Fiberglassing aluminium.

Discussion in 'Metal Boat Building' started by raf pali, Jun 26, 2013.

  1. raf pali
    Joined: Mar 2012
    Posts: 71
    Likes: 1, Points: 8, Legacy Rep: 12
    Location: Australia

    raf pali Junior Member

    Hi all.
    I am wandering of future problems that can arise by glassing directly over aluminium. Would aluminium suffer of corrosion? and aluminium oxide build-up, would it swell to break the glassing? Scenario: I want to buy a catamaran and the aluminium beams are fiberglassed right on the hulls. I curled the nose on the system but I'm not expert. Is anyone out there with experiance willing to give advise?
    Thank you all
     
  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,463
    Likes: 466, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    I have seen examples of people having coated pitted and pin-holed aluminium with glass/epoxy with unhappy results. I think it is called "poultice corrosion". I'd be more interested in understanding why the aluminium beams have been "glassed" on to the hulls rather than a mechanical fixing.
     
  3. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Aluminum can be successfully glued, though epoxy would be my choice, not typical "boat resins".
     
  4. raf pali
    Joined: Mar 2012
    Posts: 71
    Likes: 1, Points: 8, Legacy Rep: 12
    Location: Australia

    raf pali Junior Member

    Thanks Mr E. This catamaran is a 30' Wharram design, build in fiberglass and original timber beams have been replaced recently with aluminium extruded RHS. 4"x2" or 100x50 MM 1/4 wall.
    I can't say why the owner has changed the original lashing design typical of Wharram catamaran, perhaps because the sharp corners of the extrusion would cut the lashing or perhaps because the owner didn't like the lashing technique.
    In all cases I'm very dubious about the structural integrity of this cat and I am inclined to consider it an amator f...up.
    Thanks PAR. I heard of aluminium gluing however, in this case where the leverage and torque forces are so high I can't stop thinking of invisible delimitation between AL and fiberglass starting all too soon, that will produce salt pockets and corrosion
     
  5. jonr
    Joined: Sep 2008
    Posts: 721
    Likes: 11, Points: 0, Legacy Rep: 57
    Location: Great Lakes

    jonr Senior Member

    Are there examples of glued aluminum working reliably after years of salt water exposure? I know of some automotive examples, but I think that that involves minimal salt water exposure.
     
  6. gonzo
    Joined: Aug 2002
    Posts: 14,264
    Likes: 582, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Wharram designs are durable because everything is joined with flexible lashings. The wooden beams also flex with the rest of the structure. It does not look like an improvement at all.
     
  7. raf pali
    Joined: Mar 2012
    Posts: 71
    Likes: 1, Points: 8, Legacy Rep: 12
    Location: Australia

    raf pali Junior Member

    Is my point Gonzo. Why this chap has decided to change the plan so radically, is behind my understanding.
    I like the cat as it is a rarity to find a fiberglass Wharram The hulls are professionally build, thanks G. and want to buy it but, will only offer the value of the hulls and if I get it will put the work to change it back to the original plan.
    I think this is reasonable.
    Jonr: I also heard of AL gluing on airplanes but I know that the type of glues and the equipment available to those companies are not on the shelf for the average mortals.
     
  8. rxcomposite
    Joined: Jan 2005
    Posts: 2,087
    Likes: 216, Points: 63, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    There is the industry standard by 3M 2216 "Scotchweld" or Hysol 9410. Plexus also have a wide range of adhesives to choose from for a specific application. Since it is "special", it cost a bit more.
     
  9. raf pali
    Joined: Mar 2012
    Posts: 71
    Likes: 1, Points: 8, Legacy Rep: 12
    Location: Australia

    raf pali Junior Member

    Hi Rxcomposite. Thanks for the info, surely it will come handy one day for some job but, would you trust an AL boat glued instead of welded? Or would you glu AL beams over catamaran hulls? I know of airplane making companies secret products because I worked in one and those products aren't available to us. They are trade secrets until better ones are developed and the obsolete old staff comes in the market for us.
    Fiberglass for instance, was developed by NASA and became available to us long after they use it and so it goes on with all other things. practically we, the little people, are living in a primitive world in comparison
     
  10. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 146, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    The transom extension I made a number of years ago has aluminum L profiles around the inside edge of the laminate. Stainless bolts and nuts were used to pull the construction against the boat, with a layer of Sikaflex to seal it. I roughened the aluminum surface with a coarse grinding disc and drilled some extra holes to make sure there was plenty of resin under the profiles.

    After a year I tightened the nuts just a little bit, mainly to check the integrity of the construction and was satisfied with the results.
     
  11. rxcomposite
    Joined: Jan 2005
    Posts: 2,087
    Likes: 216, Points: 63, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    Good to hear from you. I came from the same training.

    No, I would not rely on gluing Aluminum because it is not meant to be. These adhesives, like plastics, are good for low temperatures. It may work on cars where panels are glued, relying on the greater surface area for bond strength.

    The difference in adhesives over epoxy glue (like epoxy mixed with cotton flox) is that these special adhesives have much higher strength and shear properties. It can also tolerate higher glue thickness, say 1/16".

    NASA declassified "advanced composites (carbon fiber, aramid, honeycomb, prepregs)" around 1980-1985. My first book about advanced composites was dated 1983. It was the private companies that developed the various manufacturing procedures and the application. It was, at the time, export of the "high tech" materials was limited to "friendly countries" and needs clearance from the US government.

    While most manufacturers would claim "proprietary technology", it is really a judicious choice of material and process commonly available in the market. It may cost an arm and a leg, or a downpayment for the Ferrari, but that is what the end user is paying for.

    GRP (Glass Reinforced Plastic) has been around for quite awhile. My BV GRP is dated 1979 and my first LR dealing with GRP, 1981.
     
  12. rxcomposite
    Joined: Jan 2005
    Posts: 2,087
    Likes: 216, Points: 63, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    There was an experiment conducted by a university professor with a group of engineering students. They were given several pieces of 2" x 4" aluminum and were told to glue them together with a maximum 2" bond length for a tensile/shear test.

    Some students, whose father worked in the aerospace microecthed the aluminum to increase the surface area. Others relied on surface preparation by cleaning.

    One student glued the aluminum strip lengthwise, increasing the bond area but has less than 2" of bond length. Just enough for grip.

    He got the highest bond strength but did not win. He was given a citation for thinking "outside the box". The 2nd highest was the microetched, pitted design. True story but lost my copy of the composite magazine.

    For our manufacturing process, all our aluminum pieces were anodized. Surface ones were bolted to the composite structure. Embedded ones were also anodized and cleaned with carbon tetrachloride/blow dried before insertion. Only surgical gloved/pure cotton gloves hands was allowed to touch the aluminum on its edges.
     
  13. rxcomposite
    Joined: Jan 2005
    Posts: 2,087
    Likes: 216, Points: 63, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    I remembered I had some thermal stress calculations before. My data, showed that aluminum has a CTE of 13x10-6 and fiberglass has 11x10-6. Thus, when the difference in temperature rise during build and actual use. aluminum will try to expand more, shearing the fiberglass or the glue joint.

    In the Area Method where the disimilar material is bolted, there is about 20% stress in the aluminum given a 0.020" diameter bolt with 6" spacings.

    A check with more recent data on composites showed that fiberglass has a CTE of 3.5 to 3.6 in/in/F. This will increase the shear in the joint considerably.

    Attached is the Excel file on the computations (example 2). feed in your own data and let us see what you have.
     

    Attached Files:

  14. pauloman
    Joined: Jun 2010
    Posts: 268
    Likes: 10, Points: 18, Legacy Rep: 151
    Location: New Hampshire

    pauloman Epoxy Vendor

    polyester resin only likes to bond to polyester resin...

    use a FLEXIBLE epoxy - I assume a thin light hull with possible flexing.

    note that many coast guard boats are aluminum and painted with epoxy.

    generally the trick to epoxy coating al is to sand to shinny (remove the oxide layer) and coat immediately
     

  15. jonr
    Joined: Sep 2008
    Posts: 721
    Likes: 11, Points: 0, Legacy Rep: 57
    Location: Great Lakes

    jonr Senior Member

    How long does epoxy paint over aluminum last on a salt water boat hull? If the answer is anything less than forever, then it might imply something about gluing aluminum.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.