5083 or 6082t6 for catamaran beam?

[resynth]

I'm replacing a soggy wooden beam on my 10 metre cruising cat and have opted for aluminium tube (substantially cheaper than mast section).
Most suppliers only sell 6082t6 grade but my understanding is 5083 has significantly better corrosion resistance?
I have found one supplier who can sell me 5083 tubing but it's nearly twice the price. If I need the 5083 that is fine but I'd rather not pay double if the 6082 will do a satisfactory job.
So my question is - would painted 6082t6 aluminium be considered reliable/long lasting in the service of a beam on an ocean cruising cat or should I pay the extra for the 5083?

Cheers
Ben

 

[Ad Hoc]

Hi Ben,

If you are replacing a beam on a multihull - catamaran - then the issue should be strength and deflection driven.
Then you address any possible corrosion issues.

Both are good for use as beams, but, like with anything in design - depends upon the final details.

First and foremost, get the strength and the deflection, in terms of scantlings you need.

 

[resynth]

Hi Adhoc,

I'm quite happy about the strength and scantlings, the corrosion issues are what I am unsure about.

Cheers
Ben

 

[Ad Hoc]

Ben
So long as you take measures to prevent any galvanic corrosion, you should be ok.
Thus be mindful of any dissimilar metals it is attached to or in close proximity and how the beams are attached and fixed to the main hulls.
I regularly use 6000 series for structure that connects the 2 hulls of catamarans.

 

[resynth]

Great, thanks Ad Hoc. I figured that as mast seem to be made of 6000 series but wanted to make sure!
I'll be bonding it in with G-flex and will bond the seagull striker on also. Should be no holes and no metals.
Ben

 

[gonzo]

Are you getting anodized tube?

 

[goodwilltoall]

Ive had problems with epoxy to aluminum

 

[resynth]

Are you getting anodized tube?

I can't find anyone who sells anodised in the size I want. Also I read somewhere that epoxy doesn't work as well with anodised. I'm going to etch prime, paint and try and keep the inside sealed.

Goodwilltoall - There are already instances of aluminium epoxied to my boat that have been there for 22 years, still holding strong so fingers crossed!

 

[Ad Hoc]

I can't find anyone who sells anodised in the size I want. ..

Not necessary.
So long as you pay attention to the connections/joints and anything that penetrates the structure and take measures to mitigate any possible galvanic action - you'll be ok.
We never use anodised ally for our cross deck structures...

 

[trip the light fandango]

There are already instances of aluminum epoxied to my boat that have been there for 22 years, still holding strong so fingers crossed

I just looked up G flex ,it sounds great, but when you say your going to glue the new beam on, do you mean into a housing/ bed with brackets, because what surface material is on the other side of the epoxy join that can manage flex and load? I get it with the post for the striker I think, also there won't be any paint in between the epoxy and the joins I'd imagine. I'm just curious, regards

 

[resynth]

Fandango - I've chopped the deck off and a bit of the inner topsides. There are 2 bulkheads at the bow with a shelf between them. The beam will be bonded to the shelf and strapped down with uni-directional running down the bulkheads. Repair deck and hull, nice fat fillet where beam enters, paint beam, hopefully job's a good 'un!

 

[trip the light fandango]

That sounds good, I've been following types of aluminium and suitability for welding, 6061 loses tensile strength if it's welded,.. I think. so the glue is interesting, good luck with the project.