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There’s a difference between a seachest and a manifold in some respects but the distinction becomes blurred in actual use imho. Often the thru hull supplying the manifold is larger than any of the hoses leading off the manifold. Commonly there are ball valves that are at the origin each of the supplied hoses. Issues of pressure differential are attenuated and if concern one can simply shut off the other drainages. Of this may limit doing multiple simultaneous activities but avoids having fluid being drawn from one draining hose to supply another draining hose. Also no device is being starved for supply. On my boat several relatively low flow devices are supplied by a large thru hull. Available flow through the thru hull exceeds potential demand. Highest flow is to watermaker and genset. Even with both in use together flow to either isn’t comprised. So you can get in trouble with a manifold but it’s unusual. Occurs only if several high flow devices are being fed by a thru hull of inadequate diameter. Of course it’s a total non issue if the valves for unused hoses are shut off or there’s a pump in those lines which allows flow only when the pump is turned on.
Still to drain holding tanks, heads, supply AC cooling water, drain sinks or bilges a dedicated thru hull seems wise. Of course the supply to propulsion engine(s) should be dedicated.
 

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Discussion Starter #42
There’s a difference between a seachest and a manifold in some respects but the distinction becomes blurred in actual use imho. Often the thru hull supplying the manifold is larger than any of the hoses leading off the manifold. Commonly there are ball valves that are at the origin each of the supplied hoses. Issues of pressure differential are attenuated and if concern one can simply shut off the other drainages. Of this may limit doing multiple simultaneous activities but avoids having fluid being drawn from one draining hose to supply another draining hose. Also no device is being starved for supply. On my boat several relatively low flow devices are supplied by a large thru hull. Available flow through the thru hull exceeds potential demand. Highest flow is to watermaker and genset. Even with both in use together flow to either isn’t comprised. So you can get in trouble with a manifold but it’s unusual. Occurs only if several high flow devices are being fed by a thru hull of inadequate diameter. Of course it’s a total non issue if the valves for unused hoses are shut off or there’s a pump in those lines which allows flow only when the pump is turned on.
Still to drain holding tanks, heads, supply AC cooling water, drain sinks or bilges a dedicated thru hull seems wise. Of course the supply to propulsion engine(s) should be dedicated.
Lots of thru hulls represent potential problems. But using a single thru hull / seacock is putting all your eggs in one basket... or so it seems.

I suppose this depends on how many devices need to either drain or draw sea water. You may be adding equipment which require plumbing to seawater. This becomes a design issue. I removed a sink and then used the thru hull to supply a deck wash pump, before. When I had a water maker it drew (shared) water from an existing galley thru hull. A sea chest was not an option and never considered... and I didn't know about the concept either. I worked with what I had.
 

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What ever floats your boat. Personally if I had infinite funds would do standpipes and eliminate thru hulls altogether. Put a small piece of foam from entrance to waterline and not even worry about freezing. Do it in metal with heated pipe wrap to have belt and suspenders. Have always had trouble with thru hulls in the past. Especially those for heads. It’s one of those things you need to exercise, replace as needed and be kind to. Then they are just another thing on the check list. All in all I don’t like holes in boats. Think the fewer you have the better off you are.
 
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