Design of wood epoxy trimaran beams

[Adrian Baker]

I'm looking for some guidance on building 2 beams for a trimaran I'm building .
1. Shape style of beams see picture re options I've considered.
2.general strength calculations and shape.
But first here's a little about the boat.
Loa 9mtr ,beam 6.8mtr
Weight target 1000kg-1200kg its mostly 6mm ply but some 9mm as well
The beams are straight, with waterstays and a stay from fore and aft of the fore beam to the mainhull for extra beam stability .
The boat is similar to a buccaneer 28 just a little longer but with the same width of 6.8
So I've done some work in reverse engineering the bucc 28 aluminium tube beams (139.7 x 4.77) using a beam compression calculator and found them to have a theoretical failure at 488kn using 2.4m as length ,1 end fixed 1 guided , with slenderness ratio less than 50 .
So converting that to wood is for me the tricky bit ,I don't want to end up with really heavy beams that I can't lift as the boat is demountable . Using a compression calculator and some wood data I've come up with this, the wood has a E of 10 GPA and compression of 42 mpa so I'd need about 120 to 140 cm^2 of area
In one of the pictures I've drawn 4 beams styles I've considered but I'm having trouble deciding which suits best and is the most efficient use of materials
Any thoughts or ideas welcome

 

[gonzo]

Converting an aluminum beam to wood is rather difficult. To start with, the materials have very different properties. A beam will have point loads at the attachments. Wood is softer and will need completely different hardware, not simply a welded tab. Backwards engineering is unlikely to work well. You need to calculate the beam as part of the new design. What is the reason to use wood? Good quality wood will cost more than aluminum sections.

 

[Adrian Baker]

Thanks for your comment, reason for wood is mostly where I live, aluminium sections in 6061 t6 or similar are very expensive and a bit hard to get,the problem of living on an island I guess

 

[Adrian Baker]

The beams on the tri built by gougeon bros Adagio are an example of of my proposal, anyone know anything about their constitution I think there strip planked wood

 

[Russell Brown]

Hi Adrian,
I wished we lived closer, I'd set you up with pultruded carbon for your beams. We ended up with a ton (literally) of pultruded carbon left over from the StratoLaunch project. I'm using it for the caps for the beams of the trimaran I'm building, plus the foils and other stuff. It's 1/8" thick and way more than double the fiber necessary for the caps on the 5" x 5" beams than I'm building.
I'd say that if you build plywood/timber box beams, some uni fiber (hopefully carbon) would help with tension and compression and that the beams could be wrapped with + - 45 % glass.
Obviously plywood & timber only beams work fine, they just have to be bigger in section, but I don't need to tell you that.

 

[fallguy]

It is out of my league to design, but I see a flaw in the approach and here, agree with Gonzo. My boat is like this one and so I want to reverse engineer is not the ideal way.

The way to design is to understand the forces and deisgn to meet. The forces and safety margin are based on the intended use and bluewater versus inshore, etc.

Another issue in beam design is dealing with rotational forces.

I apologize I can only highlight the concerns. I recommend you get the help of a NA because you will always have peace of mind in any planned seastate.

 

[oldmulti]

Hi Adrian, if you already have not viewed this link it may help. :
Trimaran crossbeam calculations https://www.boatdesign.net/threads/trimaran-crossbeam-calculations.38345/

Russell Brown suggestion is a good way to go as he is building a similar type of tri but with folding beams.

 

[Ad Hoc]

...So converting that to wood is for me the tricky bit ,I don't want to end up with really heavy beams that I can't lift as the boat is demountable . ..

That is likely what you'll end up with, unless you are able to fully justify the calculations and fixings and material properties.
Since one will inevitably be driven by the stress, the deflection and the fatigue calculations.
Wood does not preform to the same magnitude as aluminium, in that sense.
Hence i dont think you'll be happy with the result based upon your comment.

 

[fallguy]

That is likely what you'll end up with, unless you are able to fully justify the calculations and fixings and material properties.
Since one will inevitably be driven by the stress, the deflection and the fatigue calculations.
Wood does not preform to the same magnitude as aluminium, in that sense.
Hence i dont think you'll be happy with the result based upon your comment.

And this is why an idiot like me throws up the red flag because the design approach is not great.

 

[Adrian Baker]

I totally get what your saying gonzo any fallguy .I've also tackled the load on the beam from a couple of angles and via different calculations, via righting moment and waterstay load. The torsion is the only load that's I haven't really looked at yet but the waterstays do a fair bit of work in that regard

 

[Adrian Baker]

Fallguy yes I'm definitely in the red flag zone to be honest I'd like to use an aluminium section . But the only place which seem to offer is around 2000km via road and 200km across sea so freight is a killer

 

[Adrian Baker]

Russell yes you're got a point re using a non-structural foam or plywood box with carbon uni + db that's what Sam Tucker did for this beams on his 8.5mtr stealth mission ,and I've got the schedule layup he used too. I was just hoping to go down the low tech path but.....

 

[bajansailor]

But the only place which seem to offer is around 2000km via road and 200km across sea so freight is a killer

Incat is in Tasmania, and they are a very large consumer of marine grade aluminium sections in building their fast ferries - do they obtain their materials from the same place?
INCAT - Building the World's Fastest, Efficient, Environmentally Clean, High-Speed Ships. https://incat.com.au/
It might be worthwhile contacting them to see if they could spare you a suitable offcut from their recycling bin?

Re calculating the strength of your cross beams, you don't want to do it this way

 

[Adrian Baker]

love the comic thats clever I should try that....Incat are a closed shop they don't do retail sales but good thinking...I brought 2 sections down from Brisbane 15 years ago when I had a poper job to pay for it ,it costed heaps and it was a logistics nightmare and nobody would insurance them against damage while in transit.

 

[bajansailor]

Oh dear, that is a bummer re Incat - and I guess their H & S bods would not be too keen on you rummaging around in their recycling skips.

Surely there must be carriers between the mainland and Tasmania who can offer a reasonable 'Less than Container Load' (LCL) rate for freight items?
There are ferries here in the Caribbean which carry everything imaginable as break bulk cargo, usually on a very informal basis, but their distances are much less - like 10 miles rather than 600 odd miles from Sydney to Hobart.

Just thinking a bit laterally - the Sydney - Hobart yacht race starts on the 26th - what is the weather forecast like?
It is often a demolition derby - there might be yachts arriving with broken booms and masts, and there might be some sections that boats want to get rid of, which 'might' be usable as cross beams?
(Sorry, maybe I am taking this recycling thing a bit too far).