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Old 09-17-2015, 08:52 PM   #21
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OK. I'll bite. Why does it seem to make more sense?
On my current boat I have seven (count 'em, seven) through hulls. I do not close them when leaving the boat due to the hassle, except if I know I am not going to be using her for a long period. (I just replaced all hoses and clamps this year).

With a sea chest with one through hull - it will be shut every time. A no brainer. In fact insurance companies should offer a discount for a sea chest.

On top of that, to change out a through hull you need to haul in many cases. Apart from the one through hull the Sea chest switch out can be done with the boat afloat.
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Old 09-17-2015, 10:56 PM   #22
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Sea Chests ???

Pretty sure there is not a singular large bore isolation valve on the sea chests we're talking about on this thread. A steel hull like Mark's boat would be easily done, however. Most of them are a true sea chest and not an isolable manifold as you are describing.
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Old 09-18-2015, 12:40 AM   #23
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Being ignorant once again....

Do most sea chests go high enough inside the boat to be above the waterline so they can be open on the top? (or at last don't have to be sealed to withstand pressure)

Imagine how nice it would be to be able to change a through hull being able to work both "inside" and outside at the same time.
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Old 09-18-2015, 01:21 AM   #24
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Do most sea chests go high enough inside the boat to be above the waterline so they can be open on the top? (or at last don't have to be sealed to withstand pressure)
Mine does.
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Old 09-18-2015, 01:25 AM   #25
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...
To my mind a better design would allow an engine R&R with just minor pulling of a hatch in the cabin sole , and a similar hatch in the overhead.
Works for me except no overhead hatch (but I expect the engine to outlive me), leastwise no crawling.

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Old 09-18-2015, 07:01 AM   #26
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Maybe I'm missing something here(nothing new there). To change out a through hull, you need to have it "dry" on the intake side. So you would have to have the water level in the sea chest be below the intake. How do you accomplish this without taking the boat out of the water? Do those with sea chests have one big huge valve for the whole thing? I've never noticed such a thing but can't say I've really looked for it on boats I've toured.

To me the attraction of sea chests is one easy to access location and one hole in the boat.The downside is all the extra hose and hose routing. Plus the "single point of failure" issue though I perceive that to be slight.
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Old 09-18-2015, 07:41 AM   #27
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Sea Chests ???

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Originally Posted by caltexflanc View Post
Maybe I'm missing something here(nothing new there). To change out a through hull, you need to have it "dry" on the intake side. So you would have to have the water level in the sea chest be below the intake. How do you accomplish this without taking the boat out of the water? Do those with sea chests have one big huge valve for the whole thing? I've never noticed such a thing but can't say I've really looked for it on boats I've toured.

To me the attraction of sea chests is one easy to access location and one hole in the boat.The downside is all the extra hose and hose routing. Plus the "single point of failure" issue though I perceive that to be slight.

I don't think you are missing anything. In fact, you pretty much nailed it.
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Old 09-18-2015, 09:14 AM   #28
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Sea chests (that I am familiar with) have a removable cover above the water line allowing you to reach in and plug whatever outlet you want to work on downstream. Most only have 2-3 different pipe sizes and plugs to fit are normally hanging right next to the sea chest for use (might have even come with the new Defever I am familiar with...I will have to ask the owner)
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Old 09-18-2015, 10:08 AM   #29
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My post #7 shows the top of the sea chest. It is a 1/4" aluminum plate w/ 6 bolts and a rubber gasket that can be removed, it's above the waterline. Inside the chest is a 2' long perforated tube/strainer that can be removed for cleaning, which I do at lease once a year. It gets fouled up w/ algae and whatnot, not so bad that water flow is restricted.
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Old 09-18-2015, 11:26 AM   #30
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Sea chests (that I am familiar with) have a removable cover above the water line allowing you to reach in and plug whatever outlet you want to work on downstream. Most only have 2-3 different pipe sizes and plugs to fit are normally hanging right next to the sea chest for use (might have even come with the new Defever I am familiar with...I will have to ask the owner)
Ah ha, that makes sense for something like replacing a sea cock on the other side. But if for some reason you were replacing or adding an entire through hull, then it sounds like you still need to get the boat out of the water.
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Old 09-18-2015, 11:35 AM   #31
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Ah ha, that makes sense for something like replacing a sea cock on the other side. But if for some reason you were replacing or adding an entire through hull, then it sounds like you still need to get the boat out of the water.
You just block/seal the grate at the bottom of the seachest and pump it dry.
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Old 09-18-2015, 11:47 AM   #32
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Have a main engine suck air through an aircon pump and you will wish for separate through hulls. It can cause failure of BOTH engines. That single point failure thingy...

I don't see the advantage for smaller boats. Big boats with lots of sea water pumps, then it starts to make sense. Downside is longer hose runs.
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Old 09-21-2015, 02:08 PM   #33
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There are about 7 or 8 Great Harbour owners that opted to have us add our factory sea chest to their older boats. Ask ANY one of them about it and they will tell you it is the best upgrade they have ever done to their boat. The top of the sea chest does indeed extend well above the waterline, so you can remove the lid (with thumbscrews on a Great Harbour sea chest) and get in there to clean the hull screen (most owners keep an electric pressure washer in their engine rooms to blast away growth once a month or so). You can also install another seacock without hauling the boat and without pumping the sea chest dry. We've done it several times. If you're careful, you get less than a quart of water in the bilge while doing it. As was pointed out earlier, it is also pretty convenient to have all of your seacocks in one place.

The biggest advantage though is that you eliminate the need for sea strainers - and the extremely un-fun job of cleaning them! A typical seacock on a mid-size trawler is going to have an opening of about 3/4". Do the pi-r-squared math and you come up with a little less than a half square inch of surface area. Figure maybe five seacocks on a typical modern boat and you have ALL of the saltwater entering the boat through less than 2.5 square inches. That's a lot of flow - and explains why jellyfish, weeds, sand, plastic bags, etc., all get drawn into our sea strainers.

On a Great Harbour, the hull grate measures 8 inches square and is covered by an approximately 60% screen. So, do the math again - 8 times 8 times .6 equals 38.4 square inches of surface area - you're drawing the same amount of water through SIXTEEN times the area. Consequently there is almost NO suction that can be felt at the hull screen. It doesn't pick up any of the aforementioned nastiness - and a plastic bag won't get sucked up against it. In fact, the main reason to have a hull screen at all is to keep fish from swimming into the sea chest!

Before we had sea chests on the charter boats in the Bahamas, I (or whoever was on duty) had to change three or four generator impellers per season since our charterers often decided to test our shallow draft claims by anchoring overnight in three feet of water! However, after adding sea chests, the only time we ever changed impellers was at the beginning of the season.

Done properly, and with some thought to hose routing, a sea chest isn't the cheapest way to get raw water to the various engines/generators/airconditioners/watermakers/heads that need it, but it is, in my opinion, the best way.
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Old 09-21-2015, 02:24 PM   #34
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Just another reason why I want to buy a Great Harbour!
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Old 09-21-2015, 05:31 PM   #35
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Have a main engine suck air through an aircon pump and you will wish for separate through hulls. It can cause failure of BOTH engines. That single point failure thingy...

I don't see the advantage for smaller boats. Big boats with lots of sea water pumps, then it starts to make sense. Downside is longer hose runs.
Hi, Ski. Trying to envision how that relates to just having a sea chest; I assume this would be true with any shared through hull?

Our sea chest hold a lot of water and is set up with separate through hulls servicing anything connected to a pump: engines/genset/AC/raw water wash down. Each pump has it's own through hull/seacock/strainer and the large grate at the bottom keeps most trash out of the chest. A view looking down from the top. (The uppermost through hull is actually a drain for AC condensate and stabilizers and is right at the waterline.)



There are a few other below-the-waterline through hulls in the heads for discharge/drains, so there are no extreme hose runs in the ER--longest is about 8 feet to the genset.

Our chest has a very robust plexiglass top with 24 bolts holding the gasket in place (that I would only remove in dire need); it's barely visible in front of the stbd main. The glass top allows you to see the general condition of each through hull and whether you want to attempt unblocking one of them from the inside in the unlikely event one gets stopped up with jellyfish or something small enough to get past the grate. Some Defever owners have cut an access port in the plexiglas to get at the through hulls, but I'm not convinced we need one.

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