Forming Aluminium Sheet - Novice advice

[simons]

Hi, I have an aluminium boat mast with a bolt hole that has some cracks coming from it. I would like to rivet a doubler plate around the hole to strengthen it. I've attached a couple of pictures of the job to give an idea of what I'm trying to do.

My first question is what type of aluminium to use? The mast is 2mm thick. It's 40 years old and I don't know what flavour of aluminium alloy it is. The internet tells me that modern boat masts are commonly made of 6061 T6 sheet. However this type of sheet is tempered which makes it more difficult to cold form, which I think I will have to do to get it close to the curvature of the mast before riveting. Will I be able to form T6 sheet to the curve in the pic or am I better of going with a more malleable grade of aluminium and sacrificing some strength?

My second question is how best to form the sheet to the curvature of the mast. At the moment I'm thinking of creating the mast profile in wood, probably ply cut to shape and laminated, then forming the plate round it with a hammer.

Any tips or suggestions appreciated. As I said above, I'm a novice when it comes to metalworking.

 

[addjunkie]

Is it the top or bottom I cant tell from the pictures. What is the hole for?

To form sheet properly you use slip rolls or similar.

Mini Slip Rolls | Sheet Metal Roller Vice Mounted Fabrication Bender

Slip rolls for sheet metal fabrication work.

www.warco.co.uk

But Id suspect with 2mm sheet youd probably be able to hand bend round the mast then trim to size. Another option might be a machined top hat in an enlarged hole, and bonded in with epoxy.

 

[Hood]

1050 would likely do you, easy to form and good corrosion resistance, the only thing is it is soft so it depends what forces will be on it.

 

[simons]

Is it the top or bottom I cant tell from the pictures. What is the hole for?

To form sheet properly you use slip rolls or similar.

Mini Slip Rolls | Sheet Metal Roller Vice Mounted Fabrication Bender

Slip rolls for sheet metal fabrication work.

www.warco.co.uk

But Id suspect with 2mm sheet youd probably be able to hand bend round the mast then trim to size. Another option might be a machined top hat in an enlarged hole, and bonded in with epoxy.

The hole is one of two. The securing bolts for the head box pass through them. On the one hand, I don't think the load can be enormous, given the shape of the bolt (see pic). However on the other, the backstay attaches to the headbox in order for you to be able to bend the mast to suit the conditions. Max load for the backstay is 270kg.

 

[simons]

Another option might be a machined top hat in an enlarged hole, and bonded in with epoxy.

Thanks, this is an idea I will investigate further. The head of the stainless j bolt in the post above needs a nylon washer under it to prevent galvanic corrosion between it and the Aluminium mast. I'm wondering if a top hat made of something like acetal would have sufficient strength but also do the job of electrical isolation. Even If I had to change it periodically it could be a neat solution. I'll be passing by the guy who has made bushes etc for me in the past on Monday so I'll drop in for a chat.

 

[voipio]

It's difficult to see any cracks in the metal around the hole in the original images. If there is cracking, then that would suggest a considerable amount of strain on the part. Any replacement/reinforcement pieces will need to themselves be sufficiently strong, and will also have to be solidly bonded to the metal that forms the mast, probably by welding.

An alternative might be to vee out the cracks and carefully lay a TIG weld bead along them to close them up. How successful either approach would be, would depend on the mast's alloy composition, and the filler rod used

 

[RichardM]

What about just shortening the mast by 70mm re drilling the holes and reforming the slot to suit?

 

[Bullet2012]

What about just shortening the mast by 70mm re drilling the holes and reforming the slot to suit?

Oof , that would make your eyes water, having 70mm removed from your main spa

 

[waddycall]

If it’s 2mm you could probably pull it down onto the mast with a ratchet strap. When masts are shortened there’s usually a load of rivets fixing the two ends to an inner sleeve. Maybe you could cut a patch, put it in situ, fit a line of rivets down the centre line joining it to the mast, put a ratchet strap round the mast and patch, maybe with a spacer block underneath and gradually pull it down and rivet as you go.

 

[waddycall]

or use clamps with two timber packers cut to fit around the mast profile.

 

[waddycall]

Or do it properly with a big hammer

 

[Misterg]

I'd probably rivet a stainless doubler onto that to take the load from the bolts. Put something between the alloy and the stainless to stop galvanic corrosion (thin plastic, thick tape, Sikaflex, etc.) and use Monel rivets (and Duralac).

Something like 1.5mm thick would bend into place by hand (either directly on the mast or over a pipe(say) held in a vice).

The mast extrusion is probably a 6XXX alloy, so I would use the same if making an alloy doubler. Again, I don't think you'd find it too hard to bend it around by hand at 1.5 or 2mm thickness (especially if you started with a piece that was longer than necessary).

Drill small holes just after the ends of the visible cracks to try and discourage them from propagating.

What class of boat is it off?

 

[addjunkie]

I'd probably rivet a stainless doubler onto that to take the load from the bolts. Put something between the alloy and the stainless to stop galvanic corrosion (thin plastic, thick tape, Sikaflex, etc.) and use Monel rivets (and Duralac).

Something like 1.5mm thick would bend into place by hand (either directly on the mast or over a pipe(say) held in a vice).

The mast extrusion is probably a 6XXX alloy, so I would use the same if making an alloy doubler. Again, I don't think you'd find it too hard to bend it around by hand at 1.5 or 2mm thickness (especially if you started with a piece that was longer than necessary).

Drill small holes just after the ends of the visible cracks to try and discourage them from propagating.

What class of boat is it off?

That would work, after all the mast fittings for the spreaders etc are stainless riveted on.

 

[simons]

Or do it properly with a big hammer

Thanks for the ideas

 

[simons]

What class of boat is it off?

Swift18. The damage was actually caused by me trying to remove seized head bolts. I gave the spanner a bit of welly without really thinking it through. The head of the bolt was out of sight underneath. Before I realised it had twisted and cracked the hole. Next step was the nutcracker. Which in hindsight should have been the first step. I've actually done a season of sailing with it in this condition and as far as I can tell the cracking hasn't creeped. But it's a nagging concern that I need to sort. I think I have enough suggestions now to get on with the job so I thanks to everyone and bring this thread to a close.

 

[julianf]

6000 series will fracture as you bend it.

1000 series will bend fine but then not have a whole load of strength.

If you want to bend alloy, a small hammer and lots of little taps works loads better then just whacking it. You can use whatever you have to hand as a former (does not need to be perfect as, with your lots of little taps, you can keep an eye on it and adjust as required)

Again I can't comment on the structural issues as I know nothing at all about boats!

 

[Misterg]

6000 series will fracture as you bend it.

For tight bends, yes, but I’m confident a 1.5 or 2mm thick sheet would bend to the profile of that mast without any issues.

 

[Keith 66]

I have repaired several similar. My Sabre 27's mast is the original 1971 spar & had some cracks round poorly designed crosstree mountings. It is best to drill tiny hole at end of crack so it doesnt propagate further. I rolled two much larger mounts from 1.5mm 316 stainless & tig welded the crosstree sockets on.
Use monel rivets with Duralac zinc chromate paste underneath to keep corrosion at bay.
A patch over an old redundant winch mount was made from a section of old mast from the club skip cut out & annealed. Often old or broken masts kicking around boatyards & clubs as source material. Sometimes you might be lucky & find an old section slightly bigger that will just slide on.
To anneal paint a stripe of soft soap on the ally, heat with blowtorch, when the soap goes brown quench with water. The alloy will now be soft & can be bent & worked easily. It will work harden fast so you may have to repeat the annealing.
Other mast repairs can be fabricated using fibreglass & this has the advantage of no corrosion issues. Our last boats cast ally mast foot corroded away, Couldnt get a new one for love nor money so i made a new one from glass & epoxy, It took a bit of creative thinking & a three part mould made from Mdf & packing tape as release agent but it worked. 20 years later she is still sailing