Fiberglass vs Aliminium

Discussion in 'Boat Design' started by BluSky, Mar 2, 2014.

  1. jonr
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    jonr Senior Member

    If you can hydro form an aluminum hull, weld it up and then re-temper the entire hull, I would use aluminum :).
     
  2. Village_Idiot
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    Village_Idiot Senior Member

    It is extremely difficult to weld the thin aluminum that is typically used in smaller boats (under 6m). Thin aluminum burns easily and is easily stressed by the heat of the welding. You would need to spot weld small areas for short periods of time, going back again and again to add more spot welds without overheating the aluminum. Very tedious, time-consuming and all-for-naught if you falter just once.

    Thicker aluminum is much easier to weld. However, I don't see any 3m boats being made out of 0.125"+ aluminum. It'd be tough as nails, but very heavy also. What's the point of a 3m boat if it is heavy?

    Stitch & glue (fiberglass over wood) is the best method for small boat home builds. Yes, there are some very good small riveted aluminum boats out there, but they are typically built by large factories that are highly skilled in aluminum/riveting work, such as Grumman/Boeing, etc.
     
  3. dinoa
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    dinoa Senior Member

    The temper of the aluminum alloy will be compromised and its strength reduced adjacent to the weld area. How much depends on type of temper and temperatures. The welded area will be a different alloy made up of parent metal and welding filler. It will most likely take a temper differently than the adjacent sheet.

    Thin skins (think Grumman canoes) are often riveted.

    Experience with airboats have proven welded .125" thick 6061-T6 alloys under severe use without retempering.

    Dino
     
  4. tom28571
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    tom28571 Senior Member

    While I agree with most of the general material analysis here, I think it would be informative to look at home built aircraft for some further enlightenment. Aluminum competes rather well with fiberglass over foam and there are virtually no all wood planes being built. Aluminum craft are all riveted on the skin with welding only for beefier internal structure. Fiberglass is only used as a skin over structural foam due to the weight of fiberglass only skins. Wood aircraft almost always have fabric skin on the wings and tail surfaces although some plywood may show up on leading edges and fuselage skins. These comments are only for home built "experimental" rated craft.

    Keep in mind that air is not water and distributed stress is more important in the air and local slamming loads are more important on the water. Weight is even more important in the air than on the water while waterproofness is not much considered.

    For a 3m home built boat, wood appears to be an obvious choice.
     
  5. 8ball
    Joined: Jan 2013
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    8ball Junior Member

    I'd look into PVC sheet, I've seen some nice workboats built in VA out of it.
     
  6. discovery
    Joined: Aug 2013
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    discovery Junior Member

    Yes sir it is a lot more complex than that, I currently own 3 welders capable of welding ally, and am rebuilding 2 fiberglass dories. $1000 worth of consumables is pretty easy to chew through with either method (depending on the job). The background skillset of the individual is a big part of the choice. It wouldn't be much use telling a sheetmetal fabricator he needs to work in resin infused foam core, when he's much more likely to complete the job in ally.
     
  7. Kevin Morin
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    Kevin Morin Junior Member

    welding proportionally thin Aluminum for 3m hull

    The skill to weld proportionally thin aluminum, for a 3m hull, is disproportionately complex. A 0.080" or even 0.060" thickness hull is more or less 'proportionate' for (+)10' skiff or PWC hull.

    This can be done in both MIG and TIG but not by anyone without a few thousand hours of aluminum welding, and even then few (<1:10,000) welders ever try to learn to work in this scale.

    The reason is the nature of welding aluminum with either welding process. First the speed of travel is high and second the steadiness is even higher as speed of travel governs the deposition rate of the weld.

    Manual TIG is not happening. Semi-auto TIG and some MIG is the only way I can do this work and I've been experimenting with this for years.

    I think the replies have covered the issues well in regards trying this build in welded aluminum. If you don't have a couple thousand hours of aluminum semi-auto TIG; I can't expect your effort to be successful.

    I've been told by long time builders that welding this thin is simply not possible. I don't agree, but then I do see their point- its not easy and is impractical unless you have the power supply and wire feed TIG system that makes this work possible.

    Riveting marine hulls usually requires a (very) large investment in time/cost/effort to make fixtures to hold the shapes while they're formed, drilled, Cleco'd and riveted. Not common in small boat builds unless production is anticipated.

    Cheers,
    Kevin Morin
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Agreed, it is not easy and as you have pointed out takes many many man hours of experience to even get to the skill level to attempt this and successfully! But, it can be done.

    Several very good friends of mine have been LR cert for less than 1.5mm. One in particular has successfully welded 0.8mm, with an LR cert...bloody amazing!!...he did have a very good MIG set though...not cheap!! :p:D
     
  9. Kevin Morin
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    Kevin Morin Junior Member

    ad hoc,

    [​IMG]

    here's one of my smallest boats. Its 30" long and the hull is 0.060" (1.5mm) and 0.040" (~1mm) material with some pieces of 0.080" (2mm) material. [It's a memorial for a friend's son who was a metal boat fishing guide.]

    The welds were done on the bench, where I could control the position, the joint and the weld process as highly as needed, but it was still not a simple task. I think a small welded hull would require extensive fixturing to make these welds possible?

    I used cold wire feed TIG and very small electrode (0.040"/1mm) and 0.023" (0.6mm) wire to weld this model hull. Therefore, while this type of work can be done, it's not for those without practice and some expensive welding equipment.

    I hope the original poster will take the Forum's advice and work in wood epoxy?

    Cheers,
    Kevin Morin
    Kenai, AK, USA
     

  10. Ad Hoc
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    Ad Hoc Naval Architect

    Nice job Kevin...as always :)
     
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