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Old 29-06-2012, 10:40   #1
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Backing Block

Hi all,

i will be installing some bronze croco seacocks on a steel hull, instead of a oak varnished backing block i think about using thick 1/2 inch nylon + caulking compound since the flange on the groco cocks is sturdy enough,

any thoughts or advice? or schould i stick to wood or better weld a 3/8 inch backing block in steel to the hull?

thx for your tips,

tom
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Old 29-06-2012, 10:58   #2
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Re: backing block

If I were going to use wood I would use teak. Oak will rot eventually. StarBoard would be my first choice, but silicone does not stick to it.
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Old 29-06-2012, 12:16   #3
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Re: backing block

Teak if you can find it, or if you want to avoid rot, consider HDPE, which is cheaply obtained by finding a white cutting board and using a slowly moving hole saw.

If you need to radius it to the hull, you can cut grooves to make it bendy. You are basically spreading the load here...it's a glorified fender washer.

I would tend to use 4200 or similar for this job, but I don't know if it sticks to HDPE.

Normally, I would advocate steel on steel, but you don't want bronze on steel and indeed need to ensure that the bronze of the thru-hull and the seacock atop it do not "connect" to the steel hull. Much misery can ensure. In fact, it's an argument for Marelon seacocks...however, if you are determined to use bronze (and I understand why), make sure they are galvanically isolated from each other.
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Old 29-06-2012, 13:27   #4
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Re: backing block

Avoid nylon - it sucks moisture and expands. If you use nylon, make the hole bigger, otherwise the block may expand, press on the fitting, and crack.

b.
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Old 29-06-2012, 14:50   #5
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Re: backing block

Hi Lex. I would think again if I were you. Ideally you should go and buy a flange in low carbon steel, the type used in steam piping. Then go and look for a 3 piece stainless steel ballvalve that has the same PCD as the flange and the flange is now tapped with threads to accept the bolts of the ballvalve. The flange will replace the one end of the supplied ballvalve. There are 4 bolts. Get a 8 hole flange. This means that the valve can be rotated 45 degrees if the threads become damaged. Place dummy bolts into the unused holes with lanolin.

Now disassemble the whole thing and place the flange on the hull, cut both holes to the OD of a short section of 304 SS Sched. 80 Pipe.
The inner and outer corners of the hull hole and those on the flange should be chamfered 45 degrees to accept a weld.

Now weld the outer circumference of the flange to the hull on the inside, and weld the inner joint between flange and hull with low carbon steel rods, filling the groove created by the chamfering.m Re-bore to dimension.

Get a pipe of 304 Stainless that fits perfectly into the hole of the hull and flange, using 316L rods, weld the pipe ends into the flange and the hull on the inner and outer ends. The groove between the flange and the pipe end is filled. The flange is now flattened to accept the ballvalve with its sealing ring, or a gasket if you cant get the flange perfectly flat.

Here a picture to help illustrate what I am trying to describe. This is one way it is done on ships.

Next you can make a standpipe that threads into the end of the ballvalve.
Using the discarded flange from the ballvalve screwed to the top of the standpipe make a lexan cover with four bolts to create a transparent lid that ends above waterline, a tubular strainer is fitted with a clearance around the outside.
Several takeoffs are now added to the standpipe, engine, toilets, freezer, generator etc. You can remove the lid while underway without closing the seacock to clear it with a hooked wire.

Hope this helps, it sounds like a lot of work but it will help you in emergencies.
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Old 29-06-2012, 16:17   #6
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Re: backing block

Very good explanation for a steel hulled thur hull.......
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Old 29-06-2012, 19:25   #7
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Re: backing block

Quote:
Originally Posted by brutb View Post
Very good explanation for a steel hulled thur hull.......
Can't get much clearer than that.
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Old 30-06-2012, 03:05   #8
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Re: backing block

Postscript: Obviously the standpipe rises above waterline...duh.
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Old 30-06-2012, 06:12   #9
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Another vote for HDPE...and 5200
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Old 30-06-2012, 06:42   #10
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Re: backing block

hi gilana,

the seacocks will be used for the seatoilet, thanks for the very interesting explanation for creating a waterchest, i'll keep it in mind for eventual future projects, and for the rest of you fellow boating enthousiasts, you've convinced me, HDPE it is

i've been looking for local suppliers, they sell hdpe buildingplates from 2 tot 18 mm thick, should i go for the thickes size possible?
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Old 30-06-2012, 06:43   #11
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Re: backing block

'i ve also considered between seacock and hdpe backing blok a thick industrial rubber gasket, or is that too much & will the 3M sealant be suffice?
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