aluminum grade for mast

I am building a 40' sailing catamaran that will have a mast made out of 8" aluminum pipe. The mast is 40' high so two 20' lengths will have to be welded together. My question is what type of aluminum should I use?

fritz koschmann

 

I'm not sure about what grade you should use as long as it is seawater resistant.
I'm also not sure that welding the two pieces together is such a good idea, as the mast will be quite heavily stressed. Riveting the pieces together over an internal sleeve would probably be better, even though this makes a part of the mast somewhat stiffer than the rest.

 

Have you considered buying mast sections?

 

Round pipe is not good for a multihull mast, for two reasons: One, round is a poor shape for aerodynamics on a fast boat. A more oblong shape is better, such as a standard mast extrusion shape. Second, pipe is heavier wall, typically, than you need, and if there is anything that multihull sailors agree on, it is getting rid of excess weight.

In the US, you should be looking at 6061-T6 aluminum as that is what most mast extrusions are made of. It has adequate strength, corrosion resistance, and is easily extrudable. I agree about riveting and not welding. The welded strength of aluminum is always a lot less than the virgin metal strength, unless you re-anneal it. All production masts are riveted.

Eric

 

more about the boat

Thank you for your replies. The boat is a Bolger design and some photos of the building and some lines drawings can be seen at www.thegreatsea.com

The mast has no spreaders and the sail is what Bolger calls a chinese gaff. It has full battens, each having jaws around the mast. There are also three sheets. A drawing is on my webpage. I am not building a multihull for speed but for "real estate" and the ability to carry a load. The boat will be slow compared to other modern cats and it is heavier.

The idea of riveting sounds interesting but the mast would have to be smooth on the outside.

fritz

 

When done a professional spar maker, they use flush rivets just like on airplanes. Alternatively, they may use stainless steel flathead machine screws which, of course, sit flush. So a flush surface is not a problem. Figuring out how many screws is the engineering problems that has to be attended to.

Eric

 

On the grade of aluminium:

Aluminium 5000 series alloys (5083, 5086)
Aluminium 6000 series alloys (6061T5/T6, 6082T5/T6)

These are Marine grades.

On the welding: welding can weaken the aluminium, but doesn't always. It depends on the ally's condition (heat treated, hardend). But if you know the properties of the material and the stresses at the weld you can asses weather welding is an option or not.

Remember: using stainless steel in aluminium, take measures to prevent galvanic corrosion.

Hope to have been of help.


Peter