The LLMini is designed around 'easy available' materials.... 60x2 tube is much easier to get than 47x3 drill tube.
60x2 is whats used on street signs and fence posts, the 47x3 is seamless high tensile tube maybe easy to get in a mining town but not available everywhere.
Also the chassis and mast flex is critical in the sailability of a yacht, to soft it handles like mush and won't perform well, to stiff and you pop wheels easily and break welds.
There is a balance between both ends of the spectrum.

Well as it happens I have the exact tube from the exact same source, currently being used as the draw bar on my boat trailer... I only have a stick welder and it might be a bit challenging to weld the thin tube but I'll give it a go.

From what I've seen, chassis twist becomes less important if you have a flexible mast. Hence, unless you have a very stiff mast, chassis twist won't be too much of an issue. Of course this isn't to say it doesn't matter, and if you want a really forgiving yacht then max flexibility is your target. I can say that the Gal tube chassis works very well!

There is another way of doing this Flexible rear axles
I use ski axles but they could be made up with multiple layers of timber By removing and adding layers they can be varied to suit
I find the ski axles doubled up just right for me If using timber the axles need to be wider than they are deep to prevent flex forward and back
I have seen attempts to make the spine with variable flexibility but it never came to anything

All great advice on the mods there guys.

Just a heads up in Aussie, that the 60.3mm x 2.3mm "Gal tube plus" pipe, is no longer made. This was a product name for the coating used on the pipe.
So now ask for "DuraGal Plus" that has replaced it. It's only the makeup of the galvanized finish compound that has slightly changed. It is still the same "cold formed" seamed pipe, that's hot dipped galvanized. "Street sign material"
(I notice that there now seems a trend to rectangular "colour-bond patio tube", for our local street signs.)

So
Standard Duragal C350LO is 350NM. Yield Stress.
DuraGal Plus 450PLUS is 450NM. Yield Stress.

I have built chassis from both the C350 and C450NM with the same wall thickness/yacht dimensions and I cant tell the difference to be honest. (No mining here locally to try the smaller diameter drill tube, all my yachts are gal pipe.)
Both strength pipes flex beautifully when loaded up.

Hiko -what you describe was what I was thinking. The advantage over the flexi axle ( eg skis or timber) might be the camber on the rear wheels won't change as much in use, but I don't know if this really is an issue.

Since it sounds like it works quite well I think I'm going for a flexi axle - probably skis as I can get them for little$. I tried an axle like this made from laminated bamboo blind slats in T and Y frame shapes on the RC yachts and it was effective , but with some unusual bending/twisting under load, especially with the Y. Much more forgiving than a rigid rear axle.

I would like to know relatively how stiff in bending and torsion the ski type axle is compared to the spine - like has anyone loaded an axle up over some supports to measure the deflection?

Should the layers of the spring material ( ski, wood) be bonded to each other?

gofaster said..
This is interesting. My calculations show the dia60 tube to be nearly twice as stiff in torsion as the dia47. I'm tending toward making the frame on the side of lower stiffness to make for forgiving handling. It is very noticeable on the radio controlled yachts how much easier they are to sail in very strong wind if there is frame flex.
It would be interesting to incorporate an adjustable torsion element in the spine, like a torsion bar, and experiment.

Having one 60mm galtube, and 47mm drill rod chassis, I would hazard a guess and say this sounds ballpark right