Aluminum Safety Factors

Ignoring deflections for the moment…

Knowing the loads, what safety factor(s) would a boat architect use in designing an aluminum mast or cross-beam for a beach catamaran?

Would this be used typically against yield or ultimate strengths?

Which flavor of aluminum is most commonly used for boats (eg T6, etc)?

I’ve got everything else… just want to know what the industry standard is for safety factors.

Thanks for your help.

 

Can't speak to which factor of safety is typically used for your application, and I doubt that there's a "rule" for it, but in general, if the consequences of failure are very high (death) or the anticipate loads are poorly known, then the factor is higher (as high as 10). If the loads can be accurately predicted, the consequences of failure aren't severe, or weight minimization is critical, then it can be closer to 1. Start with a conservative factor, say 5, and see how that affects the design, then adjust.

FOS is based on yield strength. From a design perspective, "failure" occurs when the yield strength is exceeded.

The 5000 and 6000 series of aluminum are typically used for boat construction - each specific grade as varying characteristics such as heat treatability that may or may not apply to your application.

 

The general type of aluminium that is used is AlMg 4,5Mn for the hull and AlMg5Mn for the superstructure.
There are other suitable alloys, but most boatbuilders use the above mentioned types.

The safetyfactor?

Frankly, there are no rules except the ones that are used by the classification societies asa ABS-DNV-GL etc. But they don't apply to craft below 20 mtrs.

Also various thicknesses are used in a hull: e.g. keelplate 8 - keelsides 6 and topsides 5, superstructure 4mm

Common practice dictates mostly - if you look at the many failures in sailing yachts, than you know that the factor safety is a relative issue.
Because - what should the safetyfactor be and and what circumstances?
Therefore a good question.

One designer takes 4mm as general thickness for hull material, another one prefers to use 5mm.

If you build a 40 ft sailing yacht, generally 4mm is used. But, most hulls receive an after treatment by grinding the outer surface and easily the good part of one mm is taken away so the hull might be only 3.4 mm thick - and that is a little narrow to the edge. Therefopre 5mm is better, bot most builders don't.

Regulations within the shipbuilding circles is an odd mixture of ancient practice - old rulings - because rules did not change as materials did.

For masts and booms, the mastmaker looks at the righting moment, the different loads that are stressing the rig and they have their own knowledgwe and common practice to make things working. If they fail, we have than a mast that goes overboard.

When an exceptional rig is mounted on a ship - e.g. the mast of Mirabella V -
than the classification society will lengthly confer with the rigbuilder in order to see that the chance of breaking is minimised.

It is therefore very difficult to give a certain item a safety factor because the conditions of use varies constantly and cannot be brought into a standard rule. What remains is therefore common practice.

 

Thank you for your thoughtful comments. In many other fields, they have well defined safety factors. And basically, you are just saying in marine design, it based more on logic and the conditions.

Just to elaborate… For the SC20 project (20’ Beach Cat), I was analyzing the ~100 inch cross beams and wishing to make them out of fiberglass. Anyone else would rightly just make them from off-the-shelf aluminum Zspar and be done with it. However, this is a learning tool and as such, I want to flesh-out my fabrication and design experience. For the fiberglass design, I will be using the F.S. of 5 that we established in the fiberglass mast thread and I’ll be using constant strength concepts (versus constant cross-section) to reduce the weight. I will also make sure that it is at least as stiff in bending. We don’t want any wet noodles!

Although it may sound like it, I am not reinventing the wheel here. I am trying to rationalize all the possible loads; however, I am also double checking by reverse engineering a proven design just to make sure that my design is at-least as strong/stiff as a given design… And that design uses an aluminum Zspar beam.

For instance, the rear aluminum tube can take a central point load of 1433 lbs at yield. If I now use the fiberglass F.S. of 5 on top of this, I’d have a total F.S. of 5 times whatever they used! Makes mine a little too heavy. The buoyancy wouldn’t hold up the Land Rover required to break it.

 

That would be a nice experiment - in itself feasible to execute; and a better solution than off the shelf profiles since you can shape your profiles as required; next to that you will aquire a higher stiffness;

A would use for the hull AlMg6, that's a mil spec grade of Alu, a bit harder and that means that you may go a fraction thinner in sheet dimension e.g. 2,6 - 2, 8 mm if you apply sufficient internal reinforcement. Not welding but rivetting and gluein will give you a lighter and overall stronger hull. But that you might know fro AeroSpace.

 

You should be able to exceed the performance of aluminum in this beam design. You can add more material in the high stress areas and/or modify the cross section of the beam. You might also consider using a high modulus reinforcement like carbon in strategic locations.

The challenge will be accurately predicting the performance of the composite. With a metal the properties are well known and published, and they are uniform in all directions. For a composite there are a whole bunch of variables such as fiber orientation, type of resin ad fibers, layup method, etc. With all of those unknowns I'd suggest a higher FOS or perform some bench tests on samples until you understand the properties.

 

Absolutely......

 

types of aluminium (why US people always have to be the odd freaks - aluminum...) you should use are 5083 or 5059 (even better). These are marine grade alloys and best for sheet and plate applications. Silly gringos tend to wrongly call alloy 6061 "marine" aluminium because it has fairly good resistance to corrosion (but not as good as 5083 or 5059) and you might get away with it for structures on the deck. It is suitable for extruded and drawn products. In short, for the hull, do not waste time with anything other than 5083 or 5059. If you are building below 15 metres, d'artois gave you good numbers for plate thickness.
(hola amigo...)
If in doubt, let a professional do the maths. 5000 series aluminium is stronger than 6000. ISO standards are the best guide as all insurance companies will insure your boat (if you are crazy enought to buy insurance sale pitch) if build by ISO rules.

 

I'm sure you are right... but I have to share a little story about maxiumum strength and experts.

There's an old story in Aerospace, where some expert specified the strongest steel he could find for wing bolts for a C-5. Well, they reached their fatigue life while taxing to the runway. The wings fell off the plane when the pilot ran-up the engines.

 

Hmmm.. the discussion is about strength, while stiffness becomes an issue before strength in aluminum. Just a reminder, maybe this isn't the case and i'm talking before my turn.