• didi
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  • Registered: 2010-07-23
  • Posts: 4

Topic: through hull for sewage outlet

So I'm looking to put in a 1 1/2" through hull for the out from the head as the boat does not currently have one (only an onboard holding tank in the V Berth). Anyway, I've got a bronze sea cock and through hull to install and was wondering if anyone had pictures or could tell me where the through hull has been placed in their boats. Further more it seems that the actual thorugh hull is very long for the amount of deck it has to actually pass through and because of space I'd rather not build up a backing plate of 1 1/2" or so that would be necessary to snug down the seacock on the through hull, that leads me to my question... is it ok to cut off threads of the through hull or is it recommended you install with all the threads  and a thick backing plate?

Also I have a jabsco hand pump head and was wondering if there needs to be an additional check valve on the sea cock to prevent the water from running in through the head or if the heads one way valves tend to do the trick (or maybe it's no an issue at all?)

Ok, look forward to other peoples thoughts

Wil & Didi

  • Registered: 2006-09-20
  • Posts: 175

Re: through hull for sewage outlet

The outlet *must* have a substantial glassed in backing plate.  At least 1/2".  The thru hull can be cut down as it is a straight pipe thread, not a taper one, but you must take care not to mar the thread and get the end flat, smooth and parallel.

The pipe loops up to the deckhead, and many would recommend fitting a vented loop (with the vent plumbed to an exterior fitting on the side of the coachroof - not through the side below the gunwale - if you want to avoid both smells and flooding). 

A well and regularly maintained Jabsco with vented loops on the waste and the flush pipe (between the pump and the bowl) can be trusted, while the crew is aboard, not to flood the boat if it is always pumped dry till it bubbles, but not while the boat is unattended for any significant length of time.  As, on a C026 the rim will be well above the waterline, don't worry about leaving it on if you are just popping ashore for a meal but I *ALWAYS* shut all seacocks if I will be away from the boat overnight or longer.

  • From: Ottawa, On
  • Registered: 2005-03-10
  • Posts: 353

Re: through hull for sewage outlet

Good practice, but seeing as they are so easily accessible on my boat all seacocks get closed immediately after use - save for the engine raw water intake which gets opened at the start of the trip (weekend, whatever) and closed when I leave the boat.  That'd be a crappy one to forget to open.......

stefan_d's Website

  • Registered: 2006-09-20
  • Posts: 175

Re: through hull for sewage outlet

*YES*.  On our boat, the cooling water, fuel shut-off valve and engine battery switch all get turned off together, without any other distractions in-between, and the key removed from the battery switch. They are all turned on together as well. Woe betide the 'helpful' crew member who does one of these tasks without completing the other two.

Even so there have been a few occasions where the seacock was closed to check the filter and never got reopened . . . Not good!

Also I forgot to mention above that in really heavy weather all unnecessary seacocks should be CLOSED.

  • From: Massachusetts
  • Registered: 2006-03-07
  • Posts: 97

Re: through hull for sewage outlet

What about the cockpit drain seacocks? These are the only I don't close...

  • Registered: 2006-09-20
  • Posts: 175

Re: through hull for sewage outlet

Don't have that problem! ;-)
Early UK boat so no self draining cockpit.  There is a drain through the transom with a non-return valve to keep waves out that comes out 6" up the aft cockpit bulkhead that is supposed to help if you take a wave in the cockpit, but its a glassed in copper tube with no seacocks.

Unless you have a full cockpit cover to use when the boat is unattended, there is no point in having cockpit drain seacocks.
You'd be better off without them and with double clipped armoured hoze, but your insurance probably expects them to be there.

7 Reply by stefan_d (edited by stefan_d 2010-08-16 11:08:27)

  • From: Ottawa, On
  • Registered: 2005-03-10
  • Posts: 353

Re: through hull for sewage outlet

forgot about them - gotta leave them open, theyre double clamped with reinforced hose (the good stuff).  Candadian small vessel construction standards require that any thru hull has a positive means for closing it.  I was debating whether or not to even put valves in when i redid the thru hulls, and decided that even if i popped a hose, as long as i was on the boat, i'd probably manage to get it shut and save the thing from sinking.  i spray some krown into the valve bodies from the outside, and exercise the valves a few times before winter layup (leaving them open of course!) - and again in the spring.

stefan_d's Website

  • Registered: 2009-12-25
  • Posts: 10

Re: through hull for sewage outlet

I am planing on redoing my 1976  Contessa's cocpit. I do like the idea of drain through the transom. Could somebody post some pictures , if any available?  My gole is to make through hull free boat below waterline. Thanks.

  • From: Victoria B.C. Canada
  • Registered: 2010-09-14
  • Posts: 5

Re: through hull for sewage outlet