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Old 24-01-2010, 13:24   #1
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Thruhull Recess - Why?

Attached is a picture of two thruhulls next to eachother. I've owned a number of boats but never seen thruhulls fitted like this. Why the reinforcement? And if there's a reinforcement, why not make it a part of the hull, why fit it as a loose bit and seal it? Hull is GRP, supposed to be solid, no core.

Or is that just a strangely shaped backing plate?

/Hampus
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Old 24-01-2010, 15:00   #2
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Looks like a strangely shaped backing plate to me. It would also provide a bit more stability to the through hull seacock arrangement.
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Old 24-01-2010, 15:21   #3
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Not sure, but that's no normal Seacock...
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Old 24-01-2010, 15:53   #4
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Thank you.

The seacock is normal enough, at least that's what they look like around here. The thru hull is strange though. I'll have to change most anyway, along with the standing rigging

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Old 24-01-2010, 16:10   #5
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Hard to tell from the photo but this looks similar to to one of my thruhulls that was replaced going from metric to standard size. It is difficult to drill a new hole where an existing hole from the old thruhull is so a block is glued down inside the hull first to make drilling the new hole easier.
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Old 24-01-2010, 16:27   #6
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Thank you.

The seacock is normal enough, at least that's what they look like around here. The thru hull is strange though.

/Hampus
The seacocks around here have large strong flanges with bolts thru them. Am I missing something? Doesnt look anything like a Groco, Apollo or Buck A that I've ever seen...Those look like panhead self tappers there...Are they screw'd straight thru the hull?
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Old 24-01-2010, 16:34   #7
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Quote:
Originally Posted by Christian Van H View Post
The seacocks around here have large strong flanges with bolts thru them. Am I missing something? Doesnt look anything like a Groco, Apollo or Buck A that I've ever seen...Those look like panhead self tappers there...Are they screw'd straight thru the hull?
Never heard of them... The thru hulls here usually look like this: Untitled Page

Then you screw the seacock onto the thruhull. Seacock could look like this: Untitled Page

You do it differently "over there"?

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Old 24-01-2010, 16:36   #8
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Seacocks are supposed to be installed with a backing plate. Since seacocks are bolted as well as secured by the thru hull, they usually have a rectangular pad or one shaped to the general outline of the seacock. These look like a ball valve with a round backing plate. The backing plates are reccomended because you can put tremendous loads on the thru-hull/seacock if something heavy falls against it or someone stands on it.

Be real careful of ball valves. The ones available at the big box stores are made of brass which is a no no below the waterline. They also may have a ball made of chrome plated steel. That's something almost guaranteed to blow up on you.

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Old 24-01-2010, 16:54   #9
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Then you screw the seacock onto the thruhull. Seacock could look like this: Untitled Page

You do it differently "over there"?

/Hampus
Now I know where we differ...here's what we call a seacock: Groco Bronze Seacocks / Ball Valves

What you have linked to we call a ball valve. Maine sail on the forum has a great page I'll post a link to: Seacock & Thru-Hull Primer/Pre Information Photo Gallery by Compass Marine at pbase.com

BTW, BEAUTIFUL BOAT!!!
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Old 24-01-2010, 17:05   #10
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FANTASTIC link - thanks Christian!
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Old 24-01-2010, 17:42   #11
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FANTASTIC link - thanks Christian!
Thank Maine Sail! He's on the forum right now. He's the one that turned me on to Groco Tri-Flange adapters. Not only is he a great photographer, he knows his stuff when it comes to refitting a yacht! This is his main projects page: Compass Marine Project Articles Photo Gallery by Compass Marine at pbase.com TONS OF KNOWLEDGE THERE!!!

HAMPUS, look here: http://www.pbase.com/mainecruising/backing_blocks
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Old 25-01-2010, 03:25   #12
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That looks very common to me...many seacock’s have a round wooden plate fitted to the interior side of the hull....it would be shaped to compensate for the shape of the hull.......am I missing something here.
I have never seen a proper thru-hull installed with out one?
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Old 25-01-2010, 03:46   #13
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Quote:
Originally Posted by Christian Van H View Post
Now I know where we differ...here's what we call a seacock: Groco Bronze Seacocks / Ball Valves

What you have linked to we call a ball valve. Maine sail on the forum has a great page I'll post a link to: Seacock & Thru-Hull Primer/Pre Information Photo Gallery by Compass Marine at pbase.com

BTW, BEAUTIFUL BOAT!!!
Thank you!

It's probably just me. I mixed peas with melons. I havn't grasped all the english terminology yet and I thought that a seacock was just a generic valve that passed through the hull

I guess they choose ball valves for ease of use. The ones on the picture are used frequently but are kind of hard to get to. I'll consider replacing them with proper seacocks though Ball valves are very common here. We usually use seacocks for the engine exhaust or for the cockpit drain pipes/hoses. For toilets, zinks and cooling water intakes we usually use ball valves. then again, the water over here isn't as salty as in most other places.

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Old 28-01-2010, 11:59   #14
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That looks very common to me...many seacock’s have a round wooden plate fitted to the interior side of the hull....it would be shaped to compensate for the shape of the hull.......am I missing something here.
I have never seen a proper thru-hull installed with out one?
Ditto.....
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Old 31-01-2010, 14:49   #15
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They are there to help spread any side loading on the thru hull fitting over a wider area and prevent them breaking off from the inside if something hits them. IE an anchor gets loose in the lockr and slides into the fitting. At least thats what I've been told by a surveyor. Most cheap boats have a thruhull with the large nut flush against the hull and then the seacock threaded on. Step on one or hit it sideways and its easier to break. With a wider base it can take more load.

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