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Old 17-01-2010, 08:19   #1
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No Underwater Through-Hulls?

I'm in the pre-retirement browsing period and plan to move to the serious looking period in about a year. At the moment I'm leaning toward starting my search with semi-custom catamaran builders.

My life is filled with high maintenance items now (cars, houses, boats, wife, and one of the criteria on my list is to minimize this in my cruising life, realizing that there will be more than enough that can't be eliminated.

So my idea is to have the boat constructed with a tank that is integral to the hull. The bottom of the tank would be well below the water line and the top of the tank well above. The bottom of the tank would have an opening to the water and the top of the tank would be removable for access. Then all raw water intakes would pull from this inside-the-boat tank. Any system needing raw water that provided lift could pull from above the water line and any system that had to have gravity feed would have a through-tank fitting below the waterline. But in that case if something went wrong I would have access to both sides for repairs from inside the boat.

So what's wrong with this idea? What am I not seeing? Why isn't this standard construction?

All comments, even those highlighting my ignorance, are welcome.

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Old 17-01-2010, 08:33   #2
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Well, the input connections would presumably be:

Sink possibly?

So why not just put four hoses over the stern and cover it in a GRP moulding to hide them.

You then have the problem of what are you going to do with the output? Sink, watermaker and engine could be above the waterline but what about the heads?

You are also running long hoses around the boat, (up and down for a cat) so air locks etc. Failure on a long hose and you still sink.

If you goal is simplicity then half a dozen really good quality seacocks will outlast you, be simple to service anywhere in the world, once a year. How often are you going to clean this space out to stop green things growing inside your tank.

Your "KISS" approach is likely to be far more complicated.


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Old 17-01-2010, 08:34   #3
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Sounds good, but will take up a lot of space I suspect-
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Old 17-01-2010, 08:51   #4
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What you are describing is a sea chest. Not a new concept. Look them up on the internet an see how one would work on a boat you like.
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Old 17-01-2010, 08:54   #5
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Like tellie said, this is standard practice on commercial vessels. granted not above and below the waterline like you've suggested, but the same idea.
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Old 17-01-2010, 09:49   #6
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Thanks for the education. I figured the concept wasn't new and after I got past all of the "foot locker" sea chest links I have found some that describe this concept.

I figure this sea chest wouldn't require any more regular attention re: critters than the rest of the hull, and that could be taken care of from inside the boat so should be less onerous. It also seems like a likely place to mount underwater transducers, etc.

Pete - You raise some valid issues but this would not have to be one opening with long lines. The basic idea is to have any underwater openings be an integral part of the boat build and to be carried above the waterline for safety. If I can eliminate holes below the waterline that need to be serviced yearly by hauling the boat, then I'm all for that. My assumption is that the actual opening for the sea chest would not need to be huge, even if it supplied all of the raw water needs for the boat and the tank could be sufficiently large to make access to it easy.
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Old 17-01-2010, 22:20   #7
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And when you make the same thing for discharge it's called a stand-pipe. Stand pipes need no sea cocks.

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Old 18-01-2010, 09:55   #8
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The idea of the water well as per original poster works on cargo ships and some bigger yachts. It is a very good thing and quite common.

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Old 18-01-2010, 10:32   #9
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The Marlow trawlers use a sea chest as you describe. You can see them at a boatshow. If I remember correctly they have a plexi top - maybe so you watch that stray plastic bag get dragged into your strainer

You need all pickups below the water level. Otherwise you need perfect sealing in the chest. You also need a way for any trapped air to vent from the top so you don't get a big bubble in the chest. In general they need to be located carefully to minimize turbulence and air ingestion underway.

My biggest problem with sea chests is that it means longer hose runs. I prefer having sea cocks clustered in 2-3 easy to access spots to yards of hose snaking through dark corners of the bilge. Hoses have caused me much more maintenance and worry than sea cocks.


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