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Old 20-10-2016, 23:31   #1
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Thru Hull Flow Rate Required for Heads

I have two electric toilets in the port hull of my catamaran. They are currently connected to two separate 3/4" thru hulls.

But I am installing a new small air con and will be using one of the existing toilet water intake thru hulls to provide water to the air con (that thru hull will be enlarged to 1" for the air con).

My question is:

1. Can I connect the raw water intake for both toilets to one 3/4" thru hull?

2. If the answer to the above is NO, then can I increase the 3/4" thru hull to 1" to provide more flow for both toilets.

I am trying to avoid adding another hole to the bottom of the boat. But the boat has to come out of the water anyway to enlarge one of the existing 3/4" thru hulls for the air con. So if I really need to add another thru hull, that will be the time to do it.


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Old 21-10-2016, 00:23   #2
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Re: Thru Hull Flow Rate Required for Heads

If you don't flush both toilets at the same time I don't see a problem unless the thru-hull is a long way from one of the toilets.

I think you'd need to go up to 1-1/4" to equal or exceed the flow through two 3/4" thru-hulls.

I don't like sharing intakes. I'd just add another thru-hull with a proper bronze flanged seacock.
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Old 21-10-2016, 11:42   #3
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Re: Thru Hull Flow Rate Required for Heads

I agree with most of what HopCar said...except for increasing the size of the thru-hull to almost double its specified 3/4" size. Among other reasons I wouldn't do that is the potential to pull in animal and/or vegetable sea life that would be too large to make it into a 3/4" hole. You'd also have to install reducing adapters on both of the toilets to avoid pulling in more flush water than the toilet is designed to push out. But even if you only teed or wyed the intake lines to connect to a single thru-hull, if one toilet goes down, they both would.

So I agree 100% with his final bit of advice: install another thru-hull for the a/c pump...or if you're really opposed to putting another hole in the boat, replace one of the toilet with one designed to use onboard pressurized fresh water. Get a quote for a "conversion" from him.
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