alloy vs SS

[jambaman]

Thought i could find out this info on the web, but alas.... so now over to the experts.
I wish to build a single goal post, not the double type you normaly see, and palce two solor panels ontop. At the moment we have two posts but no cross bar. One of the posts is Alloy, 70mm OD, 8mm ID, whilst the other is 50mm OD and will be replaced. The 70mm is strong and sturdy enough to hold a cross bar and could be the simple solution. however, i would like to know if for strength to weight ratio, i could use SS tubing, and if so, what OD and inside diam, should i use.



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[Rohorn]

Hi....I fitted my cat with an alloy bridge (portique) to carry two solar panels and a genny, flag staff etc, and davits. Used rectangular section tube, 55mm times 35mm and 3mm thickness. I used two uprights each side, tapered together, and the cross span I used thin rectangular section tube as used by builders to level concrete etc, also doubled and tapered together, the lot epoxied. The span from side to side is wide....three and a half metres...and it supports a load of 150 kgs at midspan.
I expect your span to be much less. The one upright you say is 70mm OD but the I.D. you give is 8....can't be 80 so is that wall thickness? That's very strong, but how is it attached to the deck? Is it braced by the pushpit? Just cantilevered?
You don't want it waving about. Weight for weight, a good alloy (Al-Si-Mg) tempered is as strong as mild steel which is a guide. Don't mix alloy and stainless...but I expect you know all about that.
It often comes down to whats available locally. Personally I love working with rectangular alloy tubing. So precise, easy to work and bolt things to, or pop rivet, paints well or anodises, and is very strong, and looks good.

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[richardandtracy]

The answers to questions you ask are not quite as simple as you might believe.

Firstly aluminium is generally weaker than stainless.. No I'll correct that. Most Aluminium breaks before stainless. The failure stress of 6082-T6 extrusion is around 295 N/mm^2 (depending on thickness), while most 316 alloys in the softened condition are 490 to 510 N/mm^2 [stainless tube etc is usually softened]. However softened stainless starts to bend permanently at about 180 to 210 N/mm^2 while 6082-T6 extrusion does it at around 255 N/mm^2. The upshot of this is that aluminium bends permanently after stainless, but gives less warning before it breaks (7% elongation rather than 40% for stainless).

Now, stainless in the hardened condition (ie bolts), is more commonly found with a failure strength of 700 N/mm^2 (A4-70) or 800 N/mm^2 (A4-80).

Other material facts you may need to know. Stainless is 3x stiffer and 3x denser than aluminium. This means that a stainless & aluminium beam with the same shape and cross sectional area will both bend the same amount under their own weight, however the stainless section will be more highly stressed and will be able to carry less additional load before it bends permanently.
When you add extra load to a beam, the aluminium one will bend 3x further than the stainless one for a given additional load. If you increase the stiffness (by this I mean use a deeper beam) of the aluminium beam to give the same deflection as the stainless beam, you'll end up with a beam that is much stronger than the stainless one, about half the weight and giving the same deflection.

So, you pays your money & takes your choice.

As for sizing the section.. What span, what load & what deflections are acceptable?

Regards

Richard.


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[jambaman]

Thanks Richard and Roger,

I certainly asked for this didn't i!!. I will keep your breakdown for future ref.

Maybe the best way to proceed is to tell you what i want to do.

We are steel, E24E to be precise, and i wish to place two of our 4 solar panels across the stern + the wind gen + antennas, whilst the other two SP’s will have to go on either side of the cockpit.
The two existing poles on the stern are, port 2.52m long OD 50mm, wall thickness 4mm (plate on top with GPS + HF ant.) stb 2.75m long, OD 70, wall thickness 8mm (houses the wind gen). there is a plate welded to each pole which is bolted onto the top of the transom, whilst the remainder of the pole goes down to the sugar scoop. Midway between transom plate and sugerscoop, the poles have another welded plate which is also bolted onto the transom. I have practically swung on both poles and neither budge, except a little bend. All joins to the hull are insulated. The distance between each pole is 2.15m.

I wish to replace both poles as they are not tall enough to place a bar across, to 3.5m tall, then with brackets, join the two poles together with a bar, from there I will attach/design something to take the solar panels. The whole idea is that 1. I can take the solar panels off at any time with little fuss, and 2. take the bar off with relative ease.

The initial reason for asking about alloy = ss weight / strength ratio, is that I’m concerned about the weight on the stern. Presently we have a 6 man old Zodiac life raft which weighs 45kg, (+ what ever water gets inside the canister. 13kg poured out at the service in June!!). We will replace this raft with a more suitable 4 man, but they still weigh in the region of 35kg. This is placed in the cockpit, up against the transom. I know that the 70mm OD tubing would be strong enough to take on the task, but could I reduce the size of the tubing, but not the strength with SS, and of so, what would that size be?

One of the problems I face, is once I’ve decided whether to go for alloy or SS, is then to find in Greece the correct grade material. The SS available in Corfu is not of the best quality, and taking into consideration your comments about flexibility, then maybe alloy is the way to go. If this is the case, should I seek 6082 – T6 alloy, or would you advise another type?

Many thanks


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