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24-01-2010, 13:24
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#1
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Registered User
Join Date: May 2008
Location: Sweden
Boat: Between boats
Posts: 474
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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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24-01-2010, 15:00
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#2
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Registered User
Join Date: May 2006
Location: Nicholasville, Kentucky
Boat: 15 foot Canoe
Posts: 14,191
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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.
regards,
__________________
John
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24-01-2010, 15:21
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#3
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Registered User
Join Date: Oct 2008
Location: Princeton, NJ
Boat: Challenger Anacapa 42
Posts: 2,097
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Not sure, but that's no normal Seacock...
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24-01-2010, 15:53
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#4
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Registered User
Join Date: May 2008
Location: Sweden
Boat: Between boats
Posts: 474
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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
/Hampus
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24-01-2010, 16:10
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#5
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Registered User
Join Date: Sep 2008
Location: Cypress Creek, MD
Boat: Lagoon 440 Ainulindale
Posts: 62
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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.
Kirk
__________________
Kirk & Donna
s/v Ainulindale
"The future ain't what it used to be"
Tom Petty
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24-01-2010, 16:27
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#6
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Registered User
Join Date: Oct 2008
Location: Princeton, NJ
Boat: Challenger Anacapa 42
Posts: 2,097
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Quote:
Originally Posted by Hampus
Thank you.
The seacock is normal enough, at least that's what they look like around here. The thru hull is strange though.
/Hampus
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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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24-01-2010, 16:34
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#7
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Registered User
Join Date: May 2008
Location: Sweden
Boat: Between boats
Posts: 474
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Quote:
Originally Posted by Christian Van H
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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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"?
/Hampus
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24-01-2010, 16:36
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#8
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Moderator Emeritus
Join Date: Aug 2005
Location: Carlsbad, CA
Boat: 1976 Sabre 28-2
Posts: 7,505
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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.
Aloha
Peter O.
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24-01-2010, 17:05
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#10
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Registered User
Join Date: Oct 2009
Location: Australasia
Posts: 284
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Quote:
Originally Posted by Christian Van H
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FANTASTIC link - thanks Christian!
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25-01-2010, 03:25
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#12
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Registered User

Join Date: May 2008
Location: We're technically refugees from our home in Yemen now living in Lebenon
Boat: 1978 CT48
Posts: 5,970
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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?
__________________
James
S/V Arctic Lady
I love my boat, I can't afford not to!
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25-01-2010, 03:46
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#13
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Registered User
Join Date: May 2008
Location: Sweden
Boat: Between boats
Posts: 474
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Quote:
Originally Posted by Christian Van H
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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.
/Hampus
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28-01-2010, 11:59
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#14
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Registered User
Join Date: Jan 2010
Location: Margaritaville Ontario Canada
Boat: Wittholz 40
Posts: 36
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Quote:
Originally Posted by James S
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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Ditto.....
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31-01-2010, 14:49
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#15
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Registered User
Join Date: Sep 2008
Location: Toronto, Canada on Lake Ontario
Boat: Roberts Offshore 38
Posts: 1,287
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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.
Sabre
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