Different type of Alu vs Ply question

Discussion in 'Metal Boat Building' started by MCP, Sep 18, 2020.

  1. MCP
    Joined: Aug 2018
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    MCP Junior Member

    This question should have been in the "Materials" section. Sorry.

    I've been trying to resolve this in my head but can't get to a conclusion.

    I have two sets of plans and want to build a sailboat.
    I'm not concerned about the resale value or longevity. I'm also not concerned about the skill level of either material. So lets remove that from this equation.

    I want to get to the water as quickly as possible.

    But I can't determine which material will fulfill my requirements (getting to the water quickly as quickly as possible).
    I have two sets of plans and I'm happy with either. I'm also very aware that this is not short term project.

    1.) I would like to build in Alu but Ply is more readily available.
    2.) I'm much more comfortable with Ply than what I am with Alu.
    3.) Building in Ply requires many small wooden joints, fillets, sanding, fairing, sanding, sanding and sanding. It feels like a lifetime of sanding.
    4.) In my my mind, Alu will produce much quicker hull results whereas the outfitting might be a bit more work (Ply to Alu) joints.

    I've spoken to the one designer of my plans and he said the time gained in Alu hull time is lost again in outfitting whereas the Ply hull already forms part of the outfitting. I'm not sure if I 100% agree with this.

    I've been watching a builder on Youtube (Building SY Mistress) and his a approach is very simple with great results. Which in my mind makes the outfitting of an Alu (or metal) boat very simple.

    Which material between Alu and Ply will be the easiest and give quicker results over the entire build?
    (I know this question is relative, but just in general).

    I think the real question is, how much more work is a ply boat with all the fairing and sanding compared to an Alu boat. Including outfitting.

    Thanks in advance.
  2. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    I'm not sure the first two points be discounted... but you don't care so fine on that:)

    Alu will be quicker but more expensive in tooling and material. The timber will catch up in extra time preserving and painting. Alu can be unpainted- except for decks they need slip resistant coatings.
    Outfitting a .0m boat... so what & how much can you fit, the alu will be quicker depending on finish- if it's going to end up a clear finished timber interior maybe the ply wins but lining panels can be quickly templated finished and fastened in.
    Ply needs some care attaching fittings for longevity- alu just drill & tap/bolt/rivet as the application indicates.
    Go the alu if you can weld and happy to invest the equipment, go ply if you only want a few hand tools and sneak up on it.
    I'd go the alu for that size.
    Actually not sure on the size??

    All the best from Jeff.
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    One thing that rarely gets mentioned, is noise intrusion into the boat from water lapping against the boat at anchor. I'd say you would sleep better with ply.
  4. Rumars
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    Rumars Senior Member

    A lifetime of sanding is only necessary if it matters to you. Since resale and longevity don't, I fail to see why looks should. There will still be some sanding involved, but not a lifetime. Plywood is self-fairing, exactly like Al plate. You don't say what designs you choose, and it can be important when comparing, but quicker is usually what you know how to do.
  5. MCP
    Joined: Aug 2018
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    Location: South Africa

    MCP Junior Member

    "Since resale and longevity don't, I fail to see why looks should"

    Really? Everything in life is not about resale and/or profit. Sometimes life is about pursuing your dreams without having to worry about resale an losses.
    I'm building the boat for myself and will sail it until it falls apart. I have no intention of selling it.
    But that does not mean I do not want it well built and looking good....

    Thanks for your input. Noted your comment about the self fairing. That is what I like about the sheet metal.

    I have plans for a 40ft plywood cat and 40t Alu mono hull.
  6. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Gosh, you couldn't get a better 'chalk and cheese' comparison than this!

    What is your current preference - a monohull or a catamaran?

    You said "I'm much more comfortable with Ply than what I am with Alu."
    Are you competent at welding aluminium (but still prefer plywood), or would you have to hire a welder?

    If you are determined to do it yourself, without any outside help, then you should choose the material that you are more comfortable with.
    If this is plywood, and you would ultimately prefer a monohull, then look for plans for a 40' ply monohull - maybe Dudley Dix in RSA?
    Stock boat plans and designs by Dudley Dix Yacht Design https://www.dixdesign.com/designs.htm
    He also has some nice designs in aluminium.
  7. DCockey
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    DCockey Senior Member

    One factor is how the interior would be finished. Typical rule of thumb for a cruising sailboat is the interior takes considerably longer to build than the hull.
  8. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Yes really. Aesthetics are the only area that you can safely compromise in the interest of a quicker build. This will probably affect resale value and might or might not compromise longevity (depending on what materials and techniques you actually use).

    Let's compare apples to apples.
    A cabinet door can be a piece of untreated plywood or a perfectly varnished paneled hardwood affair. The door does not care if it goes into a ply boat or a metal boat or your house, the time needed to make it to a certain aesthetic standard is the same. This applies to the whole interior of the boat, including systems. Wiring a light or plumbing a sink takes the same, independent of hull material. Biggest time saver is to not have the item at all, so no cabinet doors, no kitchen sink, etc. You can of course use time saving materials, for example HPL faced plywood does not need any varnish or paint, but you still have to decide what to do about the edges. You can simply router them round and live with an exposed edge, or use a varnished hardwood trim. This is a case where you could compromise longevity for speed by not sealing the exposed edges of the ply with epoxy/varnish/paint.

    The only real difference between a metal and a plywood boat when it comes to the interior is the finish of the hull. On a ply boat you have the option of varnishing or painting the hull material itself to an acceptable finish, or use a ceiling (wich might or might not require the same treatment). One coat of paint is advisable as a minimum protection. On an Al boat you need insulation above the waterline, either glued sheets or sprayed on, and paint in the bilges. Over the insulation there usually is a ceiling in the exposed areas.

    Basicly, for a similar aesthetic result, the required work time is approximately the same for both hull materials. The real savings are to be had by having the entire interior CNC cut and prefinished before installation, and reducing it and the associated systems to a bare acceptable minimum.

    Now let's compare the outside of the hull.
    Aluminium only needs paint below the waterline, but you have to live with the aesthetics of bare Al. Plywood will need either varnish or paint to protect the top veneer layer or the epoxy.

    While both materials are "self fairing" that does not mean the boat will be fair and smooth without grinding/sanding. Even with thick plate Al you get welding distortion, experience matters a lot, plus you have welds to grind. Ply will have a lot of holes to fill. If the ply skin is laminated from multiple thinner sheets or fiberglass sheated you will have to do even more filling and sanding. So basicly speed of build comes down to builder experience and preferences. There are metal builders that can produce a perfectly fair hull with minimal grinding, and there are wood builders that can fair a similar size boat in the same time using a variable speed polisher. The owner always has the last word, and that is when he decides that "It's good enough!".

    Now let's compare apples to oranges, namely a 40' ply cat with a 40' Al monohull.
    We don't know if round or hard bilge and how exactly each goes togheter, and that is important for comparing speed of build. What we do know is that the cat is approx. 50% more boat to build simply because it has more surface area. You can check this by comparing the needed materials given in the plans. More material to install means more time. This is true for hull, furniture, systems, etc. A cat has two rudders but the mono could also have two. Same for the engine, either can have one or two. They could be inboards with conventional shaft, saildrives or outboards. What is speedier to install: a single conventional shaft or two outboards?

    If you would actually give us the details or name the designs in question so we can find them out ourselfs, maybe we could tell you something more in line with what you expect to hear. A round bilge transversly framed Al mono vs. a S&G unsheated ply cat with minimal framing both made by an amateur working with the material for the first time, the cat probably wins. Make the builder experienced in one of the materials and the balance shifts. Make the mono a thick skin single or multichine and the cat a cold molded or thin skinned affair and the balance shifts again. Make one of them a complete CNC precut kit and the other needs lofting by hand, and it could be a 2-500h difference.
    bajansailor likes this.

  9. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    There's a big difference in a multi and a mono building wise...
    Mono- build one 40' hull...
    Multi build x 2 40' hulls plus a connective structure..
    If fast to build is a high priority then go the alu mono.
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