I wonder what scale aluminum starts looking attractive.  Around here a lot of the most badass ocean powerboats are aluminum—often catamarans!  Fishing boats on lakes and at sea are both pretty commonly aluminum in all sizes.

One sheet steel boat from boatdesign.net

I have been considering building my deep-V vaka out of 3 mm Corten steel. It would weight about 240 kg and that is about twice as heavy as built from 7 mm plywood, glass and epoxy. It would be one tough little proa, and very easy and fast to build. The weight comparison is not totaly fair, since the steel vaka would be slightly larger. I try to use standard sheet sizes and minimize cutting as much as possible.
I don’t think I can weld thinner steel plates, soo I have to use 3 mm.
If I build it in 5 mm aluminum I can get the weight down to about 140 kg. That is about 20 kg heavier than plywood. It is the welding that will put a limit to how small boat you can build. 1 mm steel plate is much stronger then 7 mm plywood, but very hard to weld. The practical limit for the average amateur welder is probably around 3 mm aluminum.

But I agree with Russell. Plywood is gods gift for us proanauts. It is easy to use, lightweight, isolating from heat, cold and sound. My body reacts to the epoxy, and that is why I am learning how to weld.

Cheers,
Johannes

Galen - 25 August 2013 09:15 AM

Laurent - 23 August 2013 12:34 AM

Do you have the US National Forest Department book on woods? It is really a bible that compares the characteristics of hundred of spieces. I found it for free on line many years ago. If you are interested, I will try to locate it again…

Cheers,

Laurent

Thanks.

I believe I just found a link to different PDF chapters for that book:

http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p

Is that what you are talking about? I will take a look at it.

I doubt I would be willing to go with a heavier wood. WRC is already going to be heavier than foam. If it comes to cost I have lots of family and friends up in Washington state with many WRC trees on their land, and they said I could have them. So I am already thinking about cutting trees and getting them milled. Although that project would be a big learning curve, selecting the correct trees, and drying the timber. Sounds interesting though.

Yes, it’s that book. Chapter 5 is a gold mine…

Laurent

Galen - 26 August 2013 06:28 AM

Johannes - 26 August 2013 12:57 AM

With aluminum you always design with the “as welded” strength, and it is important to design it so there is enough structural stiffness to avoid fatigue in the plates and stringers within the intended lifetime of the hull. It is important to over-engineer all attachment-points as they are high stress areas. With aluminum it is easy to use high-strength alloy-tubes for global structure, like they do in airplanes and race-cars. I have been sketching on large diameter tubes as stringers/chinebars to give the hull a great global stiffness and ease the stresses on the plates.  I just have to figure out how to roll 120 - 150 mm thin walled tubes to the correct shape, at home without expensive equipment.
Think over-sized alu bicycle frames.
The vaka could even be made unsinkable with only the air inside those tubes..

Cheers,
Johannes

That makes sense. My trimaran had big 6066 tube aluminum cross beams that curved over the main hull and also the front beam curved back several feet. Those were a real PITA to bend and get into the right shape by eye. So I bet bending very thin walled tubes is tricky. It seems like I remember someone filling thin walled tubes with sand before bending to keep them from collapsing???

Here is a post I found about it (scroll down to see some aluminum tube successfully done this way).
http://www.garagejournal.com/forum/showthread.php?t=106491

I have done it on a much smaller scale, about 2 inch diameter aluminum tube to make a wishbone for my rig. I used wooden plugs (conical shape) hammered in and ... duct tape to hold them in place!!! Fill it up with regular construction sand. No heat. I found out that one of the key parameters is to bend over a SMOOTH and forgiving surface. Since I was not after a very accurate shape or radius… I actually used the spare tire on my trailer as the bending form… Once again, it was small tube, bending done by hand.
I am not sure I understand why you want to pack the tube so tight and put so much “pressure” inside with a threaded end and tapping, screwing, tapping, screwing, until you can’t go any further.

Yes, I did some tapping to make sure the tube was full of sand, but my reasonning was that you do want the tube to deform a bit, but you do not want it to collapse; so you need some “giving in”...

If you want some pictures, let me know, I will see what I can dig up…

Cheers,

Laurent

Rob Zabukovec - 19 August 2013 12:19 AM

Galen,

I can recommend Dave Gerr’s book “The Elements of Boat Strength*

You can calculate your own scantlings fairly easily for different methods of construction and estimate and compare approximate weights and costs.

....Rob

So I got a chance to look at Gerr’s book. It’s a great resource. But he does clarify in the front that the calculations are not for multihulls. And from other forums I have seen it posted that if you do use his calculations it will be heavy and overbuilt for a multihull. Still, it is a starting point.

Galen

Dave Gerr also says that is scantlings are conservative and aims to produce a service-able boat which will last many years…....It is an excellent starting point… what more would you expect from an experienced professional naval architect writing a book for amateur designers and home builders? You can go as light as you like from a known starting point at your own risk.

I don’t know enough detail on either of them, but I would guess that Jzerro is on the light side of the recommended scantlings and Pacific Bee would be about right, so he is not too far away especially considering that both boats were designed and beautifully built by an expert on light wood construction.

It is bizarre that you should criticise the book for producing a heavy and overbuilt wooden boat whist seriously considering building a lightweight multihull in metal.

Rob

Rob Zabukovec - 03 September 2013 02:43 PM

Galen

It is bizarre that you should criticise the book for producing a heavy and overbuilt wooden boat whist seriously considering building a lightweight multihull in metal.

Rob

Some other folks on this thread are considering aluminum, but not myself. I really don’t enjoy working with metal as much as wood. And yes, writing a book for amateur builders, it makes a lot of sense to be conservative.

And I forgot…..” Madness” also looks about right using the Lord method….....Interesting as well that Jzerro was built for himself, Pacific Bee was built later and for a client…......

It also is the only amateur’s book that I have come across that can take boat speed into account…... He does however put a 45 knot speed limit on his scantlings…!!