test) to figure out if your pump can satisfy the demand. Ballpark coolant hoses at 4 gpm max for your garden variety 3/8 hose. A quick internet search or a couple of minutes with a milk jug will give you a more exact answer.

Brett
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Old 06-03-2008, 09:31   #13
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Quote:
Originally Posted by dacust View Post
Great! That helps a lot. The only other ones I have seen were for firefighting boats and large boats. I envisioned something larger than this, but thought something smaller should work.

Thanks.

-dan
Mine will be round and made of stainless steel, but with the same features, like the transparent top, and the inlet strainer. I will have all the seawater inlets coming form this, apart from the two heads, which will have their own inlets.
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Old 06-03-2008, 13:56   #14
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3. When on plane, water is rammed into the sea chest. You coupld probably put a sea chest 10' high and it will stay perfectly full while on plane.
That's kinda what I wanted to hear. I'll just do some more research online and then decide whether I want to do it or not.

Thanks for all the replies!

(Not trying to discourage any more replies if people have something to add!)

-dan
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Old 06-03-2008, 14:01   #15
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I will have a sea chest on my boat.

Only mine will be round stainless steel. The principle is the same.
Stranded...may I suggest you stick with the fiberglass as stainless in an anaerobic environment will eventually fail. Mine is stainless and it has. My solution was to get re-welded and apply an internal coat of epoxy to protect the metal. That may be an alternative fr you as well.
Are you getting a Caliber? Great boat!!
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