Folding arm materials - Any reason not to use Aluminum?

Hi,

Newb here. Have read a lot.

I'm planning to start building a 18' folding trimaran soon (scarab 18 with some modifications to both inner structure and extended cockpit/smaller cabin).

I'm in the planning stages and material sourcing right now. My question is about the beams. Because of the tools and background I have, it's easier and probably cheaper for me to fabricate in aluminum than build the fiberglass beams. I also like the idea of building them to be "adjustable", not in that I would tweek them on the water, but that I could change the mounting points to adjust cant, approach angles, and even longitudinal position of the amas relative to the hull. That's easy to do with Aluminum. But the main reason is that they would just be easier and cheaper for me to do that way.


Is there any reason why I should not be use aluminum for these beams?

Thanks.

 

Box beams or tubes?

SInce I have seen the pictures, it can be done.

 

Which aluminum?

 

I had a couple designs in my head, but haven't put anything on paper yet.

The first would involve each beam made up of two parallel vertical plates cut to the general curved shape spaced roughly 4" apart with 2" tubing welded between them spacing them apart, or possibly a piece of flatbar between centered running the length. With the flatbar center it would effectively be like a curved piece of I-beam. The connection points for both the amas and the vaka braces would be part of the structure. I would also run the two tubes parallel to the ama to run through the beams to support the trampoline and add structure. I need to do a little work on the design before I decide on thickness, but off the top of my head, I'm thinking 1/4 - 3/8" plate.

The other options would be square or round tube of various sizes and types. 3" OD .250 wall 6061-t6 is pretty reasonable and would probably be strong enough. The big downside is that it's not as sexy if it's just straight out and doesn't have a nice flowing curve to it. I don't have access to a bender large enough to handle that size tubing.

I doubt there's much reason to spend a lot of $ on exotics. I'd plan on plain old 6061. I think I can nest all 4 arms (8 pieces) in a single 4x8 sheet, so that may be the most cost effective. I can also get them cut on pattern torch for free.

 

I'd also considered a thin wall aluminum tubing wrapped in carbon/fiberglass. That might be an option too, but I'm not sure it would save much time over the regular beam design.

 

Most aluminium beam trimarans use a waterstay to put the tube into compression its a valid approach and means you can cut down on wall thickness of the tube and hence weight you also reduce point load and reduce metal fatigue problems. In a trimaran the size your proposing I'd look at the Buccaneer 24 trimaran design and utilise a similar system (lots of threads on the multihull forum about it). There is no need to reinvent the wheel its all been done before. If you want a curved beam a composite cored or strip plank beam is stronger, lighter and less subject to fatigue issues. I thought the scarab plans included aluminium beams anyway? He will already have addressed the issues discussed previously If I recall correctly his folding system is a bit like Farrier's and uses waterstays to put the beam into compression.

 

The plans come with a beam design for building a mold and a dozen or so layers of various fiberglass fabrics. They are a lot like the farrier system. No water stays. Mostly the process seems really time consuming to build them and kinda expensive, and probably really heavy.

I'm not opposed to water stays, but I'm not sure I'll need them on this small of a boat.

Also since this boat is so small I don't have to go with a farrier style folding system to keep the folded beam under 8' for trailerability, but I do tend to like the compactness of how that design folds up.

 

You may be able to get an idea from the ian farrier tramp or eagle as it may have heen known as in the US.

As far as I know it used alumium cross beams, possiblly with a bracket added to round tubing

 

The malcom tennant sylph may have been a folding trimaran with alloy beams, which may be another idea, to consider.

 

Re Sylph, scaled from the study plans the Sylph forebeam is 100x220 rear 100x200 made from 100x50 box section extrusions and flat sheet. Has a single strut folding system like some of the Graingers. Don't know if any were ever built.

 

I took a look at the sylph. Super simple. I don't think it would work with the design of the Scarab though.


I can buy 6061-t6 in 3" .250 wall for about $300 for all 4 beams. I'm not sure there's a cheaper way to go.

I imagine the correct aluminum structure would be both lighter and stronger than fiberglass. I would think that with the right design, the steel alloys could also be lighter and stronger. I'm not sure I could afford custom double butted tube though...

 

The turbo 6 is a 6m folding catamaran by malcom tennant,that uses alloy beams.
The turbo 6 uses 100 mm sqaure tubing, with a centeral strut for a out board pod.
You may need to be increased for the scarab 18 beam.

You may me able to combine the ideas with the scarab 18. I am not sure of the beam of the scarab 18.

You may be able to use a H shaped strut, for a lower strut.

 

Ally tube is likely more predictable than a home-built glass layup.

I didn't understand the post about placing Aly into compression since it works very well in tension too, unless it was to convert bending forces into compressive ones which can also be done with a brace.

If/when you get to bond the Aly to another material there's some info in this thread -

http://www.boatdesign.net/forums/materials/bonding-aluminum-epoxy-2629.html

 

will you be using the main hull beam section? You may be able to use a strut Like an I beam with the ends noched either end to fit.

 

My reply terminology could have been better but many trimarans place the beam into the role of a compression strut using the waterstay as a tension member interesting thread by catsketcher on the subject of the twiggy beams is here.

http://www.boatdesign.net/forums/multihulls/reverse-engineering-twiggy-38602.html