Installing seacock in aluminium hull?

Discussion in 'Boatbuilding' started by DianneB, Nov 20, 2010.

  1. DianneB
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    DianneB Junior Member

    I will be installing a 1/2" bronze flanged seacock in my 1966 Lonestar aluminium boat.

    I am very tempted to fabricate an aluminium backer plate for both inside and outside the hull and bolt through the 3 layers of aluminium (with sealant), a gasket, and the seacock but it seems like over-kill. Perhaps just a backer plate on the outside of the hull to spread out the load from the 3 bolts?

    Thoughts? Experience?
     
  2. michael pierzga
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    michael pierzga Senior Member

    Why use bronze on an Aluminum boat ? Bronze is expensive and its not friendly with aluminium . Forespar sell a line of Marelon thru hulls and valves. Their thru hulls require no metallic fasteners, simply a plywood backing plate on the inside.

    http://www.forespar.com/marelon-marine-boat-plumbing.shtml
     
  3. DianneB
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    DianneB Junior Member

    Good point - I will switch to Marlon. I am just a bit nervous about all the forces being handled by the thru-hull and adhesive .;.....
     
  4. michael pierzga
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    michael pierzga Senior Member

    Its not adhesive that hold the thru hull in place. The fitting is two pieces of plastic . outside hull threaded male, thru hull insert and inside the hull a threaded female valve . you apply sealant and screw the two together. Aluminum is thin so be sure to use a well painted plywood backing block on the inside. I did not look at the Marlon page but normally they give a good " how to " instruction set. Ill look again.
     
  5. michael pierzga
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    michael pierzga Senior Member

  6. michael pierzga
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    michael pierzga Senior Member

    Its not necessary to use the flanged valve. The standard Ball is easier to install. The flanged valve is used when you think that the valve operator may unscrew the assembly by accident when operation the valve. Be sure to tell your supplier that nyou are working with a thin skin alum craft. The countersunk thru hull pictured is not suitable for thin skins. Thin skins use stand off rounded exterior profiles.

    http://www.forespar.com/pdf/techTips/M20-Marelon-Marelonplumbing-chart.pdf
     
  7. DianneB
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    DianneB Junior Member

    Thanks everybody. I am going to order my parts from Forespar

    (Still feel like I should have fasteners thru the backing plate!)
     
  8. viking north
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    viking north VINLAND

    installing seacock in aluminium

    Having converted a 30 ft alum. ships lifeboat into a motorsailer be very aware of the galvanic scale when working with aluminium. As prev. stated high quality non metalic thru hulls are excellent and just as strong as those made from metals and on my project passed the marine survey no problems. Another caution i mignt mention, wherever there is a possibility that wood will become wet or damp avoid using it next to aluminium. A type of corrosion known as poultice will set in and it is very agressive. I used a piece of 1/4 in. thick fiberglass panel (cut it from an old boat or whatever) as my backing thru hull plates or any backing plates. There is no interaction with alum., it's strong and will conform to slight to medium contours in the hull. I also bed all surfaces of the thru hull, backing plate with 3m5200 and hope you never have to remove it. You never have to worry about it leaking or coming loose. Geo.
     
  9. DianneB
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    DianneB Junior Member

    Thanks George.

    The Marlon seacock just came in yesterday but it is way too cold to install it.

    I was wondering about backing. For decking and other wet locations I have used plywood painted with marine polyester resin - a light watery coat penetrates the wood fibres and a second, thicker coat seals completely. Do you not think that sufficient?

    Dianne
     
  10. michael pierzga
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    michael pierzga Senior Member

    Be sure that the "flat" plywood block that you use is mated to a " Flat" hull plate. If the surface is curved take action to either curve your backing block or bed the block in epoxy filler to make the bottom of the block conform to your hull shape. . Well sealed plywood is the choice. No water must enter the area between block and hull.
     
  11. Brian@BNE
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    Brian@BNE Senior Member

    Dianne
    I'd suggest not using wood. even epoxied for reasons George noted above.

    Why not use the slightly flexible 'plastic' kitchen chopping 'boards' - usually white/slightly transparent, but some are coloured. I'm not sure what the material is (poly something?), but different thicknesses are usually available, you can drill/saw etc and they are inert and don't compress. Use some flexible adhesive like sikaflex against the aly to keep moisture out, but much, much less of an issue than for wood. Perhaps test for reactivity between the 'board' and adhesive, but unless you had some unusual adhesive I dont think there would be a problem.
     
  12. viking north
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    viking north VINLAND

    Dianne, Using the ply. sealed as you describe for deck work shouldn't be a problem and i wouldn't be concerned about it. Below the waterline in locations where it could be exposed to constant moisture for long periods of time it could be a problem if not well sealed especially if the wet dry cycle is long on the wet side and short on the drying out. From the treatment you described plus the bedding compound on the underside of the ply backing plate it should be ok. If you have not yet installed the thru hull why take the chance. The household cutting board idea would work but my only concern is most of them are made out of garbage plastics that could be prone to cracking under compression stress or temperature changes whereas fiberglass reinforced resin FRP is almost 100% resistant to fracturing. If the location of the thru hull has alot of curviture use several layers of thinner pieces laminated using 3m5200 or an equivalent. Wow replys from Spain and Australia and here a Newfoundlander living in Nova Scotia, Small world indeed.
    Geo.
     
  13. michael pierzga
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    michael pierzga Senior Member

    Solid wood like a chopping block will split. GRP is complicated. Don't be taking a hole saw to your neighbors boat to make a backing plate !!!! it will sink when launched.
    Plywood is the logical solution. Insure that you ply block has 100 percent surface contact with the hull form, not just its four corners, seal the wood with paint, resin, epoxy whatever and bed in sealing compound. the wood is not in contact with metal, and no water is present in the joint.

    To make a removable assembly...first cover the aluminium skin with mylar tape. Cover both surfaces...mylar tape to aluminium and plywood block face with sealing compound Mount the block, thruhull assembly, tighten...let cure....disasseble...remove mylar tape, trim cured caulking blowout,and then reassemble , You have just bounded a caulking gasket to your backing plate...you now have an easy to service thru hull assembly.
     

  14. viking north
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    viking north VINLAND

    Where did that come from, cutting a hole in your neighbours boat, shouldn't there be a smiley face there somewhere. Senior Michael, you must read my reply carefully it states the resin saturated ply should be ok, but if the thru hull is not yet installed, why not use FRP, go green , there's more than enought aroung to be re cycled in disgarded boats, auto doors, hoods, house bath tubs, showers, old motorhomes. This stuff never decomposes but does offer a cheap supply of materials. I recently bought 50 sheets 1/8 in. thick 2ft by 10 ft. form a scrap dealer that came off a demolished merry go round for $10 a sheet. Think of the boat applications i can put that to use for. Where ever it calls for flat panels i can use this in place of ply epoxy coated. One other possible problem with ply, if it becomes wet it will expand placing extreme pressure between the external hull flange and the inside nut, now picture what will happen if this wet wood freezes over the winter haulout, somethings got to give and i'll bet my bottom dollar the flange will split on a thread line as if you cut it with a saw, seen it happen on several ocassions. Just trying to give my best advise from 40yrs. of building, repairing and modifing these beautiful watercraft from a personal and buisness perpective. Geo.
     
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