New design - aluminum, not plywood

Discussion in 'Boat Design' started by JamesM, Sep 23, 2008.

  1. JamesM
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    JamesM New Member

    I'd like to take the 44' Mirage, On ( ) and use aluminum, not plywood. how hard would that be? And would it weigh more or less? The boat can be found in ( Boat Plans & Kits Catalog ) then goto ( Power Cruisers ) its at the bottom. I would really be thankful for some help.
  2. Grizz
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    Grizz Junior Member

    I would say yes, it could be built in aluminum, but it would be very expensive to try to duplicate the plans. The spec for the plywood version is multi-layered diagonal application. This is necessary to get the curved shapes you see in the forebody.

    Anything designed for steel can be built of aluminum with the understanding that centers of buoyancy will be altered, and the framing may need to be redesigned to support aluminum plate.

    Anything designed for plywood panel construction, or stitch and glue, with the exception of 'tortured plywood' found in some kayak designs, can be built in aluminum.

    Weight is an engineering problem and can usually be managed. Framing to outside of plank rather than inside of plank will go a long way to offset the weight difference if one exists. It is also possible for an aluminum boat to be lighter than a wood/cloth/resin one. Too many variables to cover all the possibilities.

    Good common sense produces decent boats if the builder is savvy.

    Good building, whatever you decide,

  3. daiquiri
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    daiquiri Engineering and Design

    This, in particular, won't be easy to replicate with aluminum - double curvatures at the bow, below the sheer line:
  4. JamesM
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    JamesM New Member

    Thanks guys

    Got any idea where i could find a boat like this one, build with aluminum? This one has the right lay out, But i would want it to be around a long time. Thats why i was going to use aluminum.
  5. PAR
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    PAR Yacht Designer/Builder

    Those look like "developed" shapes, which could be done with any sheet goods (plywood, steel, aluminum, etc.).

    On a craft this size you'd be well advised to have the plans converted to your building material and preferred construction method. In metal vessels of this size, it's more economical to use a longitudinal framing system, rather then the typical athwartship arrangements.

    Fundamental design considerations and changes such as these have to be worked out professionally.
  6. lazeyjack

    lazeyjack Guest

    I have built many boats in ALAL, (ALUMINIUM ALLOY) The one here is not long enough to work in flare at the bow, in other words the compound is too sudden Clipper effect and flare work above say 55 foot in power boats(which carry the beam further forwards, and are easier in sailing boats above say 43
    Alloy will go where ply will not, in simple curve and compound, (not flared compound) is possibel with the right equipement
    the big advantages are the tremendous strength which can be built lightly into the bottom, , 4 girdars right through which can carry the engine too
    if you a e keen you can IM me
  7. lazeyjack

    lazeyjack Guest

    here is a typical structure, this one is disp. boat but same sort thing applys
    the render here is by bhnautika in the forum, and the structure design by myself, i have found that by doing it this way i can and have given 10 years hull warrenty

    Attached Files:

  8. daiquiri
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    daiquiri Engineering and Design

    That looks to me like quite a high double curvature at the bow to be a simply developed shape. I agree with Lazeyjack on this.
    By the way - I talk about curvatures, having in mind metal sheets... while he talks about the flare, having in mind the boat. An example of seeing the same thing from two different points of view. :D

  9. kmorin
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    kmorin Senior Member

    Ply to Aluminum Converting the Design


    several points above are great advice so I'll head in a slightly different direction.

    The topsides flared section or hollow of the body plan at the forward topsides could be ignored- just skip those curves and make the sheet lay from chine to sheer - and you'll be fine in aluminum with this design.

    The rest of the hull looks like it will develop (cut from flat sheet and warp/curl/bend) from sheet metal. The easiest way to check is to buy the study plans and make a plate model- unless you're up to speed on hull modeling software which would do a faster job answering that question.

    If you build a scale model, called a builder's plate model in times gone by, then you'll know every shape change to the hull's plan needed to build this hull in sheet metal.

    As to weight comparison that is not a simple question. What you'd have to do to make an approximation is to use the Bill of Materials to see what thickness of wood is used where, compared to the full displacement (engine, fuel and water.....) so you'd have a hull weight. Then you have to do the whole boat's BOM in metal of comparable strength and find weights for that.

    The rub or large amount of time if figuring the framing and hull conversion from wood to aluminum - that is not the simplest thing to do. For example if there is 5/8" ply sheet what aluminum hull plate would you use? There is no direct, straight conversion chart for that evaluation so you have to do some calculations.

    Another way might be to buy Dave Gerr's book "The Elements of Boat Strength" and enter each of the structural framing elements into his very easy to use formulas and size them from Dave's calculations. Then take the sizes of materials in aluminum and the BOM and add up to you hull weight.

    Now you'd have a comparison of wt.s but not a comparison of hull strength.

    Generally, welded metal boats are "stronger" (not a very accurate word but it will have to do) for a given weight than wood- even glued plywood. This isn't as hard a rule if the weights are very light but is more true the heavier (read; larger) the boats being compared become. Somewhere in the 20-30' class the comparison is so far in favor of welded metal that few people would spend the time to build a wood/plywood boat because the increased in labor of wooden building is being spent on something with a shorter lifetime of use.

    By the time you're done with a 44' metal boat you'll have a hundred thousand dollars invested, plus years of your life; so I'd tend to find a marine design office and pay the ten grand up front for full package of metal plans and get the real level of support a project of this scope needs. Sounds like a lot of money but its only a few % of the whole project and its money well spent. Bruce Roberts has power boats in this range and probably has NC cutting files too? Many other designers will have metal boats in this class.

    There is nothing wrong with Glen-L's designs, plans or boats; but unless you ask him or his office to participate in the metal conversion- you're on your own for support as they're selling wooden boat plans.

    incidently, JamesM if you're not willing to give up the hollow bow area you could 'plank' it in aluminum by using long strips and seam them up- fair them off and you're there. The main reason its not done more often is that you can buy this entire area of the hull (flat) in one piece. Why bother to cut it into strips and re-weld it? The issue is one of labor versus shape gain and most folks find a bow with flam sufficiently dry in a 44'er- so they don't take time to create flare in this size hull.

    just my few cents,

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