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Old 08-12-2008, 15:21   #1
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A Smarter Seachest?

I have been thinking about this for a bit so let me float the idea past you all. I have been thinking about installing a sea chest since it would help minimize the number of thru hulls needed. Since it isn’t one of those things you see often, let me describe what I mean. I want to install one large thru hull which fills a chest of water, from the bottom of which are other smaller thru hulls to other devices such as a watermaker or saltwater pump. It can also supply the coolant water for all the heat generating devices. I’ve never seen this on a recreational boat, possibly for good reason. It seems uncommon on working boats too, but at least it’s not so rare that everyone just stares at you blankly. I would provide a link or a diagram, but I can’t find one on the web.

I suspect drag is a large argument against a sea chest. A large thru hull or a grate, as is used on the commercial boats I’ve seen, would create drag. All the ones I’ve seen actually have two grates below the waterline: cold from the bottom, hot out the top. More water flows when the vessel is moving but there is convection when the boat is stationary.

The other issue seems to be heel in smaller boats and there may be an issue of balance also. It’s too easy to rock a smaller boat and spill the water so to counteract this you need high sides and smaller volume of water.

But I think a trimaran, particularly one with a centerboard, could answer both. To me, the idea place (and I say this without knowing the dimensions involved or having a set of plans) is along the centerboard trunk. I think it would nullify the argument about the balance since the weight is much closer to center. I also think no additional drag would be induced by pulling the water from there. Unlike the commercial boats, I think the solution is to pump the heated water overboard above the waterline.

So, what do you think?
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Old 08-12-2008, 16:50   #2
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Interesting idea. I see no reason this couldn't work, but, aside from fewer thru hulls, what would be the advantage?Keep in mind that in order to feed the raw water systems you have on board, you will need a certain maximum flow. The thru hull for the sea chest will need to be big enough to accomodate this flow. So, regardless of whether you have 4-1" holes, or 1-4" hole, you wil still have the same size hole in the boat. Eliminating 3 thru hulls (using random numbers), means you will have three fewer failure points, but the severity of the failure of a single 4" hole may outweigh the benefit.
This is just my 2 cents, as I have never dealt with this type of system.
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Old 08-12-2008, 18:57   #3
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Why have a chest, why not one large thru hull that feeds a large pipe with smaller piping branching off, all with valves. I agree send the hot water overboard.

If the chest went all the way up and above to the sea level then it effectively it would be outside hull on the inside where it would be easy to get to and maintain the thru hulls, assuming you could climb down or reach down into it.
You probably could also easily clear away debris if there was just a slotted grating or something on the chest inlet at the bottom.

On the centerline would be the only place I would consider such a thing on a sailboat. Large ships that didn't roll much could put it almost anywhere.

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Old 08-12-2008, 19:36   #4
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The idea of the chest is to even out supply. Not all devices, heads , fridges, watermakers etc. are on all the time so if they were you would need that 4in hole but
if designed smartly you might use for example: a 2in hole keeping a 10gal seachest full. You would have to figure out worsest case, devices that run longer fridge, water maker , ac etc
What are the chances that watermaker and AC would run at same time? In my case never because I would be on a dock for Ac. Heads only for flushing( although some might say to keep these separate). Keep engine separate but sea chest as backup watersupply.

BTW-- The sea-chest needs to be partly below waterlevel to fill without pumps and
all output come though the top above waterline, other wise you just moved all below water thruhulls inside the boat. Go thru the top with a siphon hose to bottom also needs to be well secured , if 10 gals starts to flex thru hull , look out.

I would love to find some more info on this, I don't have one but have thought hard on it. I have 9 hole at present not including engine(saildrive) or Transducers. I have 2 1.5in holes 3 feet apart across middle of boat, either side of keel in engine room. I could build 3ft w X 3h x 1d.
Lets get crazy for a minute.
Build it right into the hull , solid and the top suffiently above waterline with an inspection hatch for maintanence/cleaning. No more sea-cocks..
Well, here comes the nurse with my meds. got to go.
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Old 08-12-2008, 19:49   #5
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This sea chest is not a new idea. It has been used on production boats but mostly power boats. The reason it fell out of favor was the fouling inside the chest which was either impossible or very difficult to control or clean compared to traditional seacocks.
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Old 08-12-2008, 21:31   #6
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Originally Posted by bmartinsen View Post
Lets get crazy for a minute.
Respectfully I think some of you are already permanently crazy coming up with an idea such as has been suggested .

Unless you have some enormous demand for seawater services (a demand which couldn't be fitted on the boats most of us have), then all one needs is a manifold off a pretty much conventionally sized seacock as jheldatksuedu suggests. The manifold can just be very simply and inexpensively made up out of fibre reinforced industrial plastic tees and nipples.

Apart from experience with other boats, on our own we have two seawater suctions - one for the engine (and it should always have its own) and one that serves everything else, including hand pumps in the galley and bathroom, toilet flushing, freezer plant condenser cooling, deck wash down pump, etc just through a manifold. Can't remember the size of the thu' hull fitting but it is around 1-1/4" at approx 900mm below the waterline.

You can fit an enormous amount of water through such a small through hull below the waterline, that even with no pump assitance at all - if you don't believe me take the hose of a seacock, open it, stand back and then panic .
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Old 08-12-2008, 22:06   #7
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Seachests are so common on commercial boats that I can't even think they are unusual.

Cleaning isn't that hard if you are clever. You make sure the grate on the outside is hinged, then bolt the other edge. Remove 2 bolts and the grate swings down. Make the grate out of solid fiberglass or s.s. plate. The diver then scrubs out the seachest and puts the grate back into place. Some ships have chemical injection systems to clean seachests (becoming less common); others rely on antifouling the seachests.

Good practice is to have the area of the inlets 2-3x the area of the connected pipes.

A sea chest with side and bottom inlets is for a "box cooler", a directly immersed heat exchanger for engines and the like. Made by companies like Fernstrum and Bloxsma.
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Old 09-12-2008, 01:29   #8
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Seachests are so common on commercial boats that I can't even think they are unusual.
In case people misinterpret you and start running away with strange ideas about putting sea chests in their little boats, maybe it should be made clear that you left out the word "bigger" before "commercial". Would be unusual to find sea chests in commercial vessels the same size as most of our sail boats and very unlikely indeed on a commercial sail boat of around 34 foot long like the original posters (in fact unusual on any sail boat).

For example, one would be surprised indeed to find a sea chest in a 45 foot charter sail boat or in a 45 foot charter fishing boat (stand up fishing or game type) both of which are commercial vessels. Even for a 45 foot fishing vessel with its demands for wash down water, refrigeration (if not relying on ice), etc it would be unusual to find a sea chest as you describe.

They only (normally) exist where the volumes of water consumed become great for engines, generators, air conditioning, refrigeration, etc especially in the case of larger high powered planing power boats where a chest may be located in the last few metres of the hull that generally stays in the water to assure supply.
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Old 09-12-2008, 01:39   #9
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kai nui: can't 4 x 1in holes be replaced by 1 x 2in hole (equivalent area) ??
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Old 09-12-2008, 02:31   #10
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Yes a 1 x 2" dia. hole has the same capacity as 4 x 1" dia. Holes - the area of a circle varies directly as the square of its radius.
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Old 09-12-2008, 03:54   #11
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I know the mention of 2" has come about in the course of comparing areas rather than any recommendation so the below is not criticising the mention of 2" - am just using 2" as an example to give a feel of the size that might be suitable.

To give an idea of through hull size I have had a look at some bilge pipework drawings here for some biggish powerful power boats - found one which has 2 off 100 imp.gal/minute fire pumps each on a 2" suction. That is a fair big squirt of water from each pump - for comparison a deck wash down pump on a typical pleasure sail boat, which is likely to be the biggest consumer by far, would be very lucky to use 1/25th as much as one of those pump's (ie 4 imp.gal/min).

So unless one is using an impossible amount of water for a small boat a single through hull servicing all seawater services (apart from engine which should always be on its own) can be much less in area than a 2" dia ones area, that even allowing for the possibility of constriction from fouling.

Our own which services all consumers apart from the engine, and includes servicing a deckwash pump, is from memory 1-1/4 inch dia - at most it is 1-1/2 inch (I'll check tommorrow when I am on board) - and approx 900mm below the WL. With everything running there is not even any noticable drop in the water level in the strainer between the through hull and the distribution manifold from suction head loss.

I hope that gives some idea of what is likely to be appropriate in my view. A seachest with multiple suctions off it is way overkill and will only introduce more maintenance and risk (well in my opinion anyway - I am just a simple soul ).
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Old 09-12-2008, 05:24   #12
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Check out Caliber's brouchure... http://www.caliberyacht.com/Download...s_Brochure.pdf they have a picture of their Sea Chest about half way through.

I made a SeaChest out of PVC. Added a bunch of T's (4) to feed the generator, Hand pumps for galley, aft Lavac Head, DC refrigeration cooling, and a spare.

Biggest problem so far is getting the hoses in the right order to ensure constant water flow for the generator.
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Old 09-12-2008, 06:03   #13
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Quote:
Originally Posted by jheldatksuedu View Post
Why have a chest, why not one large thru hull that feeds a large pipe with smaller piping branching off, all with valves. I agree send the hot water overboard.

If the chest went all the way up and above to the sea level then it effectively it would be outside hull on the inside where it would be easy to get to and maintain the thru hulls, assuming you could climb down or reach down into it.
You probably could also easily clear away debris if there was just a slotted grating or something on the chest inlet at the bottom.

On the centerline would be the only place I would consider such a thing on a sailboat. Large ships that didn't roll much could put it almost anywhere.

Jon
This seems a practcal solution, and not so uncommon...I like the idea of having it's top well above the water line...it could have a cap that could be removed and then cleaned out with a big bottle brush...or a pole with a rag wraped around it.
Ad some compessed air or pressureized water inlets and you've got all sorts of possibilities.
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Old 09-12-2008, 09:28   #14
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I thought of the same thing when I was building. I only discounted it because I didn't know as much then as I do today. It only needs to be long enough (tall enough) to go above the floorboard level to clear sea level. I considered putting it in the engine room or the other sub-cockpit areas for ease of access. Your other idea of accessing the water through the centerboard trunk was also considered (and is still worth considering), but I was concerned about accessibility for repair if a blockage arose. It would definitely be worth considering if one wanted to remove a bit of underwater turbulence.

I finally decided to go with three thruhulls, a 1/2" for the head intake forward, a 1" for the engine and any other intakes, amidship, and finally, a 3/4" aft for the galley sink, refrigeration cooler, etc. I realized that the honking great centerboard slot was contributing to far more turbulence than the dinky thruhulls, and that my thruhulls were placed in locations that were very easy to access and maintain. Ultimately, I chose laziness over other considerations.
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Old 09-12-2008, 09:44   #15
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Well...my Tayana 52 had a sea chest and it is a wonderful idea.
The large through hull connected to a sea cock and a 1.5" hose which then fed a stainless steel cylinder with 6 valved connections off of it. Inside the cylinder was a strainer basket so all water reaching the outlets was clean of debris. The top of the cylinder was a flange with 8 holes in it and fitten on top of that was a rubber gasket AND a drilled plexiglass cover. This was all snugged down by 8 bolts with wing nuts.

The sea chest fed engine, generator, 2 a/c's, and a wash down pump with one spare outlet.
The best thing was that I could look down on the chest and SEE the water flowing and the large strainer was easy to get out and clean and lasted a lot longer than the small in line strainers would otherwise have been. An added bennefit was only one through hull to secure and an easy way to winterize...just keep filling the chest with the pink stuff while the through hull was closed!
We DID have problems with the stainless body corroding at the welds. This was cured by an epoxy interior coating. If making one from scratch today I would be inclined to fabricate it out of fiberglass.
Hope this is helpful.
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