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Old 12-03-2014, 07:46   #1
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Seacock backing plate yes/no?

1973 Catalina 27. I'm changing out the original through hull and gate valve for the sewage. (Has been corroded shut for years).

I'm putting in a real brass seacock with flange and need to bolt the seacock to the hull. Hull is 7/8inch thick where I drilled. Do I I need to make a backing plate for seacock or just go ahead with bolting in seacock then screwing in through hull. Seacock/through hull size is 1.5 inch.

Thoughts? I don't want to haggle with fiberglassing a backplate in if hull is thick enough. Click image for larger version

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Old 12-03-2014, 08:02   #2
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Re: Seacock backing plate yes/no?

See: Seacock & Thru-Hull Primer/Pre Information Photo Gallery by Compass Marine at pbase.com
and: Replacing Thru-Hulls and Seacocks Photo Gallery by Compass Marine at pbase.com
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Old 12-03-2014, 08:14   #3
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Re: Seacock backing plate yes/no?

Thanks Raindog I've already read these articles and am using them as an excellent guide. I'm wondering however if I can skip the backing plate since my hull is 7/8inch thick, or almost one inch.
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Old 12-03-2014, 08:23   #4
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Re: Seacock backing plate yes/no?

If it was a smaller thru-hull I might go without the backing plate but I think for an 1-1/2" or bigger I'd put a backing plate. I can't give you a good reason, it's just my gut feeling. If it was a 3/4" thru-hull I wouldn't bother.

You might consider using a Groco Flanged Adapter with a Groco Ball Valve instead of a real flanged seacock. It makes changing the valve very easy.

Link: Groco Flanged Adapter IBVF
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Old 12-03-2014, 08:26   #5
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Re: Seacock backing plate yes/no?

I would. If you ever sold your boat they wont think you forgot to put one in. It would give it a finished look. I would put one on my boat.
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Old 12-03-2014, 11:36   #6
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Re: Seacock backing plate yes/no?

I was dubious about "countersink(ing) the machine screw heads into the hull's exterior" in RainDog's link as soon as I realised what was meant. Later I noticed what looks like a crack - I think - joining two of the holes in this image:

Dry Fitting The Seacock photo - Compass Marine photos at pbase.com

Wouldn't a bronze "backing plate" on the outside of the hull as well as one inside - with screws and through-hull passing through it - be more secure? Chamfer the plate & use CSK's if you want.
Pretty sure I've seen that arrangement before. Blakes maybe? Can't remember now. 63 if you must know.

@ Moderator: could we have an "old geezer shaking a stick" smiley please?

@ Jado27: try to stick with bronze (your new-to-you seacock looks right) not brass - your original looks like a brass gate valve.
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Old 12-03-2014, 11:50   #7
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Re: Seacock backing plate yes/no?

You know born you strike up an excellent point. We put backing plates everywhere else on a boat, why not with through-hulls? I'm surprised manufactures don't do this already with brass studs fixed to an externally imbedded plate or ring to bolt down a seacock or flange inside the hull.

As of now I want a good job done with minimal sanding and fiberglassing and metal work. This is project 1 of 10 and I'd like to get boat back in water. It's been on stands 9 months while I've been in Afghanistan.
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Old 12-03-2014, 12:03   #8
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Re: Seacock backing plate yes/no?

Me thinks backing plates for seacocks is a holdover from wood construction where planking was a uniform thickness from gunnel to keel. With glass that thick, there is no need for backing. Through bolting will take side loads off the through hull for ultimate strength. If you use plywood for backing, saturate it with epoxy to ameliorate delamination. Personally, would use fiberglass sheet. Don't like Starboard because it lacks rigidity and don't think it does much to spread the stress. Of course, the old gate valve without any additional reinforcement through bolts, has worked fine for quite awhile.

That's a pretty impressive hull thickness for a 27' boat.
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Old 12-03-2014, 12:57   #9
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Re: Seacock backing plate yes/no?

I'm with roverhi on this one. I think it would be pretty difficult to impose an abuse load large enough on a proper seacock mounted on 1" glass to cause damage to the hull. A wood backing plate may even make things worse due to it's compressability alowing some movement resulting bending moments that would otherwise be reacted in tension and shear, but that's splitting hairs. What I would do (and have) is pre bed the base first with epoxy and chopped glass to eliminate any point loading, then install with life caulk or what ever sealer you prefer.
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Old 12-03-2014, 13:05   #10
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Re: Seacock backing plate yes/no?

Groco now sells their own version of a backing block See Groco.net and click on the image to the far right. They have pressed in inserts to accept mounting studs or bolts. The very fact that Groco is selling these lends the utmost credence to not needing the mounting bolts to go all the way through the hull. I did my new thru-hulls per Main Sail's description. Used 3/4" G10, cut circles with a 6" hole saw in a big drill press. Drilled/Tapped holes for mounting studs. Thickened epoxied in place. Is *very* strong. I much prefer to do a job only once.
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Old 12-03-2014, 13:18   #11
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Re: Seacock backing plate yes/no?

I'm not sure I'd worry about it. If you do it you dont need to glass the backing plate. Just goop it liberally with 5200 and let it squeeze out. Ditto for not using the backing plate, the 5200 will create it's own level surface and harden. I've done it both ways. Not sure what a backing plate gains you on a hull that thick? the hull isnt going to flex when you pull the handle! hmmm... I guess you are bolting the seacock, one thing a backing plate does is alow some compression of the plate from the bolt pressure, so if bolting maybe use one. Beware, bronze bolts can break pretty easy, easy on the tightening. I like to make them semi snug and then tighten a touch more after the 5200 sets.
7/8 thick on a Cat 27.... really? wow! whodathunkit
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Old 12-03-2014, 13:21   #12
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Re: Seacock backing plate yes/no?

I hope it is actually a real bronze seacock.
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Old 12-03-2014, 13:51   #13
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Re: Seacock backing plate yes/no?

Yes the seacock was off an old hinkley or something and is defiantly brass.

Back when Catalina first started making yachts in 72 they didn't know much about fiberglass I'm told and so went overboard, I've heard of 1inch thick hulls found on similar lengths around these older years.

My backing plate is 3/8inch fiberglass cut out of an old boat in the shipyard. I really like the idea of mounting the bolts to the plate and then glassing the whole plate in. Should be plenty strong.

I know this has been asked before but with backing plate glassed in and seacock bolted to it. wouldn't lifeseal be fine for the seacock and through hull vs 4200 or 5200?
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Old 12-03-2014, 15:08   #14
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Re: Seacock backing plate yes/no?

I agree with roverhi that the backing plates are not needed for strength. They are a sort of throwback to wood boat days when planks needed to be reinforced. With a hull as thick as yours you can get by without them. I do recommend that the saecock be bolted to the boat though, you can do this the simple way by drilling from the outside and counter sinking flat head machine screws and filling over the heads with putty. It looks like you hull would be think enough for this and I have done this with great success. If drilling more holes in your bottom bothers you, you can do it by fitting the machine screws to a backing plate and gluing that to the inside. This has been done for many years on thiner hulls and works well. If interested I have a video series about replacing seacock on a Lagoon 38 I did this winter. Installing Seacock Part 4, Final install - YouTube
If it were me I would go simple and put the bolts in from the outside, I did this on my own boat and it has worked fine.
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Old 12-03-2014, 15:32   #15
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Re: Seacock backing plate yes/no?

Quote:
Originally Posted by Jado27 View Post
Yes the seacock was off an old hinkley or something and is defiantly brass.
That's too bad, brass is a terrible material for skin fittings or ball valve in salt water.
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