Lightest attainable weight for 18ft 5.5m alu boat

[Zent]

Hello. First post on these forums.

I'm looking into building a aluminium boat, mostly to keep myself busy, but also to have a nice alternative to my present boat.

I own a poseidon 510 fish which I bought because it's lightweight (fuel prices here are the highest in the world, so a lighter engine is a big us)
But I'm not all that impressed, mostly due to the rather flat bottom, but it's rather lightweight, 380kg bare boat and does do quite well with a 60hp outboard, 25 knots at full blast, and it's great to take the wife and dogs out camping due to ample storage.

What I would like is is an appr. 18 foot max boat of a modern design that is as light as can be. I'm thinking of 3mm alu plate thickness all around, thinner where possible, minimum amount of frames as reasonably possible and looking into using modern composite panels under the deck for stiffness to see if there's weight to be saved there.
Additionally, any storage, seats, console would be made of carbon fibre. (a neighbour does carbon fiber construction)

In short, I'm looking for info what the lightest possible design would be for a theoretical cookie cutting bare hull ALU boat using all the tricks in the book.

No, I'm not going to build it, this is just a thought exercise as of yet.

Thanks in advance.

 

[jehardiman]

That is a very long row to hoe.

Really, there is no simple answer. This is because of the classic risk (i.e. weight)-cost-schedule design trade-off triad (i.e. "quick, good, cheap...choose 2") . Even if you just say "aluminium" and minimum weight; you still have to answer all the cost and fabrication questions. If cost is no issue, then I'm sure, for a cost less than the ESA's yearly budget, you could design and build something really slick and light out of honeycomb aluminum panels. The design alone will only take 6 to 9 months...and a day rate of ~1000 euros (ROM estimate; -50%/+100%).

Kind of a sarcastic reply above, but you really spend 90% of the time and money in engineering trying to wring out that last 10% of capability. A few years ago I pointed out why there wasn't a huge amount of design money going to make ships more efficient; for a 10% increase in design and build cost, you would need a 5% increase in fuel efficiency (which is a huge increase today) to break even. Going for the lightest hull is not going to be just a 10% increase in cost.

 

[fallguy]

I have to echo and add to the great reply.

The idea of lightweight and aluminum has some obvious problems. Most people building or buying in aluminum want a durable hull. So using aluminum for lightweight, while not perhaps an oxymoron, is close to one.

And the tradeoff, super obvious.

 

[Zent]

Obviously there are trade-offs. Otherwise every alu builder would grab the opportunity to save on material costs.

But then a hobby builder with a lot of spare time and money doesn't have to worry about added complexity, extra labour hours, material costs, practicality and profit margin. And is more inclined to experiment. And in my case specifically, I do not care if the end result is prone to deforming or buckling, what the resale value is or even if I have to dump the end result straight into the trash even if I try to build it.

Anyway, I made a rough sketchup plan, and the skin plating alone would weigh around 200 to 250.
That's obviously excluding any added weight of the welds, scantling, fittings, etc.

So I'd say the lowest practical weight would be at least 300kg empty ship. Thoughts?

And sarcastic replies are more than welcome too.
While I do not have ESAs budget, I'm sure I can exceed the annual military budget of mincronesia.

 

[fallguy]

That's just it.

Aluminum skins were oftentimes 0.064" or more or doubled to 1/8", or 1/8" in even 14 and 16' boats. Now, an 18' boat is often built with 0.100" material bottoms, 20' with 0.125. But you can find cheap 18 footers with 0.064 bottoms. Personally, I'd want a minimum of 1/8" in an 18' skiff.

Here is a large manufacturer here with hull bottom and freeboard thicknesses of various designs. Hope it helps.

https://www.lundboats.com/content/dam/lund/general/2023_Lund_Redbook.pdf

 

[Zent]

Thanks for the resource @fallguy
Makes a great reference source.

So it's not out of the realm of possibilities at all then. I thought I was nuts even thinking about such low thickness when everyone recommends at least 4, preferably 5 mm minimum.

Checking your source it seems a lot of weight reduction can be achieved by the interior alone as even these boats seem to be heavier than their material would suggest.

 

[fallguy]

Most of these boats use a plywood floor which is heavy. Remember these boats are also fitted out for fishing and include lockers and livewells, carpet in many. So, anytime you add a plywood lid or box with a plastic tank inside; it adds weight.

But aluminum is not likely the lightest way to build. Probably get lighter building something like a Panga 18 or 20 with okume, no sole, but monocoque, sheathed in glass. That can be powered by a dink motor say 25hp and maybe gets 10nmpg or better, depending on how used.

 

[rangebowdrie]

But then a hobby builder with a lot of spare time and money

Ok, I found the operative sentence, now to proceed.
1, Build a plug
2, Lay-up carbon fiber on plug, vacuum bag.
3, Add foam core, vacuum bag.
4, Lay-up carbon fiber on core, vacuum bag.
5, Fair and paint.
6, Enjoy non-sinkable boat.
I may have over-simplified the process, but that would give a lightweight structure that would have good stiffness.

 

[Zent]

Oh, I've toyed with the idea to build a fiberglass boat, many many times
But here in Asia sourcing quality materials isn't easy and it's not the best climate for epoxy.

Last but not least, like I said, my present boat is quite light, but the hull is and many parts are razor thin and its just scary to think a lighter boat would be even thinner. No thanks.

What I like about steel or aluminium is that with every weld or cut, you're progressing.
Unlike building a plug first, or building a wood core to be glassed later.

And then I did ask for the lightest attainable weight for an aluminium boat, as that is my material if choice, and not for the lightest attainable weight for carbon, because if weight is the issue, carbon fiber is king, but not quite so practical for an amateur.
Like I said, carbon will be reserved for the interior, small pieces that I hope to outsource to my neighbour, who does not have the facilities for an entire hull.

And last but not least, why do I want to build a lightweight boat in aluminium?

Because I can.

 

[fallguy]

Oh, I've toyed with the idea to build a fiberglass boat, many many times
But here in Asia sourcing quality materials isn't easy and it's not the best climate for epoxy.

Last but not least, like I said, my present boat is quite light, but the hull is and many parts are razor thin and its just scary to think a lighter boat would be even thinner. No thanks.

What I like about steel or aluminium is that with every weld or cut, you're progressing.
Unlike building a plug first, or building a wood core to be glassed later.

And then I did ask for the lightest attainable weight for an aluminium boat, as that is my material if choice, and not for the lightest attainable weight for carbon, because if weight is the issue, carbon fiber is king, but not quite so practical for an amateur.
Like I said, carbon will be reserved for the interior, small pieces that I hope to outsource to my neighbour, who does not have the facilities for an entire hull.

And last but not least, why do I want to build a lightweight boat in aluminium?

Because I can.

ultralight does not necessarily correlate to fuel efficient...

Other factors like length to beam and top speed also contribute, as well as hull deadrise, etc.

 

[Rumars]

Just theoretically of course, but if I had an obliging neighbor I would have him make me all the framing in carbon fiber and glue it into a Al skin. This would allow one to go as thin in the skin as one dares (certainly under 1mm). Of course, the next question would be why fiddle around with the Al at all, just ask the neighbor what the maximum size is that he can produce, divide the hull in appropriate panels, and glue it all together into a boat.

Back to Al, the limiting factor is stiffness, so the thinner the skin gets the more framing you need. By this time frames/stringers are not plate anymore but omega/top hat sections, so there is a theoretical point when you stop reducing skin thickness, since otherwise the flanges start touching and you actually have a corrugated plate as an inner skin, with the outer one used as hydrodynamic fairing.
From a production perspective unless you have laser welding equipment you will need to resort to rivets or glue, and you better be a good tinsmith.

 

[fallguy]

Just theoretically of course, but if I had an obliging neighbor I would have him make me all the framing in carbon fiber and glue it into a Al skin. This would allow one to go as thin in the skin as one dares (certainly under 1mm). Of course, the next question would be why fiddle around with the Al at all, just ask the neighbor what the maximum size is that he can produce, divide the hull in appropriate panels, and glue it all together into a boat.

Back to Al, the limiting factor is stiffness, so the thinner the skin gets the more framing you need. By this time frames/stringers are not plate anymore but omega/top hat sections, so there is a theoretical point when you stop reducing skin thickness, since otherwise the flanges start touching and you actually have a corrugated plate as an inner skin, with the outer one used as hydrodynamic fairing.
From a production perspective unless you have laser welding equipment you will need to resort to rivets or glue, and you better be a good tinsmith.

how would one glue an Al skin to carbon frames? Just curious, and great post as well. Making the plate super thin is not going to be ez.

 

[Zent]

ultralight does not necessarily correlate to fuel efficient...

Other factors like length to beam and top speed also contribute, as well as hull deadrise, etc.

I am well aware of that.
That's part of the fun in building a boat. What works, what's practical and what can I actually make with the limitations of being an amateur.

I'm thinking a hybrid V shape with a low deadrise in the stern but sharp bow. The bow shape would ideally be of a semi axe bow design. Haven't fired up a 3d designer yet to look if it's result in something that can be built.
There's a lot of research done on hull shapes and their effects and a lot of resources freely available.

Obviously it would be a wet ride in rough weather, but ideally suited for a short nasty chop we usually have where I live.

Of course, the next question would be why fiddle around with the Al at all, just ask the neighbor what the maximum size is that he can produce, divide the hull in appropriate panels, and glue it all together into a boat.

Because, like I mentioned in my opening post, it's something to keep me busy, a project to work on.
And secondly, while that neighbour is a carbon manufacturer by trade, he does not at present work as such, but does smaller projects. So he doesn't have the facilities nor the time to work on a whole hull.

As for not going too thin, 3mm seems to be the thinnest I can practically go as an amateur, but I'll have to experiment before I even start thinking about building anything.

For below deck framing I had laminated foam core in mind for strength and subdivided water tight compartments.
For the freeboard hull I'll see if I can get away with using the bulkheads mostly for strenght/stiffness.

 

[Rumars]

how would one glue an Al skin to carbon frames?

Methyl methacrylate adhesives (most known brand is probably Plexus), the same thing that is used to glue fiberglass structural grids in production yachts today. Epoxies also work but require special preparations to remove the oxide layer (sanding with wet epoxy) and a careful selection to match elasticity.
Adhesives also work for metal to metal bonding, eliminating the heat induced distortion and reduced strenght from welding (wich are the primary problems when working with thin sheet). Examples (just a few) are Gelignite wich had its Al frames glued into the explosive formed Al hull, the hydrofoils on russian ferries and the frame of the Lotus Elise car.

That's part of the fun in building a boat. What works, what's practical and what can I actually make with the limitations of being an amateur.

Well, you can build an insulated shed from cheap local materials (straw/adobe), install a window air conditioner and build with carbon. This doesn't mean I have something against Al, far from it. If Al is what you like, that's what you should build in.
Almost anything works as long as you are willing to learn, spend money, and have unlimited time. You have to decide what's practical for you.

As for not going too thin, 3mm seems to be the thinnest I can practically go as an amateur, but I'll have to experiment before I even start thinking about building anything.

You can go lower than that, but welding becomes more difficult, so the limitation is your skill as a welder.
Traditionally thin Al boats were riveted, today you can also glue. Riveted canoes were made with 0.8mm skins with a few omega frames and extrusions for keel and gunwhales. Riveting a lot of framing into a boat is expensive, so faster boats (wich have larger loads) used thicker plate, like the abovementioned 1.6mm. Of course there were also canoes with 1.2mm and 1.6mm hulls, simply because the customers desired them to be more rugged.

You have to decide how you value your working hours, manufacturing and riveting a bunch of thin ribs is time consuming, as is straightening plate after welding.

 

[Zent]

You can go lower than that, but welding becomes more difficult, so the limitation is your skill as a welder.

So yeah, let's stick to 3mm for now. It's nice to know that lower is an option, but welding skill is the limitation here.
Riveting, no experience at all there and I'd leave it to the professionals.

All things considered I'm going to order a smaller boat kit to get some practice anyway and get a feel for the material.