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Old 15-08-2011, 18:03   #16
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Re: Thru Hulls on a Steel Boat

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Originally Posted by lorenzo b View Post
if you use a threaded nipple make sure it's #80 so it will not warp from the welding.
For people not familiar with pipe scheduling (#80) see picture. I also use seamless pipe. More information on Saunders valves can be found @

http://www.vandpsolutions.com/files/saunders_typea.pdf
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Old 15-08-2011, 20:08   #17
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Re: Thru Hulls on a Steel Boat

Brent Swain, who's been building and living on steel hull boats for about 30 years, recommends welded in 316 pipe for thru hulls, with 316 ball valves. Yes, I am very familiar with the galvanic scale. Brent says that in the real world these are the most trouble free, long lived thru hulls available. I asked him about galvanic problems, he said that after 10 or 15 years you might see a little bubbling on the paint immediately next to the thru hull. In in other words, very little galvanic trouble.

If you bring the thru hull up inside the hull significantly, ie as a stand pipe, it should be supported by a bulkead or other structure to protect it from impact.

Regards, Paul
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Old 15-08-2011, 21:09   #18
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Re: Thru Hulls on a Steel Boat

more information
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Old 15-08-2011, 21:12   #19
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Re: Thru Hulls on a Steel Boat

I have a standpipe in my boat. Teed off it are the four sisters: engine intake, A/C cooling, head seawater supply and galley seawater supply. There's a Marelon outlet for the galley sink, and a Marelon outlet for the head sink.

There are no other below WL holes in the boat. I am making the exhaust (stb. side, short run) larger in diameter and will install a "sailing shut off" to avoid the issue of backsiphoning at heavy heels, and I will be rerouting all fuel and water tank vents to the pilothouse roof instead of the gunwhales, which I find of limited sense.
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Old 15-08-2011, 21:32   #20
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Re: Thru Hulls on a Steel Boat

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I have a standpipe in my boat. Teed off it are the four sisters: engine intake, A/C cooling, head seawater supply and galley seawater supply. There's a Marelon outlet for the galley sink, and a Marelon outlet for the head sink.
In some case it could be that the engine suction may be such that to deprive other tee-off or worse bring air into is cooling.
Where do you get the seawater for flushing the toilet? A common line for toilet flush and galley sink is not very hygienic.
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Old 16-08-2011, 03:57   #21
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Re: Thru Hulls on a Steel Boat

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I've decided to go with marelon, and at the same time shift the locations of all the thruhulls to readily accessible locations. As many have noted, they need to be exercised regularly to prevent seizure, and some of mine haven't been touched since installed 33 years ago. The most difficult ones to relocate will be the cockpit drains, as they are way back under the cockpit and to climb in there requires a trained snake with arms. I will be moving them up to the front of the cockpit so I can reach them via the normal engine access point. As a benefit, this will also prevent the water from shooting up 4 feet high every time I hit reverse.
Have you considered straight sections of mild steel pipe from the cockpit to the hull, simply welded each end. No need for seacocks. That's the setup on my steely. It's been that way, untouched, for 30 years and still appears sound. Just a suggestion.
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Old 16-08-2011, 09:03   #22
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Re: Thru Hulls on a Steel Boat

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In some case it could be that the engine suction may be such that to deprive other tee-off or worse bring air into is cooling.
Where do you get the seawater for flushing the toilet? A common line for toilet flush and galley sink is not very hygienic.
You think? They are separate T-offs, and there is no way for "backflow" from the head to get back to the standpipe, and the galley seawater intake is below the head intake, anyway.

As for the engine intake depriving the A/C, they are never on at the same time, as the A/C is strictly shore-power. In addition, the depth of the standpipe below the waterline means there's no shortage of water in that column.

I believe standpipes make sense for any ocean boat for the same reason I think simple hoses leading to the deck are better than a siphon break in an exhaust system of the spring and plastic piston/vent type: Fewer moving parts subject to corrosion, fewer holes in the boat, ease of access.

Others may not concur.
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Old 16-08-2011, 09:05   #23
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Re: Thru Hulls on a Steel Boat

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Have you considered straight sections of mild steel pipe from the cockpit to the hull, simply welded each end. No need for seacocks. That's the setup on my steely. It's been that way, untouched, for 30 years and still appears sound. Just a suggestion.
That's what I have on my sailing helm, which is more or less a 4 x 2 x 15 inch deep footwell. There are two down-angled three-inch pipes leading aft to the relatively high transom. The footwell drains in seconds, and the three-inch pipes are also structural in supporting the "box" itself. Clever and old-school.
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Old 18-08-2011, 06:28   #24
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Re: Thru Hulls on a Steel Boat

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You think?
Others may not concur.
The other day my wife was chatting with an old cruiser, many times round the world he was explaining to her how he got very sick after washing is dishes with sea water.
Toilets and mainly the electrical ones have a tendency to re-circulate, in still water what come out come back in. I too have T-offs and know the limitation of such system but the inlet for the toilet is separate from the rest and the other side of the keel. Also it is not so much the depth of the standpipe but it size that matter.
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Old 18-08-2011, 10:48   #25
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Re: Thru Hulls on a Steel Boat

I do not have an electrical toilet, but a Lavac. I would not use seawater when the boat was still, I would use water from either my 50 gallon "rain collection tank", or my 150 gallons of municipal or RO water, made, again, from seawater collected offshore.

The saltwater in the galley pump is purely for "dirt removal" where fresh water is not required. In fact, I am going to rig a method to flush out the toilet lines with fresh water (probably from the "rain" tank" and probably by the simple method of filling the head sink with fresh water and letting it pass through the entire head plumbing) in order to suppress critters and to rinse out particulates, so to speak.

The toilet inlet has a physical filter and one-way valve. It would be difficult indeed to pump sewage backwards in this system, especially uphill past the loop. While I certainly appreciate the potential problem here, I have a fairly high level of confidence that I do not need to alter the current standpipe set-up. If I'm wrong, it's a relatively straightforward operation to T into the existing head sink drain and draw my head water from there, as it is below the waterline, as is the galley sink outlet.
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Old 18-08-2011, 19:10   #26
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Re: Thru Hulls on a Steel Boat

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The toilet inlet has a physical filter and one-way valve
It is interesting to see how macerated sewage will pass a fine Surflow filter.
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Old 18-08-2011, 19:19   #27
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Re: Thru Hulls on a Steel Boat

Have you considered a sea chest?
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Old 19-08-2011, 03:08   #28
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Re: Thru Hulls on a Steel Boat

Gavinised thick walled pipe or nipple welded direct both ends, with reinforcing triangles welded between pipe and hull.
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Old 19-08-2011, 12:36   #29
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Re: Thru Hulls on a Steel Boat

Plain 'ol steel with a little extra wall thickness.
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Old 20-08-2011, 14:31   #30
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Re: Thru Hulls on a Steel Boat

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Have you considered straight sections of mild steel pipe from the cockpit to the hull, simply welded each end. No need for seacocks. That's the setup on my steely. It's been that way, untouched, for 30 years and still appears sound. Just a suggestion.
How can you successfully clean and apply protective coatings inside a pipe? I found it to be a problem even with short length.
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