At what length do aluminum sailboats make sense?

Discussion in 'Metal Boat Building' started by catahoula, Aug 1, 2022.

  1. catahoula
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    catahoula Junior Member

    So I understand 3/16" or 5mm aluminum plate is approximately the practical lower limit for welding aluminum (if you don't TIG it), so with that in mind, at what size do aluminum boats start to compare reasonably in weight to a glass/foam core boat? On a smaller boat, using a heavy 3/16" plate or the hull shell would allow lighter scantlings, so there should be some compensation there. But practically speaking, where is the "break-even" point? 25 ft? 35 ft?

    Let's say a "performance-cruiser" type average monohull sailboat, with welded construction, and disregard the cost difference (though obviously cost is always a factor)

    And maybe aluminum is never truly weight competitive to cored composites, but I'm thinking about where the local deflection of a given panel stops becoming the limiting factor--obviously aluminum doesn't compare well for dinghies, etc.

    Thanks in advance
     
  2. jehardiman
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    jehardiman Senior Member

    It is not a length, but a Statement of Requirements (SoR). As has been said and proven, you can build a lot of boat cheaply and quickly with "two guys and a Binks gun". There has to be a reason to select the cost and complexity of aluminum fabrication over that of wood or glass until you are well up into the 50+ foot range. And by then you have to justify it against steel fabrication.
    Realistically, it is never about aluminum being superior as a fabrication material because it isn't. What you need is a justification for aluminum as a construction material.
     
  3. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Nope.
    It is down to the skill/experience of the welder, and the quality of welding set they use.
    Colleagues of mine can MIG down to 1.8mm thickness, and all LR certified.

    The rest.. what JEH said.
     
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  4. bajansailor
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    bajansailor Marine Surveyor

    The Pen Duick 600's are all of 6 metres (a tad under 20') in length, and were built of welded aluminium in the 70's - they must have been competitive in price against the small GRP sailing yachts of the day in France, as 350 of them were built over a 3 year period.
    There is one here in Barbados, and I had the pleasure of crewing on her in the Mount Gay regatta here 11 years ago.
    Pen Duick 600 - voilier du chantier Chantier Le Guen-Hémidy - Fiche technique Bateaux.com https://www.bateaux.com/plaisance/voiliers/pen-duick-600-REFlaeIteLqwMg,

    SailboatData.com - PEN DUICK 600 Sailboat https://sailboatdata.com/sailboat/pen-duick-600

    Quite a few aluminium Sarum 28's were built in England in the 70's - they were of multi-chine construction, and were offered in varous stages of completion by the Builder. Here is some info about them :
    SailboatData.com - SARUM 28 Sailboat https://sailboatdata.com/sailboat/sarum-28

    And here is one that was for sale with Yachtsnet, but is now in their archives, with lots of nice photos :
    Sarum 28 archive details - Yachtsnet Ltd. online UK yacht brokers - yacht brokerage and boat sales http://www.yachtsnet.co.uk/archives/sarum-28/sarum-28.htm

    Ovni Yachts in France now only build bigger aluminium yachts, but in the 70's they were building 31' cruising yachts, like this one currently for sale -
    1976 Alubat Ovni 31 Cruiser for sale - YachtWorld https://www.yachtworld.com/yacht/1976-alubat-ovni-31-8358577/

    I have been involved in the design and build of a few aluminium commercial boats here (including the cat in my avatar photo) - and aluminium is ideal for customised 'one off' construction to suit a particular purpose or usage.
     
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  5. catahoula
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    catahoula Junior Member

    jehardiman- I understand that aluminum may not be better for a given SOR; I'm coming at it from a fabrication standpoint. I've done a ton of composites fabrication (for a side business) and a ton of steel welding/fabrication (for a previous job and other projects), and only a little aluminum work, and I find that working with metals is much more pleasant, to me. I thought it would be an interesting discussion point. So the justification would be enjoyable fabrication but lighter than steel, and of course durability and not having to paint things as much is nice. But just a theoretical question-- I'm many years from seriously considering building a boat.

    Ad Hoc- The 3/16" figure came from Dave Gerr's Elements of Boat Strength. I'm sure it can be reduced with care. I regularly MIG weld 4130 steel down to .035" through choice of wire size/settings and careful use. But even if possible to MIG 1.8mm aluminum panels, it would be too thin to resist denting in normal use, no?

    bajansailor- thanks for all the links. Pen Duick 600 at a D/L of 156 is pretty competitive with composite!
     
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  6. catahoula
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    catahoula Junior Member

    For example, I like the designs of Roberto Barras (Roberto Barros Yacht Design - List of plan http://www.yachtdesign.com.br/ingles/projetos.php), and as best I can tell he starts showing aluminum designs at 32'. That's the PopAlu 32, and it calls for 5/16" plate skin, with emphasis on a heavy skin to reduce framing needs. By Dave Gerr's Scantling Number method, the same boat would call for 1/4" plate on the hull bottom. 3/16" aluminum plate on the hull bottom would be suitable for a 22-24ft boat of average proportions, per the scantling number method.
     
  7. bajansailor
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    bajansailor Marine Surveyor

    That PopAlu 32 is an interesting boat - but 8 mm hull shell thickness everywhere does seem to me to be rather excessive.
    BG Yacht Design - Pop Alu 32 http://www.yachtdesign.com.br/ingles/projetos/pa32/desc32.php
    It is all very well saying that you reduce the amount of internal structure required, but you will still need to have a certain amount of welded framing in the hull to attach your interior outfit to.

    You could also consider the Van de Stadt Vita 30 which can be built in multi-chine aluminium -
    Van de Stadt Design - Vita 30 https://www.stadtdesign.com/designs/stock_plans_sail/vita_30
     
  8. rnlock
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    rnlock Senior Member

    When I was a kid, we had an 18 foot aluminum runabout. The hull was very tough and survived quite a bit of abuse, though I can't say the same about all the parts of the boat. If I remember correctly, it was riveted, and didn't have any compound curvature that would require some kind of press. I don't know how they make such a boat, but I wonder if it couldn't be done by an amateur.

    BTW, I've seen a nice rowboat in stamped aluminum, same way they do canoes. MUCH nicer to row than the usual tin motor skiff, since it came up out of the water in the back instead of dragging a transom. It was fairly light, as the aluminum canoes were. It might have been 10 or 12 feet long. But that's not really within amateur construction techniques, I think. Come to think of it, if you made a vented concrete form, sealed the blank against it, put the other part in water and used the correct amount of dynamite, an amateur MIGHT be able to make something similar. ;-) Unfortunately, the internet thinks "Grumman rowboat" means tin motor skiff, or maybe a square stern canoe, or maybe a regular canoe. Never an actual rowboat.
     
  9. Rumars
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    Rumars Senior Member

    As usual, it's always about the compromises one is willing to make. Let me exemplify on a thoroughly modern boat, the Speedlounger 8500 Day Sailing re-defined. - Speedlounger https://speedlounger.com/ Hull, deck, cabin and framing are 4mm, cockpit and benches 3mm Al. Light displacement of this 8.5m (~28ft) boat with 46sqm (495sqft) of upwind sail area is 2150kg. To achieve it the designers gave her very low freeboard, a very short cabin and the insulation is a special paint. Therefore she is designated as a daysailer and only achieves CE category C, barring the owners from competing in many popular events, even if they are willing to put up with an extremely wet boat. Making her a cat. A racer-cruiser is possible by increasing freeboard and a longer cabin, but for keeping the weight in a competitive range, it would be necessary to use thinner plating and more complicated framing. The builders can do it, but the price would probably double and nobody would buy her.
    As is you get the performance and price of a similar size modern racer-cruiser in a tough low maintenance package, but at the expense of space, dryness and ability to compete in many races.
     
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  10. gonzo
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    gonzo Senior Member

    My 17 foot aluminum Grumman canoe weighs 70 lb and is rated to 850 lb of cargo. It competes nicely with canoes made of exotic materials. Fully loaded it weighs 920 lb. A carbon fiber canoe will probably weigh 890 fully loaded. The small difference is not an advantage when running aground or hitting tree stumps. The aluminum canoe will get a dent, while the carbon fiber canoe will crack and sink.
     
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  11. jehardiman
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    jehardiman Senior Member

    Sigh..., there is a lot more to it than that. Depending on what you hit and how how you hit it will dictate the failure. I can easily penetrate (or take apart) an aluminum canoe with a carbide tipped ball point pen, or fracture a composite canoe with a large rock. However, both could be made seaworthy with a roll of EB Green.
     

  12. comfisherman
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    comfisherman Senior Member

    The Craze in Bristol Bay boats recently has been to go super light on the skins of the houses and superstructure. Back in the day it'd be quarter or 5/16 bottom with 3/16 house. Pulse Mig has allowed folks to step down to 1/8th inch. Pretty easy to pick out which boats went with it they're either corrugated or you can see the warping from sticking together to the skin lines inside. I swear a stiff breeze makes the reinforcement line show through a problem we didn't have with 3/16 and quarter. Right or wrong the pulse Mig is allowing relatively easy heat management and Fabrication.
     
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