Floating Island, more thoughts Opinions please.

My fabricator has submitted the design below, using 4 mm aluminum.
2250 mm wide pontoons, with support baffles every 1900 mm.
Back of the envelope calculations tell me each unit can support 2,624 kg at a submerged depth of 1.5 meter.
We think we can use 12 meter long, 150x50 aluminum extrusions, 3 mm thick, spaced at 24 inch center, to create a platform 24m long by 12 m wide, with a 2.4 m overhang on both sides and a 2.4 m spacing between the pontoons.
This platform will be used in sheltered coastal waters, mild seas
Is the 4mm thick enough or would 5 be better?
Would 200 mm high extrusions be better/stronger than the 150mm?
I appreciate every comment.
Thank you.

 

The structure that has been proposed is most likely insufficient, although nothing can be said. Of course, if you don't care about weight, 5mm plates are better than 4mm and 15mm are better than 4mm. If you don't care about weight, the bigger the reinforcements, the better.
Longitudinal reinforcements are missing.
The spacing between frames could be too large.
The frames probably needed to be formed by a web with a flange.
You do not want, I am sure, the pontoons to be submerged 1.5 m. Therefore, the information they have given you in that mater is not very useful.
If you do not want to regularly sail with this craft, completely rectangular cross sections would be preferable. I like better the sections that you showed us in your other thread.
In short, there are many elements in this area that should be analyzed with much more attention. And keep in mind that the best structure is not the strongest but the one that effectively meets, and with a sufficient safety margin, the SOR of the device.

 

Thank you

 

The vee bottom adds draft and expense with no advantage. A flat bottom would make more sense. The drawing is missing the most critical elements, which are the structural attachments.

 

If you want some help during movements of the island; the best way would be to upturn the front. There is little value in the vee as others have said; basically lost displacement. Most of this is outside my wheelhouse because the critical piece is the cross structure and so I would normally not speak as not qualified to specify it and do not believe I did on the other thread.

But I’d build this like a garvey or pram hull on the bows.

I might be inclined to also consider angling the sides a bit. This would provide more width for structural connection, reduce the spans for the beams, and offer some reserve displacement for overload; though not significant, and also provide a different behavior for waves hitting the sides; I believe a beam sea hitting a flat side will shoot striaght in the air and wash over the thing while an angled side would break the sea. This, of course, can be alleviated other ways, like the deck sitting above it all, etc. Unless sorely mistaken, the trapezoid shape may also be easier to fab as net net there is more space. But, this is not my expertise; just an idea.