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Black Water Thru-Hull Location

5397 Views 48 Replies 14 Participants Last post by  eherlihy
My boat does not have a thru-hull for the black water output. I plan to drill a hole, and install the thru-hull and seacock. How far down relative to he waterline should the hole be drilled? Thanks in advance for the help.
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The boat is a 1984 Bene First 35. Currently has a bag, but we will be converting to a rigid tank within the next year. The head is on the port side just behind the v-berth. The tanks is in the v-berth. The pump out is on the starboard side and about even with the back of the v-berth.
On a boat like this the typical setup is to have a "Y-valve" which directs the outflow to either the deck pump out fitting, or to a diaphragm pump or macerator, and then to a below the waterline through hull which can be secured in NDZs. All of this equipment is typically installed under the V berth, keeping the hose runs as short as possible. This was the factory setup on the Pearson 34, Cal 33, and O'day 34/35.

An issue with this is that the line can loose vacuum through the deck fitting and "Y-valve", making it hard to pump out. One solution is to put the pump before the "Y-valve", but you run the risk of someone activating the pump without opening the through hull...:puke

Another solution is to plumb a second pump out fitting into the tank, and eliminate the "Y-valve" entirely, and the likelyhood of vacuum leaks.

One last tidbit to minimize smelly hoses; make sure that any and all hoses are run so that they drain back into the holding tank, or through the through hull.
If you are planning to install a through hull, then I hope that you have read Maine Sail's EXCELLENT tutorial on the correct way to install backing plates. Do a search for the link.
Why am I not seeing a backing plate? :confused:

It should look something like this:

- or this (Photo credit to Maine Sail / Compass Marine)
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Stumble / Greg's post is spot on. In the case of backing plates, bigger (in terms of area) is always better.
If I had the $$$ I'd like to see my entire hull backed by G10. ;)
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